The Secrets of Prof. Verschure's Rosetta Stones

[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?

 

[Andy Resnick]

This sample is from Honstjern:

Honstjern carbonatized damtjernite-like explosion breccia. This sample was obtained a few hundred meters away from the Tveitan carbonatized damtjernite-like explosion breccia. As before, this is a melanocratic aphanitic rock with nodular texture, many nodules have a core containing relic amphibole.

Carbonate vein and veinlets. Although the gross texture also 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. Many of the nodules have a complex interior structure:

Large feldspar xenocryst with amphibole inclusions, appears to have a dark carbonate reaction rim.

Groundmass is extremely fine-grained, consisting of carbonate, possibly biotite flakes, abundant apatite crystals, and innumerable minute rounded inclusions.

Edit: Ha! I forgot the mention the whole reason I selected this sample... Even though these two rocks (Honstjern and Tveitan carbonatized damtjernite-like explosion breccias) look identical and located so close to each other, K-Ar dating models place the two samples 200 million years apart. I'm not sure that puzzle was ever solved.

 

[Andy Resnick]

The Fen complex (about 1 square mile in size) has enormous diversity of rock types (I've counted between 15-17) and is the type locality for at least 6 of those. Here are 2 samples from type locality Holla, Telemark county, Norway:

Silicocarbonatite (Hollaite) from type locality Holla, Norway. Sometimes referred to as pyroxene søvite.

Euhedral prisms of pyroxene, anhedral grains of apatite, minor biotite set in a groundmass of coarse-grained anhedral interlocking crystals of calcite.

Euhedral prisms of pyroxene, anhedral grains of apatite, minor biotite set in a groundmass of coarse-grained anhedral interlocking crystals of calcite. The pyroxenes are pleochroic in shades of light yellow-green to apple-green, published analysis claims exist on the acmite-diopside-hedenbergite series. Unlike pyroxenes found in all the other rocks at Fen, the silicocarbonatite pyroxenes are only weakly zoned and show little grain-to-grain compositional variation.

There are a few euhedral prisms that I can't identify:

On the carbonate background, there are three distinct prisms: an elongated on emerging from the upper left corner and two stubby prisms along the lower third of the image. There are also some smaller fragments scattered. I think they are pyroxenes and the anomalous birefringence could indicate jadeite.

In one of the two samples, pyroxenes are enriched in Fe³⁺(dark green color) compared to the other.

 

[Andy Resnick]

This sample is (most likely) a Søvite.

Søvite is a carbonate rock of variable composition. The rock was called Søvite by Brogger (1921) after Søve farm and agricultural college. Ankerite and dolomite occur in variable amounts and in places can become quite dominant, in which case the rock is called rauhaugite. The mode of formation of the Søvite is one of the central problems of the Fen area and is important for the understanding of the genesis of the area as a whole and of peralkaline rocks in general. As best I can tell, the evidence that this rock is magmatic comes from how it occurs- in dykes and “cone sheets”- and the existence of an active carbonate volcano (Ol Doinyo Lengai, in Tanzania). The contrary evidence is that some rocks at Fen (the carbonated damtjernites, for example) indicate that Søvite could result from hydrothermal metasomatic replacement of silicates with carbonates. This particular sample has only zoned phlogopite (clear to orange pleochroism) and carbonate. Lack of apatite in noteworthy, and the phlogopite is more birefringent than "normal"- here, the birefringence extends to high third order, possibly due to the presence of titanium or iron.

I have to note that since I have no way to (optically) verify that the carbonate is calcite instead of dolomite/ankerite, this rock could in fact be rauhaugite.

 

[Andy Resnick]

I recently came across this website describing the history of the Fen complex (as a mining site) which told me the origin of all these samples:

"[From 1967-1971] Research group for rare earths (FSJ) carried out the first systematic survey of rare earths in the Fen Complex, mainly in Gruveåsen where the old iron mines were located. This was one of several periods of mapping and exploration activity in the Fen Complex during the Cold War. None of the campaigns ended with new mining, but contributed with research and increased knowledge of the field, which has become important later."

Another example of søvite:

Søvite is a carbonate rock of variable composition, occupying a large area in the eastern part of the Fen area. Mica minerals (typically phlogopite and tetraferriphlogopite), magnetite, pyrochlore and apatite are found in the Søvite in variable quantities. This particular sample has zoned tetraferriphlogopite (identification from the gold-yellow and orange-pink pleochroism) and minor amounts of apatite on a background of subhedral carbonate, likely calcite.

