The Secrets of Prof. Verschure's Rosetta Stones

[Andy Resnick]

TL;DR Summary

petrology

(Edit: since the thread title was changed, this first sentence is too cryptic: the original title referred to a Tool song....)

Besides being a favorite song by a favorite band, the thread title is a straightforward play on words. This summer, as a present to myself for being promoted, I purchased a collection of thin sections that I believe comprise the research materials of Prof. Rob Verschure, who at the time was faculty in the Geological Institute in Amsterdam.

What changed this purchase from eccentric (although, at $2 per sample, also very affordable) to something more elevated is that Prof. Verschure published his findings on many of these samples, primarily in:

https://www.ngu.no/FileArchive/NGUPublikasjoner/NGUnr_380_Bulletin_70_Verschure_35_49.pdf

and

https://research.vu.nl/en/publicati...-parameter-for-metasomatism-and-its-applicati

i.e. I have a "Rosetta stone" for explosion breccias, carbonatites, and damtjernites. Many of the collected samples have been fully characterized, for example this thin section:

Sample Hor 1 has been classified as a calcite-bearing clinopyroxene-hornblende lamprophyre that has been dated to 313 Ma. This sample contains abundant augite and brown hornblende.

This sample (Fen 23) consists of zoned augite (I think...) and carbonates, dated to 594 Ma:

Many of the samples are carbonatites, but there are also many examples of schists and gneisses; here's a schist with pronounced folding:

I'm hoping to learn quite a bit as I get more familiar with the variety of samples...

 

[Andy Resnick]

I realized that while the microstructure of these rocks are interesting, the mesoscopic structure is often more photogenic- here are some samples photographed at 1:1 using a 55mm Micro Nikkor:

The other advantage is that I have the label in the image so I can more easily keep track of what I want to image at higher magnifications....

 

[Andy Resnick]

Here's my first 'official' presentation of one of the samples:

This is an example of a "Tveitan carbonatized damtjernite-like explosion breccia". It was obtained from UTM coordinates ⁵356-⁶⁵419, is 3.71% Potassium, 0.0888 ppm Wt of radiogenic ⁴⁰Ar (5% of which is 'atmospheric' ⁴⁰Ar) 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"

The primary feature (I would call it a 'phenotype') I have come to associate with damtjernite are the rounded masses ("pelletal lapilli"). Here are a couple of higher-magnification views of this feature (crossed polars):

and, from a different sample:

The origin of these has (apparently) been a longstanding mystery- I found this paper claiming to 'understand' them as 'fluidised spray granulation':

https://www.nature.com/articles/ncomms1842

definitely not intro geology!

 

[Andy Resnick]

Next up, I was able to reproduce a few images appearing in:

Verschure, R. H. & Maijer, C. 2005: A new Rb-Sr isotopic parameter for metasomatism, ∆t, and its application in a study of pluri-fenitized gneisses around the Fen Ring Complex, South Norway. Norges geologiske undersøkelse Bulletin 445, 45–71.

Here's a piece of the published image (this is an open-source paper, btw.)

and mine (I have 3 of the 4 above samples):

Here's the relevant part of the figure caption:

"Fig. 10. Photomicrographs of fenitized gneisses. […].

(c) Ma 88: Biotite with reaction rim of massive Na-pyroxenes and alkali feldspar due to Fenitization-1. The rim was formed where biotite was in contact with quartz (clear) and not where biotite was in contact with feldspar (turbid). Other biotites within the same sample (not visible on this photomicrograph) appear stronger or even completely replaced. Location of the sample: Small quarry for road material near Steinsrud on road Holla-Steinsrud, Økonomisk Kart Foreløpig Utgave 1971 (BV 030-5-4) coordinates: 51502- 656890.1.

