The Secrets of Prof. Verschure's Rosetta Stones

[Andy Resnick]

This sample is part of a group all labeled 'Fen 6':

This heterogeneous rock is a carbonatite. On the left is crystallized carbonatite (probably calcite) with trace amounts of apatite. On the right is a carbonatized magmatic breccia, possibly a (carbonatized) damtjernite-like explosion breccia. In between is a narrow transition zone, likely re-melted and re-crystallized carbonatite; evidence of melting seen from some crystalline carbonatite grains at the boundary:

Different fields of view were used for PP and XP to highlight the selected, deformed, grain.

The transition zone itself consists of granular carbonatite and some apatite. The granular carbonatite looks like soap bubbles at high magnification:

The interface between re-melted carbonatite (upper part of image, below) and the breccia is marked by the appearance of opaques:

Breccia consists of K-feldspar (some altered) and phlogopite phenocrysts in a groundmass of granular carbonatite, apatite, and opaques. The large altered xenocryst has a large equant massed phlogopite corona:

While many of the phlogopite phenocrysts have severe dislocations:

 

[Andy Resnick]

This sample is a myrmekite:

The large anhedral grains are pyroxene and K-feldspar; what appears to be a groundmass is the vermicular intergrowth of quartz and feldspar:

Getting decent high-magnification images of the myrmekite was, in places, challenging because the vermicular texture can be highly three-dimensional. Optically, that translates to an optical thickness (proportional to the integrated refractive index n(x,y)= ∫n(x,y,z)dz) having large spatial variations. In XP, this creates a photogenic effect:

But PP imaging worked better using a low-NA lens and (digitally) zooming in for the desired magnification:

In a few places I could really zoom in:

 

[Andy Resnick]

This is another sample that was included in a publication:

Unlike many of the other samples in that publication, I have 8 samples identified as "69 Fen 33", 5 of which are not covered- the finished rock surface is exposed to air. I'm sure what to do about those- they appear very different without the canada balsam to sort-of index match, but I don't want to damage the samples by mishandling them or mounting them poorly. All 8 have slightly different compositions, so it's not clear which one corresponds to the actual published data:

Sample is a weakly fenitized gneiss: 20% quartz, 25% perthite, 35% plagioclase, 10%-0% anorthite. 10% biotite, 5% hornblende, accessory amounts of opaques, apatite, zircon, and allanite. Fenitization-1 product is 1% aegirine, Fenitization-2 products are 1% arfedsonite, 1% stilpnomelane and 1% carbonatite (in veins).

Oops- I hust realized I forgot to add scale bars... most of these images were shot with a 16x objective, FWIW.

I could locate grains of apatite, but regarding zircon- I don' think there is any in this particular sample (Fen 33 ii), but it does appear to have high-relief grains that fall on the clinozoisite <—> epidote axis: clinozoisite is colorless, epidote is ‘fluorescent’:

Zircon also has very high relief, so perhaps these are indeed zircon.

I think this is a grain of allanite (because of the radial fracturing):

But there isn't any halo of radiation damage (I couldn't find any such halos in any of the Fen 33 samples), so...?

Many of the biotite grains are either highly degraded or have a fine-grained corona when in contact with quartz, no corona is present when in contact with feldspar and precipitated opaques. I really like this example because if you look carefully, the plagioclase twinning is visible 'through' the corona:

Not sure how that happens...

What I assume are altered biotite flakes have been replaced with 'stuff', the only thing I can positively identify is stilpnomelane (finally, I spelled it without having to look it up!)- it's the furry ring surrounding a grain of quartz:

In some places, the quartz grains are loaded with minute inclusions: some are spherical (gas? liquid?), many more are not. And there's this chunk- no obvious fractures or twinning present, but there are hints of 120/60 degree axes and apparent zoning (unfortunately, hinting at 90 degree axes), so maybe this is hornblende:

Ideas on if/how to handle uncovered specimens are welcome, as are suggestions on the unidentified minerals!

 

[Andy Resnick]

If it helps, here's a pair of images from a different Fen 33 sample showing grains of apatite (ovoid, top left), altered biotite (top right), perthite (center), zircon/clinozoisite (center and lower left), and hornblende (?) (left and bottom):

In the very center is a crack/fracture in the sample (black in XP).

 

[Andy Resnick]

This sample is a weakly fenitized carbonatite:

Most of the fenitized samples I have were originally telemark gneiss (the country rock), so this one is of particular interest, as it demonstrates some chronological ordering between the carbonatite intrusion event and the fenitiztion event. According to the relevant paper, there were 2 fenitization events; the first one was a high temperature dehydration reaction while the second was a low temperature hydration and carbonation event- both are types of contact metamorphic events. This sample emplaces the carbonatite intrusion after the 1st fenitization but before the 2nd. Unsurprisingly, most of the sample is carbonatite (90%), but there is a reasonable amount of apatite as well (5%), in its characteristic anhedral, vaguely glomerular, habit.

The metamorphism is seen when examining biotite grains; these are surrounded by a corona of massed anhedral carbonatite grains:

In several places, the biotite has been replaced by chlorite and quartz:

And located within the carbonate (I suspect it's all calcite) are random cabonatite grains that have a relatively lower birefringence so they stand out from the crowd:

According to the paper, this sample is 90% (primary) carbonatite, 1% biotite, 5% apatite, 1% opaques, and the low-temp hydration-carbonation metamorphism produced 3% chlorite and accessory amounts of rutile, stilpnomelane, quartz, and fenitized carbonatite; the remarks state "
W_F2carbonatite; F₂Chl,F₂Qtz,Stp veins; Bt⇒F₂Chl,F₂Qtz,F₂Carb rims/veins"

I didn't see any rutile or stilpnomelane in this section, and I also wonder what a carbonated carbonatite would look like?

 

[Andy Resnick]

I chose this sample, possibly showing the chilled margin of an intrusion into country rock, because of the high concentration of sphene (a titanium-bearing mineral):

The magmatic component of this sample has a relatively high concentration of sphene (more than 10%) in a matrix of carbonatite, apatite, pyroxene (augite?) and heavily sericitized orthoclase. A small vein and smaller veinlet, both full of calcite, pass through the magmatic portion. Here's a pair of images of the veinlet as it passed through sericitized orthoclase:

The calcite veinlet is surrounded by some intermediate mineral, but I can't identify what it is.

The country rock (Telemark gneiss) has been strongly altered into perthite and chessboard albite:

Of interest (to me) are the sutured/interlobulated grain boundaries.

in the above, the lower left portion is sericitized orthoclase. I believe the light blue and yellow tints are evidence that the thin section is slightly thicker than 30 microns, but alternatively I may have inserted a 1/4λ plate. In any case, another interesting feature of this sample are the sphene grains near the margin boundary; the metals appear to have precipitated, turning the grains opaque:

In the above, it's clear that the group of sphene crystalline grains have become opaque (one is partially opaque). Looking at one in reflection shows possible phase separation of the metal oxides:

My first thought is that the dark grey corresponds to an iron oxide (siderite? magnetite?) while the light colored grains are titanium dioxide. But it's not clear that's the case, as the dark grey could also be ilmenite.

Ilmenite is supposed to be highly bireflective, but I'm not sure I can perform that technique. Epi-darkfield does not preserve the polarization state, and epi-brightfield was difficult due to reflection off the coverglass. I was able to to cancel out reflection from the coverglass by using crossed polarizers, but I think enough light passed through the sample, reflected off the bottom of the glass slide, and then passed back through the sample so that I was essentially observing transmitted birefringence.

 

[Andy Resnick]

This sample is prominently featured in: https://www.ngu.no/filearchive/NGUPublikasjoner/NGUnr_380_Bulletin_70_Verschure_35_49.pdf.

As presented in the paper, this sample originated in Hönstjern (UTM coordinates ⁵354-⁶⁵418) and is a carbonatized damtjernite-like explosion breccia. Using K-Ar dating on biotite, this sample was assessed resulting in a calculated age of 578 Ma. This sample is one of two given considerable text in the paper. Quoting the paper (I will insert my images here and there:

“Two explosion breccias produce ambiguous and conflicting age data:

In the Bamble region two carbonatized damtjernite-like explosion breccias have been studied, the Hönstjern breccia and the Tveiten breccia. They are situated less than 0.5 km apart, in an area dominated by anatectic paragneisses with intercalated amphibolites and metagabbros (Morton et al. 1970). The breccias lie about 10 km W of the nearest exposure of Permian intrusives of the Oslo Graben. The breccias are very similar; they consist of a wide variety of xenoliths and xenocrysts in a very fine-grained groundmass consisting mainly of carbonate, green biotite, opaques and apatite. Among the xenoliths three groups can be distinguished: (1) small, rounded fragments (up to 0.5 cm in diameter) of ultramafic, occasionally porphyritic rocks with phenocrysts of biotite or brown hornblende;

(2) larger, angular fragments (up to 10 cm in diameter) of crustal gneisses, amphibolites, granites and metagabbros; and (3) occasional fragments of a similar damtjernitic breccia. Many of the xenoliths and xenocrysts are strongly altered, but the abundant apatite phenocrysts and the cores of biotite phenocrysts and perthite xenocrysts do not show any alteration.