The presence of tetraferriphlogopite places the source location at either one of two quarries separated by just 450m: the Cappelen quarry (Søve Mines) or the Hydro quarry at Fen.

At some crystal orientations, high magnification views of the phlogopite booklets are striking- intricately patterned second-order interference colors.

Could this be an artifact of the grinding/polishing preparation? As another example, the birefringence pattern in those four adjacent large phlogopite crystals (shown above) is continuous even though the crystal orientations are vary discretely.

 

[Andy Resnick]

Another søvite:

Based on the distinct pleochroism patterns (gold-yellow and orange-pink for tetraferriphlogopite cores and gold-dark brown for the mantles), this sample seems to have tetraferriphlogopite mantled by some other variety of phlogopite.

My first guess, titanium-bearing phlogopite, is incorrect because that is found in Damtjern, and the phlogopite in those samples looks different, so it’s probably iron-bearing phlogopite.

The sample most likely has niobium-bearing pyrochlore (koppite) mostly converted to columbite.

While reading, I discovered that in addition to niobium-bearing pyrochlore (koppite), there are also rare earth element (REE)-bearing perovskites (knopite)…. pyrochlore-koppite/perovskite-knopite sounds like a strange double naming coincidence… in addition to those two, there is also a niobium-bearing perovskite (dysanalyte) and all three are likely present within the Fen complex. Plentiful rounded apatite crystals, and where there are small aggregations of fibrous amphibole (possibly stilpnomelane?) the carbonate is dominated by a mist of minute inclusions.

Opaques include illmenite, magnetite, and pyrite.

 

[Andy Resnick]

Moving on to another type-locality rock type, this is a meltegeite:

Melteigite, likely from type locality Melteig Farm, Nome, Telemark, Norway. Melteigite is part of the Ijolite series (urtite - ijolite - melteigite - vipetoite/Jacupirangite) which mainly consists of aegirine-augite and up to 30% nepheline. In fact, most of the rock types in the Fen complex are defined in terms of the mix of nepheline, pyroxene, and/or carbonatite: other than the disturbed crustal gneiss, there is rarely quartz or feldspar present in the rocks. Here, the nepheline has been completely altered. Nowadays, this rock type is classified as a melanocratic nephelinolite.

This sample has anhedral titanaugite (some with complex zoning) and subhedral nepheline (mostly pseudomorphed to chlorite , muscovite and epidote), minor amounts of carbonate and elongated, rounded, apatite crystals.

The presence of epidote implies a hydrothermal alteration process. In the following epi-darkfield image are common alteration products of nepheline: blue is sodalite, yellow (cancritine), and near the cancritine is a grain of apatite, kind of green-grey (to my eyes).

The chlorite shows anomalous birefringence: brown is typical of optically postive crystals, blue is typical of optically negative crystals)

 

[Andy Resnick]

Moving to the end of the ijolite series, this sample is a vipetoite:

Vipetoite, likely from type locality Vipeto Farm, Nome, Telemark, Norway. A local name for a variety of pyroxenite containing abundant titanian augite and hornblende with biotite, primary calcite, and occasional albite and nepheline. As pointed out by Sæther (1957) the name was actually misspelt by Brögger in the belief that the locality was named Vibeto instead of Vipeto. The correct spelling should have been vipetoite but vibetoite is in common usage. Synonymous with Jacupirangite, a variety of alkali pyroxenite consisting essentially of titanian augite with minor amounts of titanomagnetite, nepheline, apatite, perovskite and melanite garnet.

Rock texture is allotriomorphic granular, consisting of anhedral grains of pyroxenes, amphibole, carbonates, and abundant apatite.

Some grains have a poikilitic texture, pyroxene oikocrysts enclosing a variety of apatite, biotite, and carbonate chadacrysts.

Opaques likely pyrite and ilmenite (often associated with leucoxene).

 

[Andy Resnick]

Posting a day early today, this sample is an urtite/ijolite:

Urtite/ijolite adjacent to fenitized gneiss. The cutoff between the two is a nepheline concentration of 70%: more than that is an urtite, less than that is ijolite (and still less is melteigite, then vipetoite). Country rock component is fenitized gneiss, primarily perthite and chessboard albite in highly lobulated grains. Within the urtite component, there is a prominent carbonate veinlet. Nepheline largely altered to muscovite/sericite, pyroxene often altered to amphibole.

Large pyroxene grain partially altered to secondary amphibole (hydration reaction).

A relatively high concentration of sphene (possibly more than 10%)and as the boundary is approached, sphene is oxidized into opaques (illmenite and leucoxene).

Nepheline has been completely altered, mostly into muscovite. Near the carbonate veinlet are some photogenic features; here there is carbonate (calcite?) that transects altered nepheline:

 

[Andy Resnick]

This sample is (IMO) one of the highlights of the collection:

Rødberg. An altered carbonatite composed of hematite-stained granular calcite, dolomite and sometimes ankerite. Rødberg is only found in one location over the entire earth ( https://www.mindat.org/min-48029.html ). Rødberg is a source for Thorium and results from hydrothermal alteration of carbonatite. Perhaps surprisingly, it took me a long time to identify this sample because I am color blind- both PP and XP images, to my eyes, are very nearly black and white (or greyscale):

It wasn’t until I imaged using epi-darkfield that the vivid red color and distinctive texture became apparent:

Also, while the geological context of the word ‘pneumolysis’ is archaic (referring to petrological alterations due to gas emitted by magma), the medical use of the word is not, where it means “the surgical procedure of separating adhesions that bind the lung or lung tissue to the chest wall or other structures.” Indeed, the texture of hematite in this sample does remind me of alveoli, small structures within a lung.

 

[Andy Resnick]

For this sample (and the next few), I made use of the text “Igneous Rocks: A Classification and Glossary of Terms. (Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks), R.W. Le Maitre (Ed.)”, to match samples with lists of type-locality Fen rocks provided in a few publications. This one is Kamperite:

Kamperite. Type locality: Kamperhoug, Fen Complex, Telemark, Norway. Defined in “Igneous Rocks: A Classification and Glossary of Terms” as a local name for a medium- to fine-grained highly potassic dyke rock composed of almost equal amounts of orthoclase and biotite with minor oligoclase. It would now be defined modally as a K-feldspar syenite on the QAPF diagram.

The biotite is a late crystallization product. Some biotite grains have small inclusions of apatite, zircon, or epidote (possibly piemontite).

Minor opaques, possibly illmenite or magnetite.

 

[Andy Resnick]

This sample is Ringite:

Ringite. Type locality: Ringsevja, Fen Complex, Telemark, Norway. A local name for a coarse-grained carbonatite containing aegirine and alkali feldspar, considered to be a mixture of carbonatite and syenitic fenite.

Fenite (named for Fen, the type locality) is an alkali syenite found in the peripheral parts of the Fen area primarily consisting of alkali feldspar (here, likely albite and anorthoclase) and aegirine (which has replaced the original biotite during fenitization). Fenite can have a highly variable composition reflecting the variety both mineral chemistries and geologic processes that have occurred, and I will work through the many different fenite samples in due time. Meanwhile.... ringite. One of the interesting (to me) aspects of this sample is the ease with which the alkali feldspars can be distinguished, using either PP or epi-darkfield imaging.

The albite is mostly clear while the anorthoclase is milky/cloudy. This sample has several metal oxides, including (I think) lucoxene:

in addition to iron oxides:

The red hematite has an appearance similar to rodberg, here it is confined to the interstices of carbonate grains, again mimicking alveoli.

 

[Andy Resnick]

Continuing to identify a "legion of obscure rock types named after equally obscure European villages", this is Sannaite:

Sannaite. Type locality: Sannavand, Fen Complex, Telemark, Norway. A variety of lamprophyre composed of combinations of olivine, titanian augite, kaersutite (kaersutite is a dark brown to black double-chain calcic titanium-bearing amphibole) and Ti-rich biotite phenocrysts with alkali feldspar dominating over plagioclase in the groundmass which also contains nepheline. Defined within the lamprophyre classification, this set of samples appear to be sequential thin-section preparations from the same rock:

This is evidenced by, for example, the prominent (pseudomorphed) nepheline phenocryst in 52 ii and 52 iii and rounded amphibole phenocryst in the lower edge of 52 i and middle of 52 ii. These samples are strongly porphyritic with large phenocrysts of both amphibole and (pseudomorphed) nepheline, there are also, based on the hexagonal habit, smaller phenocrysts of (pseudomorphed) biotite. Any biotite that was present has been pseudomorphed to chlorite with inclusions of (relatively) coarse-grained leucoxene.