(d) Fen 125: Biotite replaced during Fenitization-2 by radiating needles of bluish Na-amphibole, subsequently partially replaced by fine- grained magnetite and hematite. The replacement occurred where biotite was in contact with quartz. Other biotites within the same sample appear unaffected or completely replaced. The sample was taken 1750 m south of the Fen Complex, far outside the zone of Fenitization–1. Location of the sample: HSP post M00059, Økonomisk Kart Foreløpig Utgave 1971 (BW 020-5-1) coordinates: 138965-51753. […]

(f ) Fen 33’’’: Biotite with a rim of a massive aggregate of blue Na-amphibole pseudomorphing Na-pyroxenes. Stilpnomelane (brown radiating flakes) occurs at the edge of the blue Na-amphibole aggregates. Location of the sample: Håtveittjørn Section 375 m from the contact with the Fen Complex. "

For me, this kind of information is really useful- not for the technical detail- I don't yet have enough knowledge to fully appreciate them- but rather (1) I learn how a geologist looks at samples, and (2) for the experience observing how color is used as a identification tool- I can't rely on color names to identify the mineral. For example, now when I read "bluish Na- amphibole", I understand what that color looks like to my color-deficient eyes. That holds especially for green/brown minerals.

Also, "stilpnomelane" :) .

 

[Andy Resnick]

This sample, "TVE 15", was obtained from UTM coordinates ⁵356-⁶⁵419 and is classified as a "sheared gneiss":

(All images are taken with crossed polarizers). This sample is one of a group of "crustal xenoliths from the carbonatized damtjernite-like explosion breccia near Tveitan, Bamble region". This particular sample is considered "country rock", rock that was already present during the explosion event and is dated to 1.4 Ga based on ⁸⁷Sr/⁸⁶Sr measurements.

It's clear there is something unusual about this sample- notice the elongated bright 'streaks' throughout the rock. These are sheared/strained quartz grains:

Note the presence of 'slip planes', which are visualized as undulose extinction. These generally lie parallel to the strain direction. The major components of this sample are quartz, K-feldspar (perthite) and plagioclase. Much of the plagioclase feldspar has undergone either sericitization or albitization. Minor amounts of biotite are also present (for example, there's a small biotite crystal located below the center of the second image).

I tried to calculate the pressure required to deform the quartz crystals, assuming the crystal started as a cuboid and was deformed via shear, but I got numbers that are way to large- about 10⁶atm. Clearly, slip planes reduce the yield stress and there are other considerations I am yet unaware of.

In any case, I started thinking about grain deformation/recrystallization dynamics and wondered about feldspar grains- quartz always has clearly demarcated grain boundaries, but feldspar does not seem to. For example, look at the (microcline) feldspar grains located just below the smaller deformed quartz crystal, left of center:

There are two bright-ish 'arcs' that separate the grains, and I have seen this particular pattern repeated in other samples. A closer look shows that the boundaries between feldspar crystallites have a much different appearance than quartz:

In contrast, neighboring quartz grains are (almost) always separated by a dark line:

The last image is of multiple microcline crystallites with a large feldspar grain that has undergone sericitic alteration, also showing symplectic alteration (symplectite):

Does anyone know of a good reference that discusses various grain deformation/recrystallization processes? The next sample I want to show has me thoroughly confused....

 

[Andy Resnick]

Today's sample is "Fen 59", mostly because I get to use a bunch of weird words....

This carbonatite sample was obtained from the Håtveittjørn section of the Fen complex, an inferred distance of -230m from contact with the country rocks... not sure how to interpret a negative distance. As I mentioned, this sample was a little confusing. At 1:1 zoom of the above image, it's clear that most of the material is a carbonate (for example, calcite and dolomite), but there are also what appear to be numerous subhedral and anhedral oikocrysts enclosing (most likely) anhedral carbonate chadacrysts:

What little I have read about pokilitic textures typically state that chadacrysts are euhedral, so one confusion that I still have is understanding how these formed. The other challenge was identifying the oikocryst mineral: neither quartz nor feldspar nor pyroxene nor garnet nor leucite. here are PPL (parallel polarizers) and XPL (perpendicular polarizers) views using a 4x objective:

In the PPL image, I decentered the illumination to improve the relief. The carbonate is grey and the oikocrysts are clear and in higher relief. It took some effort, but I finally identified the oikocrysts as apatite- the appearance of apatite in thin section for every kind of rock *other* than a carbonatite is "small crystals, hard to detect". Note also the carbonate vein and the small green/brown (remember- I don't do colors very well) crystals.

