The Secrets of Prof. Verschure's Rosetta Stones

[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: