Unlocking 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)