This rock does not appear to contain olivine or augite, so my assignment of sannaite could be incorrect. Even so, alkali feldspar >> plagioclase in the groundmass and possibly nepheline (altered to paragonite) in the groundmass as well.

The nepheline phenocryst has been pseudomorphed into fine sheaves of paragonite (muscovite).

Pac-Man is amphibole (kaersutite?) while his meal and eye is amphibole and/or biotite that has been altered to chlorite and leucoxene.

Possibly two small grains of anatase?

 

[Andy Resnick]

Switching over to a better-studied rock type: Fenites.

Fenite. Type locality: Fen Complex, Telemark, Norway. Fenite is an alkali syenite and is found in the peripheral parts of the Fen area with highly variable composition. From an economic perspective, fenites are enriched in rare earth elements and so are an attractive object to study. Metasomatism (chemical changes due to fluid interaction/transport) of the country rocks by the Fen Complex was named by Brøgger “fenitisierung” (fenitization) and the metasomatized gneiss “Fenit” (fenite), and this process has been observed at other localities on the earth. It has been shown that multiple (at least 3) fenitization events have occurred at Fen, with each instance corresponding to a “pulse” of magmatic-derived fluids that transport materials through brecciated rock, changing the chemistry by both leaching and deposition. Because metasomatism is highly dependent on both the original rock composition as well as the fenitizing fluids, fenites display a wide range of appearances and compositions (similar to the variability of lamprophyres at Fen).

The first fenitizing event (fenitization-1) produced aergirine and sodic amphibole at the expense of hornblende and biotite in country rock, the second event formed stilpnomelane and other oxides at the expense of the fenitization-1 minerals, and the third, caused by interaction between rocks and groundwater-derived hydrothermal fluids infiltrating the eastern part of the complex, led to oxidation of ferrocarbonatite into Rødberg.

This particular sample has been classified in a paper as exhibiting both weak fenitization-1 and weak fentization-2. Green pyroxenes (aegirine) and blue amphiboles (riebeckite, and arfvedsonite) are considered the most prominent fenitization minerals.

Fenitization-1 is characterized by the breakdown of primary hornblende and biotite. Both minerals show incipient formation of fine-grained reaction coronas, with an outer zone of aergirine and an inner zone of albite and/or microcline. The coronas occur only where hornblende and biotite crystals were in contact with quartz.

Primary feldspars (perthite and plagioclase), remain largely unaltered, except for the development of turbidity by very fine-grained mineral matter. Fenitization-1 minerals can range from less than 5% (weak) to greater than 10% (strong) and exceed 90% of the total rock volume.

Fenitization-2 is a later process resulting in the replacement of Fenitization-1 minerals, primary gneiss minerals, and also minerals of the Fen Complex (Ijolites and carbonatites). Fenitization-2 produced low-temperature crystallization of hydrous minerals (Na-amphiboles, new greenish biotite, new chlorite, sericite and stilpnomelane), carbonates, quartz and opaque minerals. Fenitization-2 minerals are fine grained and seldom exceed 10% of the total rock volume.

In the above images, transformation of the (green) pyroxene into (blue) amphibole can be seen, as can the growth of a few brownish stilpnomelane flakes.

Stilpnomelane often forms "bushy" borders with quartz:

Apatite, allanite, sphene and zircon occur as accessory minerals:

For the above images, starting at the top left-of-center is a grain of apatite. Moving around clockwise, we see the palimpsestic (a new favorite word!) replacement of (likely) hornblende into aegirine and albite (Fenitization-1) as well as opaque oxides (fenitization -2). Continuing around is a large grain of (ilkely) Allanite-(Nb), in the lower left corner is a square crystal of (likely) zircon, and more towards the center, an anhedral grain of (I think) sphene. The center of this image is dominated by a grain of perthite (and a crack in the sample). A small flake of biotite is in the upper left, and the lack of a reaction corona provides supporting evidence for the identifications of both apatite and feldspar.

I have at least a dozen thin sections that are uncovered (bare rock surface, no Canada balsam or coverglass) and so cannot be used for optical minerology. I’m not sure what to do with them- my priority is to not damage the samples- but I have been thinking about applying a thin film of immersion oil to the rock surface in order to hold in place a coverslip. The index of refraction of immersion oil very closely matches that of canada balsam.

