The Secrets of Prof. Verschure's Rosetta Stones

[DrDu]

Nice pictures! However, I am generally missing pictures taken with only the polariser without analyzer. PP and epi-darkfield are fun, but not of diagnostic value. The olive-green interference colours in the pictures of the metamorphic biotite could be anomalous interference colours typical of chlorite.

The mineral on the last picture could also be anorthoclase, which also shows microcline-like structure.

 

[Andy Resnick]

This week, I'm going back to a sample that was published:

One reason I wanted to examine this sample is that I felt I needed a 'sanity check' on my progress; the other reason has to do with the prevalence of a particular mineral in this sample, arfvedsonite (see also here).

According to the paper, this sample's petrology is:

"S_F2-S_F1 gneiss; along cracks Carb and Aeg -> Arf. Sample obtained 306m (Inferred distance) from contact with country rock.Primary Gneiss: 2% Quartz 40% Perthite, accessory Apatite, Zircon, Sphene. Fenitization-1 (high temperature dehydration): 15% Aegirine, 24% microcline-chessboard albite ‘matrix’. Fenitization-2 (low temperature Hydration-Carbonation): 5% Arfvedsonite, 2% Opaques, 10% Carbonatite dispersed, 2% Carbonatite veins, accessory albite.

Recall: S_F1 means "strongly fenitized -1" and S_F2 means "strongly fenitized -2". These designations are given based on the amount of certain minerals (and their alterations) that are present.

So: here are some images to enjoy! The first pair of images shows aergirine on the upper left side and bottom right corner, chessboard albite in the top middle, some feldspar, and arfvedsonite showing the anomalous blue bifrefringence in addition to the usual first-order color.

This next pair of images is mostly arfvedsonite with some calcite, feldspar, and a grain of seriticized feldspar along the right edge. A line of aegirine runs vertically just to the right of center. Again, the arfvednosite shows both first-order bifrefringence color in addition to anomalous blue:

The next photos are just to show off how photogenic this sample is:

And this last photo is of perthite with some needle-like inclusions- I'm guessing aegirine or possibly mica.

 

[DrDu]

Very nice slide! You should also try to identify the pleochroism of the arfvedsonite. From your picture, one already recognizes a change from light blue to light green parallel or perpendicular to the longitudinal extension. How is your polarisatior oriented? (NS or EW)? Do you also find a typical "head-cut"? What are the pleochroitic colours there? The picture is from W. E. Tröger, Optische Bestimmung der gesteinsbildenden Minerale, Teil I Bestimmungstabellen, 5th edition, Schweitzerbartsche Verlagsbuchhandlung, Stuttgart, 1982. The book is an absolute must have!
Furthermore, I highly recommend a look at
https://homepage.rub.de/olaf.medenbach/download/mineraloptik/
although only in German.

 

[Andy Resnick]

Very nice slide! You should also try to identify the pleochroism of the arfvedsonite. From your picture, one already recognizes a change from light blue to light green parallel or perpendicular to the longitudinal extension. How is your polarisatior oriented? (NS or EW)? Do you also find a typical "head-cut"? What are the pleochroitic colours there? The picture is from W. E. Tröger, Optische Bestimmung der gesteinsbildenden Minerale, Teil I Bestimmungstabellen, 5th edition, Schweitzerbartsche Verlagsbuchhandlung, Stuttgart, 1982. The book is an absolute must have!
Furthermore, I highly recommend a look at
https://homepage.rub.de/olaf.medenbach/download/mineraloptik/
although only in German.

Thanks for the feedback! I was busy last week working with my solar eclipse images, these are excellent questions/suggestions- thanks!

 

[Andy Resnick]

This week's sample is another trachyte:

I chose this slide due to the presence of 'something' I'm hoping someone can help identify, that is also present in many other samples. I would describe this sample as having:

Phenocrysts of altered sanidine, altered nepheline, minor amounts of altered biotite in a groundmass of acicular aegirine, platy trachytic feldspar, and ‘granules’ of feldspar (?). Minor amounts of cabonatite. Nepheline phenocrysts are heavily altered to mica (?), almost appears like glimmerite, with a rim of granulated feldspar (?)