A closer look at some of those smaller crystals (surrounded by carbonate) in PPL and XPL is next. Make note of the birefringence of the crystals, the crystal on the left edge that is clear, and the corona of very small crystallites throughout:

So now we go to the relevant paper. The composition of this rock was reported as 60% carbonate, 15% biotite, 20% apatite (confirmation!), 1% quartz, 4% chlorite, and accessories of opaques, rutile, allanite, stilpnomelane, and dispersed carbonate. Since the crystals above are clearly not biotite, there's more to the story: "Fenitization"- an alkali metasomatic event.

In the "Remarks" data column, this rock is characterized as "M_F2carbonatite; Bt ⇒_F2Chl, _F2Qtz,_F2Carb rims", and now I can understand what I am looking at: a carbonatite, moderately Fenitized twice.

The original biotite, which appeared 583 Ma ago during the volcanic event, was subsequently altered first by a metasomatic Fenitization (F1) event that broke down the biotite, and then altered again (Fenitization-2), a late hydrothermal process replacing the F1 minerals, producing new fine-grained crystals of chlorite, quartz, and carbonates. And indeed, in the images it can be seen that the biotite crystals were replaced with chlorite (preserving the micaceous habit) and quartz, and also contain a reaction rim consisting of small carbonate crystallites.

Phew! That's a lot. Just getting warmed up, tho....

 

[Andy Resnick]

This, according to the paper, is a sample of 'melteigite', obtained from the Mjølteig A section of the Fen complex, -114 meters from country rock:

One thing I am learning is that the names of these 'exotic' rocks are often in flux- another name for melteigite is 'nephelinolite'. Melteigite comprises one end of the ijolite series, a type of plutonic rock consisting primarily of cliopyroxene and nepheline. Melteigite is at or near the melanocratic end, with the lowest fraction of nepheline while urtite, the most leucocratic, has the highest fraction of nepheline.

This sample is 'officially' described as a "strongly fenitized (F2) meltegeite, with coarse-grained zonal aergirine with titanaugite coronas". Without this, I would have absolutely no way to make sense of the images. For example, here's a pair (PP and XP) using a 4X objective:

I can maybe recognize a zoned-aergirine crystal in the center and (probably) a Ti-Aug crystal in the lower right. Other than that....???

Here's an image pair taken with a 16X objective from a different location:

I'm slightly more comfortable making mineral identifications here- for example, the central (zoned) aergirine crystal is partially surrounded by chlorite. I can maybe see a partial corona of crystallites, but I can't identify them. Perhaps surprisingly, there are virtually no opaques in this sample.

According to the paper, this sample has 63% aergirine, 3% each of biotite and melanite, 2% each of titanite, apatite, and carbonate. The F2 process created 9% each of chlorite and sericite, I'm guessing by altering the nepheline since I couldn't find any nepheline crystals in the sample. Carbonates make up the the remainder.

I can't find much information about ijolites (or melteigite or urtite). On one hand, I found "Ijolite is a rare rock type of considerable importance from a mineralogical and petrological standpoint.", but no references to back this statement up. By contrast, I also found "Ijolites are common alkaline rocks composed predominantly of nepheline (30–70 modal %) and clinopyroxene (~ 40 modal %), mainly diopside and aegirine-augite." So.... ?

In any case, it never fails that when I read the word 'foid', I giggle.

 

[WiredBuilder]

I'm getting the hang of identifying minerals, like those central aergirine crystals surrounded by chlorite. It's kinda cool that there aren't many opaques in this sample.

Now, about the composition, it's like 63% aergirine, with 3% each of biotite and melanite, and 2% each of titanite, apatite, and carbonate. That F2 process threw in 9% each of chlorite and sericite, probably by messing with the nepheline since I couldn't spot any nepheline crystals. The rest? Well, carbonates take up the slack.

I've been digging around for info on ijolites, melteigite, or urtite, but it's been a mixed bag. Some say ijolite's this rare deal with mineralogical clout, while others call it common alkaline rock mostly made up of nepheline and clinopyroxene.

 

[Andy Resnick]

Lately I've turned my attention to microcline, a form of feldspar, because I'm fascinated by the iconic 'tartan plaid' twinning pattern.

My impression is that the crystallization kinetics of feldspar is quite complex, resulting in a range of different crystal forms (monoclinic, triclinic) depending on temperature. I don't yet understand the details of albite/pericline twinning, but the twinning pattern seems (to me) a type of phase transition similar to spinoidal decomposition - a long-wavelength crystallization process (in contrast to the short-wavelength nucleation process).

One way microcline is altered (hydrothermally, for example) results in spontaneous unmixing of the two components (Na-feldspar and K-feldspar), forming two minerals called 'perthite' and 'antiperthite', depending on which component is the host mineral. Here's an example of perthite:

The thin lamellae are sodic feldspar residing within a larger K-feldspar crystal. The larger blobs are either (I think) orthoclase (center and bottom center) or sericite (right edge, bottom right), another type of feldspar alteration product.

It's possible that a third type of alteration results in "chessboard albite"- this is (I think) an example:

Maybe that would look better with crossed polars :)

Feldspare and quartz can also spontaneously unmix via spinoidal decomposition, forming myrmekite (a symplectite):

I'll end this post with a mystery:

Three microcline and a sericite-altered feldspar crystals surround a central crystal that may seem to be microcline, but notice the twinning does not follow the albite-pericline twinning laws. albite-pericline results in twinning patterns oriented at (nearly) 90 degrees, but the central one shows twinning at 120 degrees, so I don't (yet) know what this mineral is.

Suggestions are always welcome :)

 

[Andy Resnick]

This is a sample of damtjernite from the type locality Damtjern:

This is a complex rock:
"Damtjernites: are feldspathoid- and/or alkali feldspar bearing [ultramafic lamprophyre], characterized by olivine, phlogopite and clinopyroxene macrocrysts and/or phenocrysts, and groundmass phlogopite/biotite, clinopyroxene, spinel, ilmenite, rutile, perovskite, Ti-rich garnet, titanite, apatite and primary carbonate, with essential minor nepheline and/or alkali feldspar." (https://www.alexstrekeisen.it/english/vulc/aillikite.php)

Is that all? :)

The rounded features (ocellar/pelletal lapilli structure) are what I refer to as a 'phenotype' (characteristic) of this rock, and shows that it was formed in a manner similar to (for example) kimberlites- both are hypabyssal facies. At higher magnification we can see these rounded structures more clearly:

While it's possible to discern crystals of apatite (upper and lower right), carbonate (left center, within an ocellus (?), possibly phlogopite/biotite (top center), the enormous amount of opaques really obscures everything. Here's another example taken with a 16x objective, the field of view is about 0.5mm on a side:

The third image uses epi-darkfield illumination, which I tried out of desperation and am quite pleased with the result. Although biotite is supposed to be present in this rock, I think all of the biotite has been altered. The red iron oxide stands out, even to me :). You can also see a highly reflective grain on the left side, probably a metallic grain.

Here's a small crystal of nepheline and 'biotite':

The groundmass is really inhomogeneous (1:1 crops) and full of tiny crystallites:

 

[Andy Resnick]

This is another lamprophyre from the Horte region:

In many ways, it is similar to the sample Hor 1 shown above in post #1. Hor 2 is also presumably dated to 313 Ma, and displays many zoned clinopyroxene (augite) phenocrysts:

The next pair of images show a small clot of augite showing twinning. If you look carefully, you can see the twinning even in parallel-polarized light:

I found this interesting feature in one of the phenocrysts:

It's not a surface feature (scratch/dig), but buried within the section, I assume some sort of fracture pattern.

Also visible in the sample, several crystallites that have completely altered to sericine:

Technically, speaking, petrographic information about this sample has not been published (Hor 1 has, tho). However, the minerals present (as best I can identify) mirror the composition of Hor 1. In any case, the paper states: "Locally, euhedral sericitic aggregates are observed (allegedly pseudomorphs after nepheline)."

The next sample(s) I'll post are also igneous rocks containing altered nepheline- phonolites (tinguaites). I have a series of about 10 of these, and none of them have any published information, so I'm still trying to figure out some of the minerals.

 

[Andy Resnick]

As I mentioned above, the set of Fen samples contain about 10 examples of phonolites. I spent some time learning about the QAPF diagram, and can now classify these as phonolites/tephritic phonolites/foid-bearing trachytes with some confidence. Here are 3 samples of a phonolite:

You can see, maybe not that well since the image is so small, two primary types of phenocrysts: feldspars with Carlsbad or Braveno twinning and nepheline that has been altered to muscovite and (I think) a zeolite. In addition, there are some pyroxene phenocrysts. The groundmass consists mostly of plagioclase and aegirine.

These rocks don't occur in the Fen complex, but rather as 'dikes and spatters' outside the area. A good reference paper (with a map) is here: https://njg.geologi.no/images/NJG_articles/NGT_59_2_115-124.pdf, this rock used to be called 'tinguaite'.

Something interesting about these rocks (and also the tephritic phonolite samples) is the spatial arrangement of aegirine needles- the claim is that the crystals form a 'flow pattern' illustrating the magmatic flow. However, I don't know of any evidence for this- the needles just seem to have arranged themselves as if oriented by flow.

Higher magnification views of the altered nepheline and groundmass:

I think the low-birefringence "bubbly stuff' is a zeolite. I also found a small phenocryst of (I think) amphibole and magnetite, the highly reflecting bits:

 

[Andy Resnick]

This sample is an igneous breccia, because I found something that I can discuss in class (Optics of Materials) next semester:

Most of the phenocrysts/xenocrysts are feldspar, I'm not sure if the right hand side is the chilled margin with country rock or if it's a mega xenocryst. In any case, I found something interesting- a small inclusion within a feldspar crystal:

These are 1:1 crops taken using a 16x objective. Here's the inclusion taken with epi-darkfield:

I'm not sure what the inclusion is, but I don't think it's an opaque. If you look closely, you can see radiating lines, these are (I think) slip planes in the feldspar crystal. The reason I think that is because of the view with crossed polars:

What you are seeing is the "photoelastic effect", also called stress birefringence. This is caused by the relationship between the (tensor-valued) crystal strain and the (tensor-valued) refractive index. The inclusion is (somehow) exerting stress onto the feldspar crystal, straining the feldspar and creating the optical effect.

This same phenomenon is also used in acousto-optic components, which are devices that use the photoelastic effect (usually with ultrasound and Tellurium dioxide,TeO₂) to modulate either the propagation direction or the frequency of light.

I'm not sure what the origin of the stress is here, but the halo around the inclusion (visible in PP) could indicate a particular mechanism. Photoelasticity is complex to model but easy to demonstrate- usually molded plastic things: forks, protractors, etc. Next semester I can also show this!

 

[Andy Resnick]

This post will tell a (hopefully nonfictional, rather than fictional) interesting story that starts with some observations of carbonatite samples. I’ve already posted images of a few of these interesting rocks, here are 2 more examples:

Carbonatites are mantle-derived igneous rocks. Carbonatite petrogenesis is still actively researched and current interest in carbonatites rests on mining and extraction enterprises: carbonatites are enriched in Nb and rare earth elements.