In addition to the many samples of fenitized gneiss showing various degrees of fenitization, I also have samples of fenitized carbonatites and fenitized ijolites. Stay tuned!

 

[Andy Resnick]

Another fenite:

Fenite. Type locality: Fen Complex, Telemark, Norway. This sample was been classified as having moderate fenitization-1 and weak fenitization-2, so it preferentially shows the effects of fenitization-1 metasomatism. Moderate Fenitization-1 is characterized by the nearly complete breakdown of both hornblende and biotite by the formation of wide Na-pyroxene + albite- microcline reaction coronas.

Na-pyroxenes (aegirine) also form irregular patches and vein-like aggregates.

Primary plagioclase is partly replaced by fine-grained, clear aggregates of chessboard albite, this feature in particular is also diagnostic for fenitization:

Sæther (1957) called the formation of clear chessboard albite and microcline aggregates ‘matrix formation’. Primary quartz and microcline perthite are still present. Adjacent K-feldspar grains show albitization following along the boundary between grains (intergranular albite). Interestingly, each half of the albite seam has the same crystallographic orientation as the *opposite* K-feldspar, as can be visualized by inserting a quarter-waveplate when imaging XP:

The mechanism for this process (“dissolution–precipitation”) is the subject of a considerable body of literature, most of which I don't fully understand (yet).

 

[Andy Resnick]

Continuing with examples showing increasing levels of fenitization:

Fenite. Type locality: Fen Complex, Telemark, Norway. This sample was been classified as having strong fenitization-1 and weak fenitization-2, so it should preferentially show the effects of fenitization-1 metasomatism. While this sample’s fenitization-1 minerals comprise 24% of the total rock volume, future samples will show fenitization-1 minerals comprising up to 95% of the total rock volume. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase. This sample still has primary gneiss minerals (74% of the total rock volume, according to the paper). However, biotite and hornblende have been completely broken down. Coronas of Na-pyroxene surrounding chessboard albite may be present, but Na-pyroxene generally occurs as veins and irregular aggregates. The original gneissose texture is still recognizable in places. The strongly fenitized-1 rocks also contain clear, euhedral alkali feldspar crystals forming a matrix of chessboard albite and microcline without preferred orientation.

Fenitization-2 minerals are also present, comprising 2% of the rock volume. One example (below) is the breakdown of aegirine (Na-pyroxene) into arfvedsonite (sodium amphibole).

I used the large language model "NotebookLM" for optical criteria to distinguish riebeckite from arfvedsonite- both are sodium amphiboles, differing only in that riebeckite has 2 Na ions per unit cell while arfvedsonite has 3. Based on the training materials I uploaded, NotebookLM provided a few differences, primarily in terms of pleochroism. Based on the program output (and the original paper :) ), the blue amphibole in this sample is arfvedsonite.

 

[Andy Resnick]

Fenite. Type locality: Fen Complex, Telemark, Norway. Fenitization-1 minerals comprise 51% of the total rock volume of this sample, having been classified as having strong fenitization-1. In addition, it is classified as weak fenitization-2, so it should preferentially show the effects of fenitization-1 metasomatism. The macroscopic texture has been described as a “mortar texture”, with large phenocrysts of microcline perthite within a fine-grained groundmass of chessboard albite.

Strongly fenitized-1 rocks contain clear, euhedral alkali feldspar crystals forming a matrix of chessboard albite and microcline without preferred orientation. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase. Of the primary gneiss minerals, only microcline perthite remains (46% rock volume). What began as thin intergranular albite seams has now become thick bands of chessboard albite. The original gneissose texture is barely recognizable.

Na-pyroxene now occurs entirely as veins and irregular aggregates:

Fenitization-2 minerals comprise 1% of the rock volume. Three of those minerals: opaque oxides, calcite, and stilpnomelane (the thin seam), are shown here:

Lastly, a poorly-exposed photo of chessboard albite with a quarter wave plate (mica plate) inserted:

 

[Andy Resnick]

Continuing with strongly fenitized-1 gneisses:

Fenite (fenitized gneiss). Type locality: Fen Complex, Telemark, Norway. This sample’s fenitization-1 minerals comprise 83% of the total rock volume. This sample was been classified as having strong fenitization-1 and weak fenitization-2, so it should preferentially show the effects of fenitization-1 metasomatism. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase. Of the primary gneiss minerals, only microcline perthite remains (15% rock volume).