Here's a sequence of altered nepheline, showing the 'glimmerite' and granules of 'something' on the rim:

Whatever that foamy stuff is, it's also present in the (altered) sanidine:

The little bits of carbonatite are nicely photogenic- I identified these as carbonatite due to the high birefrengence- which also helps create a 3-D appearance:

 

[DrDu]

The white rim looks like quartz to me.

 

[Andy Resnick]

Very nice slide! You should also try to identify the pleochroism of the arfvedsonite. From your picture, one already recognizes a change from light blue to light green parallel or perpendicular to the longitudinal extension. How is your polarisatior oriented? (NS or EW)? Do you also find a typical "head-cut"? What are the pleochroitic colours there?

I had a few images on hand to show pleochroism- here's a pair of PP and XP images with the sample rotated 90 degrees between the sets. On the upper pair of images, the arfvedsonite is along the left half and bottom half. The (according to my eyes) green-blue pleochroism is pretty cool :).

About your other questions- the polarizer is oriented (I think) NS, but that's a guess based on the angle marking on the polarizer mount (zero degrees). What does "head-cut" mean- a slice perpendicular to the long axis? There are a few square-ish/diamond shapes that have a strong green -> pink color change....? I can try and get some decent images of those.

 

[Andy Resnick]

The white rim looks like quartz to me.

I was wondering if that's the case. Is it an alteration product? My understanding is that these rocks are all very silica-undersaturated.

 

[DrDu]

Yes, if it is quartz, it can only have formed diagenetically.

 

[DrDu]

I had a few images on hand to show pleochroism- here's a pair of PP and XP images with the sample rotated 90 degrees between the sets. On the upper pair of images, the arfvedsonite is along the left half and bottom half. The (according to my eyes) green-blue pleochroism is pretty cool :).

View attachment 343673

About your other questions- the polarizer is oriented (I think) NS, but that's a guess based on the angle marking on the polarizer mount (zero degrees). What does "head-cut" mean- a slice perpendicular to the long axis? There are a few square-ish/diamond shapes that have a strong green -> pink color change....? I can try and get some decent images of those.

The orientation of the polariser is best detected by the pleochroism of biotite. Biotite, cut perpendicular to the sheets, appears dark if the polariser is oriented parallel to the sheets, and bright if perpendicular.
And yes, a "head cut" (my translation of the german "Kopfschnitt") is a slice perpendicular to the long axis. You should see the characteristic cleavage pattern with 60 and 120 degree angles and also pleochroism, which is different, at least in one direction, from the colours seen along the longitudinally oriented crystals.

 

[Andy Resnick]

The orientation of the polariser is best detected by the pleochroism of biotite. Biotite, cut perpendicular to the sheets, appears dark if the polariser is oriented parallel to the sheets, and bright if perpendicular.
And yes, a "head cut" (my translation of the german "Kopfschnitt") is a slice perpendicular to the long axis. You should see the characteristic cleavage pattern with 60 and 120 degree angles and also pleochroism, which is different, at least in one direction, from the colours seen along the longitudinally oriented crystals.

Last night I realized I need to be more careful in my polarizer description/analysis. The polarizer may indeed be oriented NS, but then the analyzer is EW, so for PP imaging I may need to be more specific about which polarizing element is in the path... stay tuned!

 

[DrDu]

Last night I realized I need to be more careful in my polarizer description/analysis. The polarizer may indeed be oriented NS, but then the analyzer is EW, so for PP imaging I may need to be more specific about which polarizing element is in the path... stay tuned!

Usually, when talking about PPL, you assume that the polarizer is present, but the analyzer has been removed.