Since time immemorable, if I showed you a rock primarily made of either calcite or dolomite (or a mix of the two), you called it ‘marble’. AFAIK, the first people to claim that they found a “limestone rock of magmatic origin” was Högbom in 1895 and Brøgger in 1921, who published his findings regarding the petrology at the Fen Complex. Their claims were disputed for decades until the 1960 eruption of Ol Doinyo Lengai in Tanzania, confirming the igneous origin of carbonatites.

Being curious, I wondered what Brøgger presented as evidence- and I couldn’t find anything other than language like “carbonatite appears in intrusion-like formations usually associated with magmatic activity”…. pretty weak evidence. To be fair, the report is in German, of which I am not fluent. But I really couldn’t find any published evidence to support published claims of magmatic origin (before 1960 or so).

So, I wondered ‘what kind of evidence would demonstrate an igneous origin?’ I wondered if any accessory minerals (mostly apatite, but some clinopyroxene and mica is also present) could be considered evidence. In my (limited) readings, I recall that of all the Fen rock samples that I have, the only ones that are definitively magmatic in origin are the damtjerites and phonolites- I’ve posted examples of those earlier. Damtjerite has characteristic ‘ocelli’ or “pelletal lapilli”, ovoid-shaped features, and these appear in several of the Fen samples. Looking carefully at Fen 3 ii, I found one along the bottom edge:

A closer view with epi-darkifield illumination:

Zooming in further to the reaction rim surrounding (I think) biotite (augite? unclear.) shows some of the crystallites:

The ovoid structure would not have been preserved during a metamorphic process. Therefore, the carbonatite and ovoid formed at the same time, so the carbonatite is of magmatic origin. Being largely ignorant of petrology, this seems to me to be strong evidence.

An earlier post described the trouble I had identifying apatite, because most descriptions of apatite in thin section go like this: “Apatite is obnoxious in thin section. It often plucks out in grinding, leaving a little hexagonal “apatite wuz here” hole in your section.” Not the case with carbonatites- apatite is a major accessory, typically forming anhedral crystals (which is something I still don’t understand- how can a crystal be anhedral?).

That got me thinking “What is different about apatite in carbonatite compared to apatite in other rocks?” What if the apatite crystal was not wetted by silicate melt, and so is not tightly bound to the rest of the rock; grinding could indeed pluck out the weakly-attached apatite crystal. But if apatite is wetted by the carbonate melt, it would remain in place after grinding.

That implies that the carbonatite melt and silicate magma are also immiscible. I then wondered if I can find evidence of that in a sample? Sure enough, looking closely at Fen 1 b (image reposted here for continuity):

Reveals, at the light-dark boundary- first the XPL transmitted, then the epi-darkfield reflected:

In transmitted light, it’s hard to make sense of what we see- two different microcrystalline (cumulate?) masses. The more I examine this sample, the more interesting it becomes… [continued on next post]

 

[Andy Resnick]

Closer inspection of the boundary does indeed show features that would be present if the two fluids were immiscible:

Remember, the light colored phase is opaque in transmitted light, while the dark colored phase is translucent. I think it's also important to point out that the phases are not glassy; then the sample would be uniformly dark in cross-polarized transmitted light.

Elsewhere in the sample, at higher magnification (40X):

The tiny specks are microcrystallites of an opaque; most likely magnetite or hematite but pyrite is a possibility. The larger yellowish flakes in the center are larger crystals of the opaque, and the very large opaques at the top center of the sample (see previous post) are crystals as well- images of those will have to wait for a post dedicated to this really unique sample.

Going back to the literature, my idea of immiscibility is not absurd- indeed, this is one of the current carbonatite petrogenesis theories. That gives me some confidence that I am (slowly) learning the subject :)

 

[Andy Resnick]

It's been a while since I've added to this thread because I've been struggling to better understand the mineralogy/petrology of the Fen region. For example, fenitization is a highly variable metamorphic process associated with carbonatites. The learning process has really opened my eyes to the scientific content of my samples- not just the minerals present, but also the petrogenesis.