[Note: transparent apatite and green-brown biotite phenocrysts in the dark-colored groundmass]

[Note: perthite along the top, what I think is quartz along the bottom, and carbonate veinlets. I thought perthite resulted from metamorphic processes and would be considered an alteration, as is the quartz (higher magnification below, XP]

Numerous veinlets of carbonate transect both the xenoliths and the groundmass.

[Note: on left is sericitized plagioclase , right is perthite, center is carbonate]

Biotite phenocrysts from the Hönstjern breccia yield a K-Ar age of 578± 20 Ma, concordant with the age of the Fen complex. The partly chloritized biotite booklets from the Tveitan breccia give much younger ages, however: a K-Ar age of 280±10 Ma and a Rb-Sr model age between about 310 Ma and 255 Ma, depending on the assumed initial 87Sr/86Sr ratio (0.702 and 0.705, respectively). The early Permian age of the Tveitan biotite is supported by four K-Ar whole-rock dates obtained from the same breccia: an age of 316 ± 10 Ma for the groundmass and ages between 500 and 380 Ma for three crustal xenoliths. The latter three ages could very well be interpreted as reflecting varying degrees of resetting of the K-Ar systems of Sveconorwegian crustal fragments during transport by the exploding magma in Permian time.

[Data for K-Ar dating is now presented. Fun fact: In 2013, the K–Ar method was used by the Mars Curiosity rover to date a rock on the Martian surface, the first time a rock has been dated from its mineral ingredients while situated on another planet… thanx, wiki! The paper continues…]

There thus appears to be a difference between the age of the Hönstjern breccia and that of the Tveitan breccia; about 580 Ma for the former and about 280 Ma for the latter. The simplest explanation is that the age difference is real, the Hönstjern breccia håving been formed in the latest Precambrian, in relation to the damtjernite volcanism elsewhere, and the Tveitan breccia having formed in the Permian and associated with the magmatism in the nearby Oslo Graben. The similarity between both breccias is then difficult to understand, however. Another explanation could be that both breccias were formed about 280 Ma ago, but that the Hönstjern breccia contains biotite derived from an older rock, carried upwards by the exploding magma. “

I’m not sure the age discrepancy was ever reconciled…. Also, note that the features described as “small, rounded fragments (up to 0.5 cm in diameter) of ultramafic, occasionally porphyritic rocks with phenocrysts of biotite or brown hornblende” are now thought to be formed by stages of fluidized granulation (https://www.nature.com/articles/ncomms1842), similar to a spray coating process.

A few remaining images: first, a grain of apatite that seems to have zoned inclusions of carbonate:

What I think is chlorite (and some quartz along the left and top edge; a small spear of carbonate radiating right and up):

And a tiny 'snowflake', imaged with epi-darkfield:

This sample really opened up my understanding of a dozen or so other samples, I'll continue presenting other samples of carbonatized explosion breccias from Tveitan and Fen (near Söve).

 

[Andy Resnick]

This sample is classified as a Tveitan carbonatized damtjernite-like explosion breccia. It was obtained from UTM coordinates ⁵356-⁶⁵419, and was dated to 316 Ma. From the paper: "[The breccia consists of] a wide variety of xenoliths and xenocrysts in a very fine-grained groundmass consisting mainly of carbonate, green biotite, opaques, and apatite. [...] Many of the xenoliths and xenocrysts are strongly altered, but the abundant apatite phenocrysts and the cores of biotite phenocrysts and perthite xenocrysts do not show any alteration. Numerous veinlets of carbonate transect both the xenoliths and the groundmass”. This sample has several angular fragments of xenocrysts and small pelletal lapilli of ultramafic rocks with cores of biotite or brown hornblende. Recall that even though this sample was obtained a few hundred meters from the explosion breccia at Hönstjern and appears very similar, the age of the two samples is completely different.

I wanted to image the pelletal lapilli which posed a challenge due to both the ultramafic nature and complex microscopic structure of the fragments. For example, here are three, showing both PP and XP views:

It's difficult to tell what is going on, even at moderate magnifications. Zooming in on one of the cores located within the topmost one:

this one has a core of biotite (which has been altered but is still micaceous) interspersed with curved branching opaques.

The microstructure can be better viewed with epi-darkfield (this is the middle row lapilli):

Here, the biotite core is surrounded by a shell of what is now (I think) chlorite and decorated by small high-index granules.

These granules are not opaque, but have a very high index of refraction and birefringence, so I suspect these are a titanium oxide. Here are additional high-magnification views of these granules and matrix, in the XP view it's a little hard to tell what is in focus and what is not:

What is surprising to me is the roughness of the granules at the microscale- I would not have expected that, even for rapidly cooled magma.

 

[Andy Resnick]

A very photogenic (I think) diorite from Tveitan:

This sample consists almost completely of cumulate plagioclase (including chessboard albite) with apatite and stilpnomelane, muscovite and chlorite (?) after biotite (if that's the correct phrasing): I mean that the original biotite crystals have, to a greater or lesser degree, been metamorphosed into stilpnomelane, muscovite and (I think) chlorite. There are minor amounts of carbonate and diopside. Plagioclase is heavily altered (‘dusty’ appearance). Clusters of euhedral and subhedral apatite grains, sometimes with interstitial stilpnomelane, are present:

There are regions of disorganized "something" as seen by very disorganized birefringence that occur near a carbonate particle: Lower left corner is stilpnomelane, center is a carbonate agglomeration.

I suppose the material could be quartz but regardless of what it is, I would say the optical evidence appears to show immiscible fluids. I want to re-visit this region and take some better images to really visualize the interface.

Other photogenic features occur within some of the altered biotite:

Suspended within what I think is chlorite are microcrystals of jewel-toned diopside and acicular stilpnomelane, possibly some hexagonal apatite as well.

An otherwise great image of biotite reminding me of a 3-D surface, marred by a dirty sensor:

Lastly, within a grain of apatite is a microinclusion (possibly diopside?) showing stress birefringence:

 

[Andy Resnick]

This sample has a lot of interesting and photogenic features:

This sample is a mixture of calciocarbonatite and altered phlogopite. Composition is subhedral equigranular carbonate (likely calciocarbonate) grains, aggregates of subhedral apatite, and zoned phlogopite (orange-tan pleochrosim) with equigranular aphanitic carbonate reaction rims (phlogopite often strongly altered).

Carbonatite grains (likely calciocarbonatite), altered phlogopite with carbonate rims, and radiating prismatic aggregates of an opaque. For that particular object at high magnification, what appears to be a prismatic habit is actually a 'cloud' of fine opaque particles, almost like a smear. Possibly an artifact of lapping/polishing? I have similar features on other samples.

Carbonatite, aggregate of apatite, and biotite with carbonate rims. Some opaques at upper center.

Zoned phlogopite (center) eroded by carbonate. Apatite aggregate on left, altered phlogopite on upper left and right side, some opaques on left.

A close-up of the carbonate rims:

Birefringence of (calcite?) is so high that interference occurs at twinning planes, resulting in visible colors.

Carbonatite in PP.

Minor amounts of radiating splays of highly birefringent prismatic crystals: because the thickness of individual prisms is less than 30 microns, I instead estimated a thickness (10 microns), indicating aragonite.

Carbonatite on left side and right side, aggregate of apatite in center, prismatic bundles of something from lower left towards upper center, and opaques scattered throughout. It may not be apparent in this shrunken image, but it's possible to see through the prisms to the apatite underneath. At high magnification a single prismatic crystal looks like this (PP and XP):

The altered micaceous mineral consists of equigranular aphanitic aggregates, similar in appearance to carbonate reaction rims, but yellow color, no pleochroism, and birefringent color consistent with members of the biotite group, so it is most likely phlogopite as well.

Top is carbonate, lower 2/3 of image is filled with a granular phlogopite-like mineral.

Minor amounts of opaques. Some opaques within the phlogopite exhibit what appear to be pleochroic halos…. I wonder if this rock is radioactive?

The catch-22 is that if I borrowed a Geiger counter and this sample is indeed radioactive, campus safety people would likely confiscate it or require that I store the materials as hazardous. I'll pursue "don't ask, don't tell", as it were....

 

[Andy Resnick]

This intriguing sample is, I believe, a myrmekite:

I'm not sure it is, tho. The sample is leucocratic with grains of what could be K-feldspar, pyroxene (enstatite?), or even olivine (yellow bifrefringence color).... or perhaps the sample is slightly thick. Many of the grains show undulatory extinction, and few of them have any recognizable fracture geometry.

Honestly, I spent more time photographic the sample than figuring out what it is made of.

The star of the show are the intergrowths, most likely quartz and plagioclase:

but on occasion quartz and (likely) pyroxene:

This sample was a lot of fun to image. I fell into a bit of a rabbit hole regarding the differences between mymekite, micrographic/granophyric intergrowths, and symplectites. For example, this sample doesn't really show 'wartlike' intergrowths indicative of a metamorphic process, the intergrowths are too rounded to be considered igneous intergrowths (micrographic/granophyric); the intergrowths are too large to be considered symplectic. Finally, intergrowths of quartz (or feldspar, for that matter) and pyroxene are nowhere to be found on the interwebs.

There seems to be a catch-all term "kelyphitic/symplectic texture" that could apply here...?

https://www.researchgate.net/public...ew_dynamic_view_of_their_structural_formation

 

[Andy Resnick]

This sample is another trachyte:

First, a slight digression- since graduate school, I have been (professionally) interested in fluid flow and interfacial energy; trachytic textures really appeal to me aesthetically. The trachyte samples have given me a reason to work with a couple of lenses that I don't have much experience with (mostly because they are difficult lenses to work with)- Plan 1.25/0.04 and Plan 1/0.04 microscope objectives (both Zeiss finite-conjugate lenses). Here are a few examples using the 1.25x objective:

While both lenses get soft in the corner, I can shoot DX to crop them out. On the other hand, the whole point of these lenses is the gigantic field of view so I can't really complain.

Ok... moving on:

This sample is a trachyte. Groundmass consists of small sanidine laths, aegerine needles/prisms, and small aggregates of granular pyroxene, likely diopside.

Suspended in the groundmass are small phenocrysts of biotite, large phenocrysts of (altered) sanidine and (altered) what was likely nepheline. Alterations are generally pseudomorphic; sanidine metamorphosed into a different tabular/platy feldspar (based on adding a lambda plate) and nepheline metamorphosed into (I think) pyroxene:

Here's what the 1λ plate adds to the visual effect:

Feldspar has a characteristic blue-orange-purple appearance, here's another image:

Lastly, a high-magnification image of the interface between the two altered minerals shown in the 2 images above:

I'll probably shoot a few more trachytes, play around with the 1.25x and 1x lenses some more...

 

[Andy Resnick]

Another trachyte:

This is a leucocratic sample. Groundmass of subparallel sanidine (trachytic texture) containing phenocrysts of feldspar, pseudomorphed altered nepheline (based on habit), and metamorphosed unknown primary mineral, now replaced by quartz and biotite:

Even though there is no evidence that the tracytic texture is related to some sort of flowfield, the trachytic texture can look a lot like streamlines, especially in interior corners:

The groundmass components can get a little tricky to identify:

Nepheline phenocrysts have been replaced by antigorite -> biotite, with a scant corona of granular quartz partially surrounding the phenocryst:

Optically, I find the metamorphism interesting- there are what appear to be polarization singularities at boundaries (black lines/curves):

These are still quite visible at very high magnification:

 

[Andy Resnick]

One last (for now) trachyte:

This sample is mesocratic. Groundmass consists of subparallel sanidine within two different matrices; both are granular, possibly metamorphosed, and identification is tricky. I think the darker matrix includes a carbonate (calcite or dolomite), while the other is a fine-grained feldspar?

The phenocrysts are also metamorphosed and I can’t identify anything other than (possibly) orthoclase and some type of carbonate. Red-brown staining in places, possibly an iron oxide?

 

[Andy Resnick]

This sample, one of three, is interesting:

This is a sample of Tveitan country rock, (according to paper) a Garnet- Biotite gneiss. I think it is a mylonite (probably a protomylonite) due to evidence of dynamic recrystallization and S-C fabric texture. In the above image, (isotropic) garnet is visible in the upper left and lower right. Zooming in on (I think) one of the S- foliation boundaries where there is an apparent singularity (smoothed out by dynamic recrystallization):

This sample has porphyroclasts of feldspar suspended in a fine-grained groundmass of feldspar. Where there is microcline, there are wartlike myrmekites (“Nearly complete to complete replacement of plagioclase takes place and leads to the formation of wartlike myrmekite in places where the replacement was incomplete.” [wiki]).

Biotite is present:

As is sericitized plagioclase:

 

[Andy Resnick]

This sample is also a Tveitan country rock, sample TVE 29:

In the paper, we find the following information:

Pegmatitic hbl-bio gneiss (location ⁵351-⁶⁵422) has been Rb-Sr dated to 0.9 Ga. Texture reported as inequigranular, fine-grained, medium grained, mesocratic, and foliated. The foliation looks really striking through the 1X objective:

Primary minerals are quartz, K-feldspar (as perthite), plagioclase (25% anorthite) with lesser amounts of garnet, biotite, and hornblende. secondary minerals include chlorite, also chlorite possibly as pseudomorphed hornblende, carbonate, and sericite. I think this is an example of pseudomorphed hornblende:

Upper left is biotite, upper middle has some opaques, upper right is hornblende (or amphibole), while the center is "something" surrounded by (I think) pyroxene (high birefringence), and the remainder of the grains are generally quartz.

I also find small isolated grains of diopside (pyroxene) and in places, what looks like microcline and wartlike myrmekite:

 

[Andy Resnick]

Fenite is a type of metamorphic rock associated with carbonatite intrusions. Fenite itself does not have commercial value, but the associated carbonate intrusions often do (rare earths, mainly). This sample, MA 68, is from the Håtveittjørn Section and was located 10m from contact between country rock and the (magmatic) Fen Complex.

(The pen markings are original) This sample has been classified as a strongly Fenitized-2 (low temperature hydration-carbonation, F2) fenite; only relic accessory zircon survives from the primary gneiss. By volume, the predominant mineral is feldspar: the modal composition is 88% mesoperthite created by initial high temperature dehydration (fenitization process, which this paper refers to a fenitization-1, or F1).

Here's a couple with a full-wave or quarter-wave compensator (I like the colors!):

At higher magnification, you can clearly see perthite unmixing texture in orthoclase grains with rims of plagioclase, the appearance reminds me of mitochondria :)

A second fenitization event (low temperature hydration-carbonation, F2) altered the F1 aegirine into a range of minerals including 2% stilpnomelane in veins; 2% F2 sphene, 2% sericite, 2% F2 biotite, 2% F2 carbonate, acc Ap; F2 Op.

Here's a set of images showing a perthite grain (center), F2 sphene (left side, high-relief agglomerated grains), stilpnomelane (the high birefringence diffuse stuff) and some opaques:

The F2 sphene was a mystery (to me) for a while; I ruled out everything else until sphene was the only plausible thing left. I made a definitive identification by epi-darkfield; sphene is highly scattering and appears bright white.

 

[Andy Resnick]

This sample, Fen 40, was obtained 195m from country rock-Fen complex contact and is another example of fenitization:

There are actually 3 thin sections generated from this rock, so I'll mix-n-match views as needed. In contrast to the previous sample, this one is strong F1 (high temperature dehydration, Fenitization-1), and weak F2 (low temperature hydration-carbonation; Fenitization-2). Let's start with low magnification:

Much of this sample is the primary gneiss: 30% quartz, 30% perthic texture in orthoclase, and 14% plagioclase. The remainder, principally biotite, was F1 transformed into aegirine and microcline-chessboard albite (Fsp). This is clear at higher magnification on features similar to what you see in the upper image, near the center- a ring of aegirine surrounding Fsp:

hmm... having problems uploading the images. I'll just split this post:

 

[Andy Resnick]

The primary gneiss also has accessory Zircon grains:

the F2 metasomatic process generated accessory amounts of arfvedsonite:

 

[Andy Resnick]

This sample is an altered melteigite: I accidentally duplicated myself, sorry... :)

Let's go with "I've learned a lot since then, so I'm re-visiting a complex sample". I have been keeping track of which samples I have posted here, this one got overlooked.

A couple of low-magnification views to show the mafic content:

Meltegite is the melanocratic endmember of the ijolite series and consists primarily of pyroxene and nepheline. This sample does not contain any quartz or feldspar, two minerals comprising 70% of the earth’s crust. All of the nepheline has been altered.

Melteigite sample was obtained -114m from country rock-Fen complex contact (negative distances means the sample was located within the pluton). The primary melteigite consists of 63% aegirine, 3% biotite, 2% sphene, 2% apatite, 2% carbonate, 3% melanite and accessory opaques. The remaining 25%, which is altered nepheline, has been low-temperature hydrated-carbonated (F2 fenitization) into 9% Chlorite, 9% Sericite, and 5% F2 carbonate (dispersed). In addition, there are 1% each of F2 opaques and F2 carbonate in veins. Most of the pyroxene grains are zoned with titanian augite as a corona:

Above, the partially altered aegirine grain is surrounded by Chlorite. Here is another image pair, showing the altered aegirine, with chlorite and calcite also present:

And another image pair, showing aegirine-augite on either side of sericite, another alteration product of nepheline:

According to this paper (https://pubs.geoscienceworld.org/ms...en-alkaline-complex-Norway?redirectedFrom=PDF), “Colorless to light brown cores of Al-diopside can be found, but the bulk of the pyroxenes are low Ti, low Al, Na-rich diopsides. The compositional zoning is one of Na and Fe enrichment along an aegirine-hedenbergite trend.” So, I’m a little confused by this second report; if “the bulk of the pyroxenes are […] diopsides”, then how can “The compositional zoning is […] along an aegirine-hedenbergite trend.” also be true? Mindat.org lists some synonyms for aegirine-augite, including Aegirine-Diopside and Aegirine-Hedenbergite, so maybe my confusion is just a nomenclature thing.