Fenitization-2 minerals comprise only 1% of the sample. Although the paper mentions ‘Fenitized-2 apatite’ is present as an accessory mineral, I think, based upon the optical properties in both transmitted and reflected light, is fenitized titanite, not fenitized apatite. The primary evidence is by comparing epi-darkfield images of this sample and of apatite, found in sample 67 Fen 23 ii- there are clear differences in appearance. The images below predominately show (I think) fenitized titanite (and pleochroic pyroxene).

This sample has a relatively high concentration of titanite- the paper claims 1%- and also claims they are xenocrysts. I'm not sure that's correct, but I'm not an authority on the subject...

Fenitization (defined as the leaching of SiO2 and enrichment of Na2O) is quite complex; I've been working through a more comprehensive paper "Carbonatite-related contact metasomatism in the Fen complex, Norway: effects and petrogenetic implications" (https://www.cambridge.org/core/jour...implications/00AA1631F359FD10A73327CB7BEEB65F). which is, to be honest, somewhat beyond my expertise. I'm working on it! :)

 

[Andy Resnick]

This sample is almost completely altered:

Fenite. Type locality: Fen Complex, Telemark, Norway. This sample was been classified as having strong fenitization-1 and weak fenitization-2, so it should preferentially show the effects of fenitization-1 metasomatism. This sample’s fenitization-1 minerals comprise 95% of the total rock volume. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase.

According to the relevant paper, the only remnant from the primary gneiss is accessory titanite (4% rock volume). That said, I believe the lightly dusted grains along the right side are primary gneiss (K-feldspar):

Fenitization-2 minerals comprise only 1% of the sample, and although the paper mentions ‘Fenitized-2 apatite’ [as accessory mineral] I think, based on the optical properties, is actually fenitized titanite.

Comparison of epi-darkfied images of apatite (67 Fen 23 ii) and these show clear differences. Worth mentioning here (for comparison later) is the habit of these titanite grains- anhedral with relatively rough edges.

Biotite present is classified as “Fenitized-2 Biotite”; optical appearance (clear to very light brown pleochroism, characteristic “birds-eye’ extinction pattern mostly absent) is markedly different from biotite found in either unaltered country rock or the phlogopite found within lamprophyres found within the Fen intrusion.

 

[Andy Resnick]

This is one of the more interesting samples in the collection:

Chilled margin of Ijolite intrusion. One side is fenitized gneiss, the other is fenitized melteigite.

The geniss was been classified as having strong fenitization-1 and weak fenitization-2, while the melteigite has been characterized as having strong fenitization-2. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase. Fenitization of the country rock (telemark gneiss), the first of several metasomatic events, was driven by the release of volatiles and fluids from the ijolite intrusion (on the other side of the sample) resulting in loss of SiO2 and Al2O3 and influx of Na/K/Ca-rich fluids (an alkaline reaction).

The relevant papers indicate that the melteigite was fenitized later, undergoing a low temperature hydrothermal metasomatic event. Evidence for this is the presence of stilpnomelane, a fenitization-2 mineral (not shown here). The melteigite is pierced by a mixture of albite, prehnite, and epidote- filled veinlets.

Nepheline (with pyroxene grains) has been completely altered to (most likely) a zeolite.

Here, the opaque shows evidence of metasomatism; it appears to be pyrite enclosed within a shell of magnetite:

In this sample, titanite (with epidote inclusions) has a significantly different appearance, as compared to the previous samples- note how the crystallites are much smoother and also potentially a slightly lower relief:

 

[Andy Resnick]

This is a very specific type of fenite:

Pulaskitic fenite. ‘Pulaskitic’ means this rock is classified as a nepheline-bearing syenite (field 7' on the QAPF diagram) composed of alkali feldspar (perthite) and nepheline (altered to sericite). From the relevant paper, “Close to the Fen Complex, a maximum 50 m-wide zone only rarely contains quartz, plagioclase or orthoclase/microcline. These rocks – fenites in the sense of Brøgger (1921) and probably similar to Kresten’s (1988) contact fenites – are mainly made up by aggregates of Na-pyroxene in a matrix of sub-to euhedral mesoperthite (in places with Carlsbad or Baveno twinning) with discontinuous intergranular rims of exsolved albite. The texture of these rocks is magmatic.”