 

[Andy Resnick]

The orientation of the polariser is best detected by the pleochroism of biotite. Biotite, cut perpendicular to the sheets, appears dark if the polariser is oriented parallel to the sheets, and bright if perpendicular.
And yes, a "head cut" (my translation of the german "Kopfschnitt") is a slice perpendicular to the long axis. You should see the characteristic cleavage pattern with 60 and 120 degree angles and also pleochroism, which is different, at least in one direction, from the colours seen along the longitudinally oriented crystals.

I had a chance to take a few careful images of arfvedsonite crystals today, I think I found an arrangement with both 'head cut' and longitudinally oriented crystals in the field of view. Here's a photo with just the (EW) analyzer present:

On the right side, the crystal is in the 'head-cut' position (I think, based on the fracture pattern), while at the lower left (and also a small one nestled in on the right) are longitudinally-oriented crystals. The same field of view, this time only the (NS) polarizer in place:

I tried to orient the sample to maximize the color changes. Here's both analyzer and polarizer (crossed polars):

And a montage of PP(both polarizer and analyzer) and XP under sample rotation:

In the XP series, I measured an extinction angle of 25 degrees... assuming I did that correctly.

Then I just had some fun, playing with color: here's XP plus 1/4 waveplate:

Replacing the polarizer with a circular polarizer (circularly polarized light incident on sample):

And then 2 photos using only a Cokin chromofilter SA: I forgot to pay attention to the exposure setting, both of these are overexposed.

 

[Andy Resnick]

This is a sample of an ultramafic rock:

I think this is either a phlogopite-hornblende-olivine peridotite or a pyroxene peridotite- I had (and still have) trouble distinguishing between pyroxene and amphibole in this sample. Major minerals: serpentinized olivine phenocrysts, horneblende or clinopyroxene phenocrysts, some zoned/twinned. Phlogopite phenocrysts. Opaques may be chromite? Not much of a groundmass, but what there is consists of small grains of phlogopite and carbonatite.

Here's a pair of images showing phlogopite, calcite, what I think is apatite at bottom. The grain in the upper right corner is unknown (to me):

An oddly zoned phlogopite:

Here are a couple image pairs of what has confused me- grains that are either pyroxene or amphibole grains, I can't identify which:

Any guesses?

Here's another grain with some (relic?) serpentized olivine:

Finally, while I've posted a few images previously of the serpentinized olivine, it's so photogenic I decided to post some more images here again:

(XP with full wave plate)

(Chromofilter SA)

(XP)

 

[Andy Resnick]

This sample is very heterogeneous with some interesting and unusual textures:

On the left side (side closest to label) consists of approximately equal amounts plagioclase (albite?), lightly sericitized K-feldspar (orthoclase?) and little to no quartz, so I classify that part (presumably country rock) as a monzonite with aegirine inclusions:

Right side is magmatic with a porphyritic texture: phenocrysts of pyroxene and hornblende, each with reaction coronas, opaques, and minor apatite in a groundmass of anhedral carbonatite and masses of skeletal phlogopite.

I'm not sure what the central grain is; it could be acmite (a fibrous form of aegirine)?

Reaction coronas (here, around a grain of hornblende) appear to consist of phlogopite pseudomorph surrounded by a shell of granular carbonatite and plagioclase, itself surrounded by a shell of skeletal phlogopite.

In between the two phases (magmatic and monzonite) is a transition phase consisting of massed prismatic and acicular aergirine, especially concentrated around macrocysts of monzonite.

How did the aegirine needles form inside the pre-existing feldspar crystals? Also, since the 'monzonite' part of my sample is only about a square centimeter, what confidence do I have that it is a representative sample of the country rock? (not much, but I'm no expert).

Some opaques in the magmatic part also appear to have a dendritic or skeletal texture. Flakes of 2 different Iron compounds (likely pyrite and magnetite) occur together- this image is reflected darkfield (in transmitted brightfield the whole frame would be black):

My suspicion is that the white mineral is aegirine, based on what other parts of the sample look like in reflected light.

 

[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.