In any case, now that I understand this sample better, I plan to present unaltered melteigite next, as there are some really interesting features.

 

[Andy Resnick]

This sample is also a melteigite:

This sample consists of subhedral elongated grains of aegirine-augite, diopside, and apatite in a groundmass of (altered) nepheline, calcite, and titanite:

Nepheline altered to chlorite and sericite/muscovite (below, surrounded by apatite grains) so this sample is probably F2 fenitized to some degree.

Altered nepheline (center, below) contains small crystals of (possibly) epidote, if true then presence implies hydrothermal alteration.

A higher-magnification XP view of the crystalline inclusions- one in the upper left has the characteristic habit:

Some calcite in veins. Opaques: grains and a few “skeletal” arrangements of needlelike grains (ilmenite? magnetite? (60 degree symmetry? 90 degree symmetry? reflecting the primary mineral?) filled with pyroxene, calcite, and cryptocrystalline (probably)TiO2. These features are really striking and can best be seen with epi-darkfield imaging:

Biotite replaced with chlorite (anomalous blue birefringence), here with (F2?) titanite (very high relief) and pyroxene (high relief), a grain of carbonatite on the lower right:

Sample lacks melanite (see below).

From a relevant paper: Ijolites and melteigites (mela-ijolites)consist of euhedral prismatic crystals of clinopyroxene and apatite set in a matrix of nepheline and minor calcite. Sphene and strongly zoned (5-12% TiO2) melanite are common accessories. Pyroxenes are pleochroic in shades of light yellow-green to apple- green. Colorless to light brown cores of Al-diopside can be found, but the bulk of the pyroxenes are low Ti, low Al, Na-rich diopsides. The compositional zoning is one of Na and Fe enrichment along an aegirine-hedenbergite trend similar to that determined for the urtite pyroxenes, e.g.Di₇₀Hd₂₀Aeg₁₀ to Di₄₀Hd₄₀Aeg₂₀. Pyroxenes from ijolites which contain melanite are richer in Na and Fe on average than pyroxenes from rocks which lack melanite.

 

[Andy Resnick]

I originally thought this sample was also a melteigite:

because it superficially looks like the other samples. However, it's actually a pyroxene-hornblendite (or a hornblende-pyroxenite), an ultramafic rock lacking quartz and feldspar that consists of nearly equal amounts of hornblende and clinopyroxene (most likely a mix of diopside and augite):

The transparent minerals are apatite and carbonate, likely calcite. While this sample is about 5% apatite, the other Fen 12 sample I have is closer to 15% apatite. But that sample doesn't have photogenic crystals of hornblende:

Both hornblende and pyroxene grains are anhedral poikiolitic containing mostly apatite chadacrysts, but there are also pyroxene and biotite chadacrysts. Since everything is sub- or anhedral, I'm not sure if these are proper 'chadacrysts' or just inclusions. Hornblende is also a hydrothermal metamorphic product of pyroxene, so pyroxene inclusions could indicate incomplete alteration of the pyroxene. A couple of higher magnification views of the central inclusions above, showing apatite and clinopyroxene:

The opaques are interesting. They are a mix of red hematite (in veinlets), Illmenite (steel grey), magnetite (yellow/gold), and (I think) cryptocrystalline Titanium dioxide (white):

A super-duper magnification view of the lower right- these grains are tiny! Solidification must have happened extremely rapidly... but it's not a glass.

 

[Andy Resnick]

Instead of discussing another sample from the Fen complex, I want to present a sample from Crabtree Mine, in NC:

The nearby town of Spruce Pine has been in the news recently; Hurricane Helene dumped about 20 inches of rain into the area around Asheville which has frankly devastated the entire region. My family and I were vacationing there 14 months ago, and we took a side trip to the Crabtree emerald mine:
https://www.emeraldvillage.com/mines-activities/crabtree-emerald-mine/

I knew the area was full of interesting mineral deposits and opportunities for "rock hounding" (https://www.mindat.org/loc-26957.html), because the prior time I visited (also Crabtree) I was about 12 years old, pretty much when this picture was taken:

That's a load of emerald ore coming up from the mine for us 'civilians' to work through. All of those rocks you see in the image are pegmatite bearing large crystals of tourmaline, mica, cancrinite, beryl... and the occasional emerald . I have vivid memories of the place and was happy to re-visit the area. Even so, I was unaware that the region supplies most of the world's semiconductor industry with pure quartz (the Spruce Pine mine). I can't imagine what the area looks like now.... we joke about how flooding refreshed the mine dumps, but when I think about the people who live and work at Emerald village, or Little Switzerland, or the 1 1/2-lane roads that snake up and down the mountains... it's bad right now.

The region's geology mostly consists of a large intrusion of pegmatite, and the sample I have is primarily plagioclase containing abundant tourmaline (schorl). Last year I sent the sample off to Van Petro (https://www.vanpetro.com/) who prepared the thin section:

I would classify this particular rock as quartz-diorite, and the plagioclase often shows deformation twinning.

Confirmation of tourmaline- top row is PP, bottom is XP:

The top row shows pleochroism, the bottom shows extinction at 0 degrees- these properties are confirmatory for tourmaline.

One odd aspect about the plagioclase is that some of the grains are sort of blue-grey, while others are sort of yellow-brown. It's hard to show in photos, but hopefully you can see the faint colors, even in PP.

And finally, a grain of (what could be) ultrapure quartz:

Anyhow, I hope the residents are able to recover.

 

[Andy Resnick]

This is an example of Damtjernite from the type locality Damtjern:

Quoting from various papers:

“Damkjernites from the type locality at Damtjern are lamprophyric rocks containing phenocrysts of red-brown titanian phlogopite, yellow-brown titanian pargasite (amphibole), and clinopyroxene set in a groundmass of brown-green ferropargasite, pyroxene, green phlogopite, manganoan ilmenite, ulvospinel-magnetiite, and calcite."

Damtjernites are not found anywhere else in Norway, but they seem to be similar to Alnöites. Superficially they are similar to kimberlites, but there are significant differences in mineral content that argues against any simple relationship. In fact, the various damtjernite dikes surrounding the Fen complex all have very different appearances.

In the above view, a large pyroxene phenocryst is in the upper right corner, directly next to a grain of titanian phlogopite (pink-purple). Also visible are several zoned pyroxene phenocrysts (brown, mantled by yellow) and calcite. I'll zoom in on the bright purple phenocryst shortly.

Starting with the groundmass:

The opaques are Ilmenite, and it's (barely) possible to distinguish the amphibole and pyroxene by end facet angles- for example, in the center several of the grains show 60-degree angles (amphibole). The groundmass is also rich in carbonates. The grains in the upper left corner and lower right corner could be amphibole overgrown with pyroxene.

Zoned titanian phlogopite seems to be one diagnostic of Damtjernite. In PP, the grains are extremely pleochroic (clear - dark red/brown) and in XP, they are a bright turquoise:

In addition to titanian phlogopite, the complex pyroxene phenocrysts are another diagnostic.

"The pyroxene phenocrysts are composed of anhedral pale green cores mantled by anhedral to subhedral overgrowths of purple-brown pyroxenes. The pale green phenocryst cores are Al-Na diopsides which exhibit a wide range in Al content coupled with a low Ti content, i.e.the pyroxenes are rich in CaAl₂SiO₆ and poor in CaTiAl₂O₆. The Al-Na diopsides are complexly zoned with respect to Al₂O₃, which either increases or decreases from core to margin within individual crystals. The pyroxenes which mantle the Al-Na diopsides and which form the ground mass pyroxenes are Ti-Al salites (salite = Fe-bearing diopside), strongly zoned with increasingTi and Al from core to margin (Table l, anal. 6-9). This zoning reflects an increase in the CaAl₂SiO₆ component at the expense of the CaTiAl₂O₆. component.

Another example of complexly-zoned pyroxene phenocryst- I checked the extinction angle to verify this is pyroxene and not amphibole:

Another odd feature found in this sample (and a few other samples) are acicular masses of pyroxene surrounding a calcite phenocryst:

I like how the calcite looks like soap bubbles...

Finally, the opaques can also have a complex character. I believe this image shows ilmenite (the dark grey) lined with pyrite or magnetite (the bright yellow) and titanium dioxide:

I've identified 40 different examples of damtjernite in the collection, I hope to spend some time going through the different localities because the samples do show significant variations.

 

[Andy Resnick]

This next example of damtjernite comes from Brånan:

While there are some differences between Damtjern-damtjernite and Brånan-damtjernite, there are some similarities. For example, the presence of zoned titanian phlogopite:

and complexly-zoned pyroxenes (top of frame, titanian phlogopite on bottom):

From one of the papers: “The damtjernite dike near Brånan (Werenskiold 1910), 20 km NNW of the Fen complex, is also similar in appearance. The rock, which has been described by Brogger (1921) and Griffin & Taylor (1975), consists of phenocrysts of zoned augite, zoned biotite and aggregates of carbonate in a groundmass of pyroxene, biotite, opaques, melanite, sericitic pseudomorphs after nepheline, apatite and carbonate. Biotite produces a K-Ar age of 594 ± 20 Ma. “

I didn't find any melanite or sericritic pseudomorphs in this sample. In any case, the zoned pyroxenes are probably the most striking thing to image, the lower right image also has a crystal of phlogopite on the left side:

Another striking feature- I like how the artist made use of negative space within this calcite ocellus:

At higher magnification, we see the dark spots are not just opaques but something else (I don't know what...):

And finally, at super-duper magnification, I found what seems to be a metal oxide (likely ilmenite) partially covering the tip of a (likely) pyroxene crystallite- note how some of the dark spots are out of focus, so this really is a three-dimensional structure:

More to come...

 

[Andy Resnick]

Posting a day early this week... another damtjernite, this one from Steinsrud:

The appearance of this damtjernite differs somewhat from the other locales:

"A diatreme-facies damtjernite near Steinsrud, 1 km SW of the Fen complex, is likewise a less dark-coloured rock. Phenocrysts of clinopyroxene, large brown hornblende and biotite, along with phenocrysts or xenocrysts of feldspar (both sodic plagioclase and alkali feldspar) and aggregates of feldspar or feldspar-quartz are embedded in a groundmass of alkali feldspar, minor pyroxene, green amphibole, biotite, opaques, titanite and quartz. Biotite and hornblende give K-Ar ages of 523 ± 20 Ma and 597 ± 30 Ma, respectively.”

The following description is also appropriate- Sanna is a nearby locale with a damtjernitic plug that has an appearance similar to that at Steinsrud:

"The small plug at Sanna (Barth and Ramberg, 1966), 7km SSW of Fen, shows some similarities to the Fen damkjernites. Here, large titanian ferropargasite megacrysts are common and phenocrystal pyroxene cores are Na-salites. However, these cores are mantled by low-Na₂O salites with higher MgO contents than the cores. An outer thin discrete rim of apple-green acmitic pyroxene is commonly present on the phenocrysts, and a similar pyroxene together with green ferropargasite forms the bulk of the groundmass. Late-stage fluids have crystallized to alkali feldspars, zeolite (?altered nepheline), and calcite. At Sanna the majority of the phenocrysts are euhedral, country-rock xenoliths are uncommon, and spinel lherzolites appear to be absent, indicating that explosive activity was not so intense as at Fen and that the magma cooled relatively slowly. The Sanna "damkjernite" lacks the characteristic red titanian phlogopite and Ti-Al salites found at Fen."

The important point here is that all the damtjernites have been assigned the same age, suggesting they all formed during the same eruptive event. But then it's difficult to understand how the compositions could be so different. For example, here's a pair of images of the groundmass:

Along the top are three grains of sphene (very high relief), and the upper middle also shows a couple of hexagonal crystals of apatite. The bottom shows a crystal of pyroxene with the green acmitic rim. Unlike the previous examples, the groundmass here is primarily orthoclase rather than calcite.

An interesting object: randomly-oriented equigranular phlogopite and pyroxene arranged in a shell surrounding (I think) thin laths of feldspar:

Was this some sort of large amygdule? A closer PP view of the feldspar shows the relief in good detail:

Another interesting feature are (apparently) zoned opaques, here in reflected light:

The center is ilmenite (maybe chromite) surrounded by (most likely) magnetite with a thin rim of granular pyroxene.

Lastly, because this is not mentioned in either published report, I found what I think is stilpnomelane as an accessory in both veinlets and radiating needlelike crystals, here just some needles:

 

[Andy Resnick]

Another damtjernite, this one from the island of Presetoya:

"Very similar in mineral composition [to a dark damtjernite dike near Gulbrandstjern], but containing less altered, zoned biotite, is the damtjernite dike on the nearby island Presetoya in Hoseivatn (Klåy 1965), which probably forms the continuation of the Gulbrandstjern dike. Biotite gives a K-Ar age of 594 ± 20 Ma.

"The damtjernite dike near Gulbrandstjern (Klåy 1965), some 20 km SW of the Fen complex, consists of abundant phenocrysts of zoned pyroxene (Ti-augite, occasionally with aegirine-rich cores), biotite (strongly replaced by chlorite and apatite), opaques, zoned melanite and apatite in a groundmass of carbonate, chlorite, epidote, white mica, opaques, titanite, apatite and minor alkali feldspar. Biotite yields a K-Ar age of 601 ± 20 Ma."

There is comparatively less groundmass compared to other damtjernites. Another zoned pyroxene:

One of the pyroxene phenocrysts contains an amygdule:

It took a while for me to identify the andradite (var. melanite). I thought garnets were isotropic, but apparently andradite can show weak birefringence:

I think many of the melanite grains have sphene cores: melanite formula is Ca₃(Fe,Ti)₂(SiO₄)₃, sphene is CaTiSiO₅, so the chemical composition is similar, and it's possible to see a grain of something in the center of the garnet:

 

[Andy Resnick]

This is the last example of damtjernite that I have from a known location (I have others generically 'from the Fen region'):

"Within the Fen complex, 0.5 km east of Sove, there is a dark, carbonatized damtjernite in diatreme facies. The rock contains abundant biotite flakes (up to 4 cm in diameter), which give a K-Ar age of 578 ± 20 Ma and a Rb-Sr model age between about 555 and 580 Ma, depending on the assumed initial⁸⁷Sr/⁸⁶Sr ratio (0.705 or 0.702, respectively)."

Note that this sample is of a carbonatized damtjernite- almost everything has been metamorphosed into carbonate (possibly dolomite, there's no clear way to tell).

Large opaques, rounded phenocrysts (pelletal lapilli of varying sizes) and angular phenocrysts (both presumably pyroxene altered to carbonate and quartz) and phlogopite (moderate pleochroism clear - light brown) crystals in a groundmass of anhedral equigranular carbonate with small blades of phlogopite and opaques.

Phlogopite crystals are rounded and often show kink banding, here with some pelletal lapilli:

A closer view of a small pelletal lapilli shows the overall structure of carbonate and quartz grains. The light brown mineral may consist of minute flakes of phlogopite?

Other (presumably) pyroxene phenocrysts have been completely converted to carbonate:

And a closer view of the groundmass:

Phlogopite and apatite are apparently resistant to the hydrothermal carbonation reactions, but some phlogopite is eroded. Here's a high magnification view of an apatite grain, showing some sort of alteration along the border:

And finally, the larger opaques can have a halo of a fine white and fine red cryptocrystalline materials:

 

[Andy Resnick]

This sample is a Damtjernite from somewhere within the Fen complex:

To recap, here are some maps of the area starting 'big picture' and then zooming in:

I have damtjernite samples from many of the identified locations: Branan, Horte, Tveitan, Degernes, Fjone, Hjolmodal, and Gardnos (Gardnos is an impact crater, not an eruptive feature) and posted images from most of these. Zooming in:

I have posted images of damtjernite samples from Presteoya, Steinsrud, and Damtjern. Zooming in to the Fen complex (this map calls damtjernite 'lamprophyre'):

Damtjern is the black smear in the lower right. Within the Fen complex, there are numerous damtjernite pipes.

The point of all this is to show that even though all damtjernite is assigned the same geological age and are all 'close to each other', implying association with the same eruptive event, their petrology is highly variable. The only real constant (AFAICT) is complexly-zoned pyroxenes. Most of the samples have titanian phlogopite phenocrysts, but not all. Most of the samples have carbonate, but not all. Most of the damtjernite within the Fen complex has olivine (lherzolite), implying a more forceful eruptive event since lherzolite is mantle-derived, but not all. Moving on...

This particular sample is highly melanocratic- possibly the most extreme of all my samples- due to an excessive amount of opaques. It's possible this sample is highly tectonized. Large crystals of phlogopite dominate this field of view.

This image shows a few features of interest. In the center is (I think) an ocellus: the center is amphibole, surrounded by a shell of carbonate (probably dolomite), itself surrounded by a thin shell of (I think) amphibole:

Regardless if this is an ocellus or not, this feature (a mafic grain surrounded by carbonate and rimmed by another mafic mineral) appears with some regularity.

There are also complexly-zoned pyroxenes:

And zoned amphiboles:

The groundmass consists of carbonatite, phlogopite, opaques, pyroxenes, amphiboles... the usual!

 

[Andy Resnick]

Here is another sample, possibly cut from the same rock:

Unfortunately, I have not yet identified all of the minerals that are present. In any case:

This sample is Damtjernite, from somewhere in Fen. Very melanocratic due to large amount of opaques.

Phenocrysts of complexly zoned pyroxenes (diopside?) and rimmed (?) amphiboles. Large crystals of highly pleochroic (clear to red-brown) zoned titanian phlogopite. Lots of opaques. Mesh-textured partially-to-fully serpenitized olivine indicates mantle-derived rock and a violent eruptive event.

Again, there are altered phenocrysts consisting of rimmed mafic mineral mantled with carbonate (dolomite?).

Groundmass is largely carbonate (likely dolomite), phlogopite, and opaques. Andradite occurs as an accessory.

Now we get to the complicated stuff- any suggested identifications are welcome!

Partially altered (mesh textured) pyroxene grain, possibly carbonate veins, reaction rim is granular carbonate with many opaques, itself surrounded by a mantle of pyroxene and/or amphibole, with an appearance of lamellar twinning in places.

Identifying this grain proved difficult- the fracture pattern could be either pyroxene or amphibole (both geometries are present), and if pyroxene, it's probably orthopyroxene based on the XP images.

Here's another example: Orthopyroxene (?) grain surrounded by a granular rim surrounded by amphibole (?):

In these two cases, I at least have some hint as to the identities. In this last set of images, I have absolutely no clue. Overall, it's a grain of partially altered pyroxene (mesh textured). However, zooming in to a small region, I found some sort of yellow-green mineral, low to medium relief, with a totally bizarre birefringence pattern:

Maybe it's a phyllosilicate, but the color and birefringence is (I think) don't really match those group members. If it is bastite, then the grain is orthopyroxene. I dunno what it is, but I found the Eye of Sauron:

 

[Andy Resnick]

The next few samples of damtjernite are from a different (and unknown) location within the Fen complex:

Damtjernite from Fen complex. Porphyritic sample featuring a large partially serpentinized (mesh textured) olivine grain with inclusions:

on a background of zoned pyroxene phenocrysts and phlogopite crystals in a groundmass of carbonate, pyroxenes, and phlogopite:

Olivine is likely somewhere on the forsterite (Mg rich) to fayalite (Fe rich) series. So far, I have been unable to identify many of the minerals in this sample. For example: Within the large phenocryst, several possible amygdules: interior is carbonate (likely dolomite), some phlogopite grains, and a thin shell of (possibly) chlorite rimmed by opaques.

Microtextures of some of the fully serpentinized phenocrysts have remarkably intricate spatial patterns of opaque microcrystals, in one there may be a small crystal of sphene.

The fibrous mineral may be phlogopite, but I am unsure.

By stopping down the condenser, the olivine reminds me of stained glass; in reflected light the opaques suggest rivers of silver.

 

[Andy Resnick]

Another damtjernite sample from the same location in the Fen complex:

Damtjernite. Large grains of zoned pyroxene and titanian phlogopite (highly pleochroic), partially serpentinized olivine (mesh texture).

Groundmass is the usual blend of carbonate, opaques, phlogopite, and pyroxenes. I had to "phone a friend" for help identifying a few minerals in this sample- they haven't gotten back to me yet, tho. In one grain of partially serpentinized olivine, there appears to be a metasomatic conversion to an unknown mineral (green? Birefringence is weird…).

Olivine is likely somewhere on the forsterite (Mg rich) to fayalite (Fe rich) series. Possibly “carbonated peridotite"? In another location, there is this feature:

At bottom is a grain of (likely) augite, at the top is a grain of altered olivine. In the middle, the fibrous mineral is unknown but very photogenic. In epi-darkfield, it's like looking at something buried under a sheet of ice:

a PP view:

And an image pair at higher magnification, epi-darkfield really accentuates the fibrous nature of whatever-this-is:

In another location, a phenocryst with fine grained prismatic texture lined with opaques and mantled by pyroxene:

Opaques appear as a fine smoke in places.

 

[Andy Resnick]

This is the final example of damtjernite from location #26:

Damtjernite from Fen Complex. Porphyritic sample featuring partially serpentinized (mesh textured) olivine grains, zoned pyroxene grains, clear -> red pleochroic zoned titanian phlogopite with minor andradite and apatite grains in a groundmass of carbonate, opaques, pyroxenes, apatite, and phlogopite:

The above image pair seems to show localized deformation/strain of a biotite grain by an apatite grain.

As in the previous post, here is another example of a phenocryst (of unknown composition) with fine grained prismatic texture lined with opaques and mantled by pyroxene… I think it was originally pyroxene, possibly augite, that has been nearly completely pseudomorphed into a moderate relief, low birefringence mineral with a halo of opaques.

Here is another odd feature: this is a high-magnification PP image of an Andradite grain apparently showing a 90 degree fracture pattern

My understanding is that andradite, like all members of the garnet family, do not have cleavage planes...? Finally, a high magnification epi-darkfield image of well-formed octahedral microcrystals, likely magnetite:

 

[Andy Resnick]

Moving on to Fen location #9:

Damtjernite, similar to 67 Fen 9 (posted previously). Upper left quadrant is darker due to damaged glass slide- there is evidence it accidentally contacted a grinding wheel. Top of sample is feldspar (syenite to monzonite composition) consisting of a mixture of K-spar and chessboard albite, no quartz evident, likely a xenolith of country rock rather than a chilled margin. Secondary large xenolith has identical composition, could be entrained piece of country rock.

Both are rimmed by prismatic aergirine. Within the damtjernite: zoned pyroxene phenocrysts.

Grains of clinopyroxene surrounded by mesh textured pyroxene and rimmed by amphibole and opaques (?):

Phlogopite phenocrysts are smaller than usual. Groundmass is mix of subhedral to anhedral carbonate, opaques, pyroxenes, phlogopite. Aggregated microgranular carbonate surrounded by opaques:

Presence of small, rounded grains of carbonate surrounded by prismatic aergirine (oriented radially);

Some large opaques exhibit hexagonal structure:

 

[Andy Resnick]

This rock is especially interesting (to me), I'll need more than one post to present all the images.

Partially carbonatized damtjernite-like breccia. Large carbonate xenocryst consisting of subhedral to anhedral equigranular carbonate, anhedral pleochroic phlogopite with possible evidence of zoning at the margins, and minor anhedral apatite. Overall, xenocryst appearance is similar to sovite.

Smaller xenoliths of monzonite and altered pyroxene phenocrysts.

Groundmass seems to be mostly subhedral carbonate with minor amounts of apatite, pyroxene, amphibole and opaques.

Sample shows a grain size gradient, with very small ‘nodules’ of altered pyroxene surrounded by… something with a lot of opaques?

Some nodules are carbonate surrounded by aegirine prisms, while others are fully carbonatized:

Continued on next post...

 

[Andy Resnick]

... A few more image pairs of altered pyroxenes and amphibolites:

Another image of the sovite-like xenocryst- phlogopite, carbonate (likely dolomite), and apatite:

[Edit] Another example of a carbonate grain surrounded by aegirine:

\[Edit]

The dolomite intrusion has been dated to 539 +/- 14 Ma, roughly coincident with dating of damtjernites (582 +/- 24 Ma) and possibly suggesting that a pre-existing damtjernite dyke/pipe was co-located with a later carbonate eruptive event that forcibly (and in places, intimiately) carbonatized the damtjernite containing material originating from the mantle (along with country rock).

 

[Andy Resnick]

This is another example of damtjernite, from a different location within the Fen complex:

Damtjernite from Fen Complex. Porphyritic sample with phenocrysts of partially-to-completely serpentinized olivine, zoned/mantled clinopyroxene, amphibole, titanian phlogopite in a groundmass of primarily subhedral carbonate, opoaques, lesser amounts of prismatic pyroxenes and phlogopite flakes. The usual :)

I had to 'phone a friend' to help decipher a large, possibly zoned, fully altered olivine grain showing two distinct serpentine group minerals:

The black veins are magnesite, the brownish mineral is (likely) Iddingsite or Saponite. Saponite is a mixture of smectite, quartz, chlorite, serperntine, talc and is green/yellowish. The formation of iddingsite or Saponite depends on O2 fugacity. Iddingsite is a common alteration of olivine during oxidation, hydrothermal and deuteric processes. It appears as a reddish-brown replacement of olivine. Iddingsite is a pseudomorph, and during the alteration process the olivine crystals had their internal structure or chemical composition changed, although the external form has been preserved. The alteration of olivine to iddingsite occurs in a highly oxidizing environment under low pressure and at intermediate temperatures.

The clear (PP) mineral, on the other hand, remains a mystery (to me). One odd optical feature is the appearance under crossed polarizers. Here are three images (PP, XP, and XP) with the sample rotated 90 degrees between the two XP images:

I can sort-of interpret the two XP images in terms of twinning and an extinction angle; for example the dark-ish blob directly above the scale bar in image 2 is located near the top center position in image #3. The 60-degree 'twinning' (?) angles present in image #3 perhaps indicate an amphibole, but who knows... Moving on:

This large opaque grain (epi-darkfield image) appears to be a mixture of (likely) magnetite and pyrite.

Aggregates of granular mineral with high refractive index (highly scattering), presumably titanium dioxide?

 

[Andy Resnick]

Next up, a damtjernite from Fen location #50:

Damtjernite. Overall porphyritic texture and mineral constituents are similar to 67 Fen 8 and 67 Fen 8 ii; one significant difference is the presence of glomerulocrysts of phlogopite; Fen 50 and 50 ii have them while Fen 8 and 8 ii do not.

In the above image, there are phenocrysts of phlogopite, zoned pyroxene, and xenocrysts of partially altered olivine (serpentinization).

This image shows altered (serpentine) olivine grains, phlogopite crystals, and in the center a clot of small phlogopite grains, these are absent from the Fen 8 samples.

Sample lacks both quartz and feldspar. Major constituent minerals: serpentinized olivine phenocrysts, amphibole and clinopyroxene phenocrysts, some complexly zoned.

Phlogopite phenocrysts. Not much groundmass is present.

When using a 1-λ (red) quartz plate, olivine’s partial serpentinization results in highly colorful veins of magnesite and serpentine

Replacing the polarizer with a Cokin P Series P171 Varicolor Red/Blue filter produces this optical effect:

Something about this image seems vile and vaguely horrific. :)

There are also grains of an isotropic mineral:

It's not clear what this is- there is a near perfect 90-degree fracture pattern, so it's not garnet. Other isotropic mineral possibilities: halite? fluorite? sphalerite?

In some of the altered (serpentine) olivine grains, the metal oxide particle distribution is highly suggestive. The spatial distribution of grains probably reflect the underlying chemical reaction-diffusion dynamics of formation:

And the serpentine itself forms really intricate patterns, down to the microscopic scale:

 

[Andy Resnick]

At this point, I have looked at almost all of the damtjernite samples I have. So now I am going through a half-dozen or so samples of (what I think is) "carbonatized damtjernite"- damtjernite that has been metamorphosed by hydrothermal processes (including carbonation), I think I previewed one earlier. As with "unaltered" damtjernite, there is considerable variation in the appearance of these samples.

Carbonatized damtjernite. Scattered rounded phenocryts, most highly altered, rounded grains of moderately pleochroic (colorless- orange) biotite (phlogopite?) in a groundmass of small equigranular anhedral carbonate grains and small (probably) biotite flakes, minor amounts of apatite and substantial opaques.

In the lower left corner of the sample, there is a rounded phenocryst that consists primarily as an aggregate of small equisized biotite grains with scattered inclusions of an unidentified mineral: anhedral low-relief grains lacking clear fracture or cleavage patterns that are either isotropic or collectively, (and unfortunately) oriented at extinction.

The core of altered and rounded phenocrysts typically consists of grains of quartz, produced via diagenesis containing myriad micro-intrusions of an aggregrated, granular, colorless, high-relief mineral with also very high bifrefringence, probably a carbonate.

In this image, a biotite covers the upper left side. On the lower right corner, there is groundmass and moving toward the center, subhedral carbonate. The center of the image is probably an anhedral grain of garnet, likely andradite.

Some smaller phenocrysts have been completely altered to an aggregate of minute calcite grains. The core is often surrounded by a shell of small subhedral grains of carbonate.

There are at least 3 different opaque minerals, two are probably magnetite and pyrite- in this sample, pyrite is a minor accessory. The third, a granular cryptocrystalline aggregate, may not technically be an opaque mineral (i.e. a metal oxide) - instead, the opacity could be an optical effect due to strong scattering. The individual crystallites are colorless and have a refractive index higher than the host mineral. Epi-darkfield imaging really helps distinguish these materials from one another.

The opaques typically have an intricate geometry, likely reflecting the chemical reaction kinetics occurring during metamorphosis.

 

[Andy Resnick]

Another example of carbonated damtjernite:

Carbonatized damtjernite, based on the distinctive porphyritic nodular texture.

Numerous rounded (and altered) phenocrysts with a distinguishable core and mantle, often the mantle (and only the mantle) is studded with opaques.

Larger phenocrysts containing a core of diagenic quartz seem to have a multilayered mantle.

Calcite (anhedral equigranular crystalline) veins with abundant opaques transect the sample. Everything in this sample is either carbonate, opaque, or quartz. Other than the veins, carbonate consists entirely of microscopic granular aggregates. Lack of apatite is noteworthy, I think.

Finishing up with some higher magnification views of the interface between quartz and carbonates:

 

[Andy Resnick]

This is a multi-part post, there are too many good images. :)

These samples immediately caught my attention because of the pronounced nodular texture.

Possibly from type locality- slides are each neatly labeled “DAMTJERN” in exceeding tiny letters (opposite the sample ID and not part of the photos above) and so I was initially confused- these samples appear to be carbonatized damtjernite rather than the sample Fen 260 I shared previously, identified as “Damtjern diatreme-facies damtjernite (type locality)” and published in https://www.ngu.no/filearchive/NGUPublikasjoner/NGUnr_380_Bulletin_70_Verschure_35_49.pdf

These samples are extremely melanocratic and consequently, difficult to photograph. Most phenocrysts present have been altered and generally appear as rounded nodules coated by magnesite and containing high concentrations of opaques, scattered among these are both large and small angular phenocrysts with a core of unaltered mineral (typically biotite or amphibole, sometimes serpentinized olivine) and mantled by aggregated granular magnesite. Some nodules contain several smaller nodules within- sort of a self-similar texture. One sample (N-6A) has a xenolith of crustal gneiss, highly altered (fenitized) K-feldspar with perthic texture. Groundmass is carbonate (anhedral grains) with high concentrations of microscopic inclusions (hematite and magnesite), small euhedral grains of apatite, small grains of biotite containing massive inclusions of fine-grained metal oxide (illmenite? spinel? magnetite?) that presumably exsolved during the hydrothermal metamorphic process. Minor amounts of pyrite and titanite.

(more next post)

 

[Andy Resnick]

More images at 1X:

And the fenitized crustal gneiss at 4X:

(More next post)

 

[Andy Resnick]

Because of the extreme melanocratic nature of the samples, transmitted light is of limited use. Reflected light is a superior imaging method here, as the different opaques can be easily distinguished by color and habit. At high magnification (40x), epi-illumination with crossed polarizers was used rather than epi-darkfield. Nodules outer coat is a thin crust of (likely) magnesite, groundmass contains (red) hematite.

First, an example of the different image modes:

The ability to see detailed structure with epi-darkfield is pretty obvious. There is an issue with white balance, as you will see especially at 4X:

I've decided the white "opaque" that I have noted on several samples is magnesite (magnesium carbonate). Here's an image at high magnification:

 

[Andy Resnick]

These next few samples are (I think) extremely complex and difficult to describe.

Carbonatized damtjernite breccia from Sove, diatreme facies. A wide variety of xenoliths and xenocrysts are present that can be generally divided into three major categories:

Here are two images of 1) angular fragments of crustal feldspar and quartz, 2) rounded nodules consisting of a granular core of commingled carbonate and diagenic quartz that is mantled by a more melanocratic granular carbonate studded with opaques.

and, along the bottom of the sample, 3) what appears to be a fragment of carbonatized damtjernitic breccia, itself containing xenocrysts of both types (1) and (2), giving the sample texture a scale-invariant appearance.

Numerous carbonate veins and veinlets:

Here's an image of a highly complex phenocryst. Upper left has plagioclase and could have been altered into “chessboard albite”, 2 subhedral/anhedral grains of quartz, possibly crustal in origin, possibly diagenic quartz bottom center, no clue about anything else.

One image- a close up view of a rounded nodule; subhedral carbonate core, possibly ferrous fracture network, and outer mantle of aggregated granular carbonate and opaques, encapsulated thin carbonate “candy coating”.

Epi-darkfield showing pyrite (center, highly reflective), hematite (red) and magnesite (white):

Finally: plagioclase grain showing twinning deformed by shear (3 degree bend) as well as some deformation twinning. Checking the literature for feldspar, I found Young’s modulus is approximately 95 GPa. 3 degrees bend corresponds to a shear strain of 0.05 (strain = tan (angle)) indicating in an applied shear stress of 9.5 GPa. If this grain were a cube, each length 0.72 mm, the calculated shear stress resulted from an applied force of (approximately) 5 kN (1100 pounds). In other words, the image shows the deformation that would occur if the grain was stood upon by a large grizzly bear, balanced on the tip of one toenail.

 

[Andy Resnick]

Another carbonatized damtjernite:

Brecciated carbonatized damtjernite. A wide variety of xenoliths and xenocrysts are present that can be generally divided into three major categories: 1) angular fragments of crustal feldspar and quartz, 2) rounded nodules consisting of a granular core of commingled carbonate and diagenic quartz that is mantled by a more melanocratic granular carbonate studded with opaques and, near the center of the sample, 3) what appears to be a fragment of carbonatized damtjernitic breccia, itself containing xenocrysts of both types (1) and (2), giving the sample texture a scale-invariant appearance.

My understanding is that carbonate magmas are somehow phase-separated from silica magmas; as if the two are immiscible. I was (and still am) curious if, when these two substances are forced into contact, I could image the result of any physio-chemical interaction between quartz and carbonate. This sample has a few interesting features….

Calcite vein transecting (crustal) feldspar and quartz. The feldspar definitely has substantial numbers of inclusions of carbonate while the quartz has comparatively few.

Two images: views of the variety of pheno- and xenocrysts present. The rounded granular phenocrysts are (likely) fully carbonatized amphibole and pyroxene grains, each surrounded by a darker granular carbonate mantle. Crustal xenocrysts (feldspar, quartz) are angular and partially altered.

Two images: predominantly biotite with groundmass in the corner. On one side, the biotite is mantled by a shell of prismatic grains of carbonate, and splitting the biotite is diagenic quartz with a thin layer of anhedral carbonate separating the quartz and biotite.

Three images: this appears to have been a xenocryst of crustal quartz that has been partially altered by carbonate. Within a thin shell of opaque-laden granular carbonate is patchy anhedral cabonate intermixed with diagenic quartz surrounding a core of quartz. Despite quartz being inert, there appears to be evidence of a metasomatic reaction, as can be seen at high magnification of the edge of the core. The core is covered with an approximately 10-15 microns thick ‘skin’ of quartz with a birefringence different from the bulk. In addition, there is a very thin (single thickness) layer of inclusions co-located at the boundary of this change of birefringence. It’s not clear what is happening.

#thinsectionthursday

 

[Andy Resnick]

Yet another carbonatized damtjernite: an embarrassment of riches!

Brecciated carbonatized damtjernite. A wide variety of xenoliths and xenocrysts are present that can be generally divided into three major categories: 1) angular fragments of crustal feldspar and quartz, 2) rounded nodules consisting of a granular core of commingled carbonate and diagenic quartz that is mantled by a more melanocratic granular carbonate studded with opaques and, at the upper right of the sample, 3) what appears to be a fragment of carbonatized damtjernitic breccia, itself containing xenocrysts of both types (1) and (2), giving the sample texture a scale-invariant appearance.

Here is a plagioclase grain showing 26 micron slip fracture. I'm unclear how to estimate the applied stress…

An epi-darkfield image of (I think) hematite/goethite and magnesite:

Myrmekite (quartz in plagioclase). "Wormy" appearance indicates metasomatism was accompanied by tectonic strains, presumably by strain occurring during the metasomatic process:

One image: a large nodule containing smaller nodules and surrounded by a variety of pheno- and xenocrysts. Mostly carbonate and opaques, xenocrysts are quartz and feldspar.

One image: some sort of diffusion process resulting in unusual strain birefringence patterns (created by micro-inclusions) within a quartz grain.

A quartz grain with undulose extinction.

This one is a bit odd: apparent metasomatic replacement of plagioclase by quartz in the presence of carbonate. Small biotite grains with exsolved opaques as inclusions.

 

[Andy Resnick]

Same type of rock as before:

Brecciated carbonatized damtjernite. A wide variety of xenoliths and xenocrysts are present that can be generally divided into three major categories: 1) angular fragments of crustal feldspar and quartz, 2) rounded nodules consisting of a granular core of commingled carbonate and diagenic quartz that is mantled by a more melanocratic granular carbonate studded with opaques and, for much of the right side, 3) what appears to be a fragment of carbonatized damtjernitic breccia, itself containing xenocrysts of both types (1) and (2), giving the sample texture a scale-invariant appearance.

Next image: plagioclase showing twinning deformed by shear (5 degree bend). Checking the literature for feldspar, I found Young’s modulus is approximately 95 GPa. A 5 degree bend corresponds to a shear strain of 0.09 (strain = tan (angle)) indicating in an applied shear stress of 17 GPa. If this grain were a cube, each length 0.75 mm, the calculated shear stress resulted from an applied force of (approximately) 9.6 kN (2160 pounds). In other words, the image shows the deformation that would occur if the grain was supporting a Volkswagon Beetle.

PP image of a biotite grain with principal cleavage planes nearly perpendicular to the microscope optic axis. I believe the fine dark lines are edges of crystal planes fractured during the sample preparation’s final polishing step:

XP view of biotite with kink banding:

Next set of images are (I think) quartz “vascularized” by opaques. The use of “vascularized” is apt given the appearance in epi-darkfield; I think the opaques here are hematite grains. The textural appearance is similar to that of Rodberg, a rock that is only found in the Fen complex.

Image pair of a carbonized nodule. The interior is commingled carbonate and quartz

Image pair of a carbonized nodule. The core is biotite, which is surrounded by carbonate and quartz, surrounded by a thin rim of opaques and melanocratic carbonate.

the lasat image pair is of a plagioclase xenocryst partially altered into quartz, in the presence of carbonate:

 

[Andy Resnick]

This sample is only slightly different- it's a carbonatized damtjernite:

Unlike the previous few samples, there are no crustal xenocrysts present.

Carbonatized damtjernite. Numerous rounded and angular phenocrysts that have been pseudomorphically altered by carbonization.

Near center, a rounded altered phenocryst (core is serpentine/muscovite, mantle is prismatic amphibole), lower left is angular unaltered carbonate phenocryst, numerous smaller phenocrysts of great variety and flakes of biotite within a carbonate groundmass studded with numerous opaques.

Many pseudomorphic grains have been altered to either a muscovite/serpentine/chlorite core with a fibrous amphibole shell or a carbonate core (aggregated granular) with fa thin rim of opaques; both types are similar to occurrences in Fen 9 and in Fen 260 (type locality for damtjernite) samples.

Right and upper center are gently rounded altered phenocrysts containing a core of serpentine/muscovite and a mantle of prismatic amphibole. Left is large carbonate phenocryst containing anhedral grains of apatite, and lower right is a grain of biotite. In addition, there are numerous smaller phenocrysts of great variety and flakes of biotite within a carbonate groundmass studded with numerous opaques.

Higher magnification views of an altered phenocryst consisting of a serpentine/muscovite core, prismatic/fibrous amphibole mantle, and thin shell of opaques:

Groundmass is primarily anhedral carbonate with some minute amphibole and biotite flakes in addition to a significant amount of opaques.

In the above image, there is visible an opaque with a pleochroic halo in biotite. In both PP and XP views, the optical effect (pleochroism, birefringence) spatially varies due to differing orientations of biotite crystals- one crystal is “en face” (basal cleavage or pinacoid face is perpendicular to the optical axis of microscope), the other large crystal is oriented at a right angle. There is a small biotite grain near the center as well, oriented approximately 45 degrees to the second grain.

Last image pair: high magnification view of an altered phenocryst, granular aggregated carbonate interior surrounded by a shell of prismatic/fibrous amphibole with a “hard-shell candy coating” (some sort of opaque oxide)

 

[Andy Resnick]

I think this is the last damtjernite sample I'll post- I have a couple more from type locality Damtjern, including one x-tra large (2" x 3") thin section, but it's time to post some of the other rock types.

Carbonatized damtjernite. Sample consists of a variety of altered phenocrysts in a groundmass of anhedral carbonate, opaques, and minor amount of small biotite flakes. Some phenocrysts have a core of chlorite, with or without an inner core of carbonate (likely dolomite), surrounded by anhdreal carbonate and opaques. Opaques are varied, including magnetite/goethite/illmenite, hematite, leucoxene, pyrite. Biotite flakes show signs of alteration, including the presence of diagenic quartz.

Epi-drkfield image of opaque grain, showing 2 distinct minerals with a vermicular/wormy texture. Presumably illmenite and leucoxene.

Epi-drkfield image showing a variety of opaques in a groundmass of carbonate (clear) and biotite (dark green, upper right corner). Opaques are predominately iron and titanium oxides.

Epi-drkfield image showing a variety of opaques in a groundmass of carbonate (clear) and biotite (dark green, upper right corner). Opaques are predominately iron and titanium oxides (black/red/white) and iron sulfides (yellow/gold)

Image pair: at top, three rounded altered phenocrysts (chlorite and opaque-studded carbonate). Bottom right, carbonate phenocryst (subhedral grains of carbinate); bottom left is a complex altered phenocryst. Groundmass is primarily carbonate and opaques.

Image pair: on right, a rounded altered phenocryst (chlorite and carbonate); lower left is a rounded grain of biotite, also on left a large opaque

Image pair: image is dominated by a partially altered (maybe just fractured?) biotite grain; center of grain is primarily diagenic quartz. and tip of grain is muscovite.

Image triplet: PP, XP, and epi-darkfield of groundmass. Transmissive minerals are carbonate and a flake of biotite (lower center) which is dark green in epi-darkfield. Epi-darkfield shows two different opaques, the dark one is probably an iron or titanium oxide while the white grains are probably leucoxene. Some of the white grain clearly show an octahedral crystal habit.

Image triplet: PP, XP, and epi-darkfield of an altered phenocryst: anhedral carbonate core surrounded by chlorite and mantled with carbonate and opaques (leucoxene).

Image pair: (right) 16x/0.35 lens epi-darkfield and (left) 40x/0.9 lens epi-brightfield (XP), showing the imaging effect of numerical aperture on both image detail and depth of field on finely textured material. The 16X image was re-scaled so the field of view matched that of the higher magnification lens. The leucoxene structure has an appearance similar to viscous fingering.

 

[Andy Resnick]

This sample is from Tveitan, a few miles from Fen:

Tveitan carbonatized damtjernite-like explosion breccia. Melanocratic aphanitic sample has a nodular texture, many nodules have a core containing relic amphibole.

One large (possibly crustal) fragment has plagioclase and chlorite.

Although the gross texture resembles carbonatized damtjernite samples 74 Fen 50 N-6A, N-6B, and N-6C; there are significantly fewer opaques, biotite phenocrysts are absent, and chlorite is present. Feldspar xenocrysts, some with microperthite texture, appear to have a carbonate reaction rim. Groundmass is extremely fine-grained, consisting of carbonate, possibly biotite flakes, abundant apatite crystals, and innumerable minute rounded inclusions.

Not sure what these inclusions are, possibly epidote?