A few websites have been particularly useful for me during this learning process:

http://www.alexstrekeisen.it/english/index.php
https://www.rockptx.com/

I'm sure there are others as well. The basic "problem" is that nearly all the samples I have are very uncommon and missing from online collections. For example, I have found no online examples of carbonitization- a contact metasomatic process associated with carbonatites involving CO₂ and H₂O interactions with country rock. Here are two examples (out of a dozen or so that I have):

When Brogger described the Fen petrology, he named about a dozen based on the type locality: "a legion of obscure rock types named after equally obscure European villages" (from http://www.alexstrekeisen.it/english/vulc/lamprophyres.php). These include ringite, vipetoite, hollaite, fenite, sovite, rauhaugite, meltegite, and damtjernite. While these names were still in use in the 1960s (according to the references I have), almost all been renamed.

Two rock types that kept their names are fenite and damtjernite. Here's a fenite:

(the blue circle was already there and is marking pen on the coverslip)

and here's another sample of damtjernite:

These 4 examples all quite complex rocks, and I expect to spend time exploring these (and other examples) at higher magnifications. Stay tuned!!

 

[Andy Resnick]

This week, my understanding of petrology has been "2 steps forward, 1 step back"... I'll explain shortly.

Here's an example of fenite, one of about 4 I have:

I'm not exactly clear about this, but these rocks seem to be described in terms of a gradation starting from "Telemark gneiss" through "fenitized gneiss" to "fenite". I have another dozen or so examples of fenitized gneiss that have greater or lesser amounts of a characteristic texture, here from the above sample:

What you predominantly see are individual grains of (micro)perthite rimmed by albite, and this texture seems characteristic of high-temperature fenitization. At higher magnification, I'll show some perthite grains and an example, in the center, of 'chessboard albite':

Now for the step backwards. Perviously, when I switched from crossed to parallel polarizers, what I actually did is remove the illumination-side polarizer.... so it's not really 'parallel polarizers'. I suspect most published PP images I have seen are imaged the same way.

So, being an optics person, finally realized that I should be honest and use 2 parallel polarizers. Besides making pleochroism significantly easier to visualize, I am seeing minerals exhibit pleochroism that should not... leaving me more confused that I was before.

Here's an example of what I mean, using a tourmaline-bearing pegmatite from the Crabtree Emerald mine in North Carolina:

This is imaged using XP, and you can see the large tourmaline crystals (the sample was cut transverse to the crystals) embedded in what appears to be plagioclase crystals. Tourmaline has high pleochrosim, and to show that I'll present 2 PP images, between which the sample was rotated about 45 degrees:

The tourmaline crystal (likely schorl) shows high pleochroism as expected. Unfortunately, so does the plagioclase! I imaged the sample using XP, again with 2 sample orientations:

Imaging this way, the sample behaves 'normally'.

So, I'm confused. Any ideas?

 

[Andy Resnick]

This sample (in center) is Harzburgite and consists of nearly 50/50 olivine and orthopyroxene:

There are a few interesting features to take note of. First, several of the olivine crystals show undulose extinction. Second, this rock has undergone partial serpentization, giving and excellent clue to help identify the minerals. This can be seen below, in a location where serpentine veins run through both olivine and pyroxene grains (PP and XP):

The central crystal (green? brown?) is olivine (Forsterite) while the orthopyroxene (Enstatite) is in extinction. Notice how the serpentization reaction proceeds differently depending on the reactant mineral, as seen at higher magnification below, in PP and XP: Enstatite is on the right, Forsterite on the left.

At even higher magnification, we can observe possible evidence of exsolution lamella in the pyroxene (likely Enstatite-ferrosilite):

 

[DrDu]

This sample is an igneous breccia, because I found something that I can discuss in class (Optics of Materials) next semester:

View attachment 335924

Most of the phenocrysts/xenocrysts are feldspar, I'm not sure if the right hand side is the chilled margin with country rock or if it's a mega xenocryst. In any case, I found something interesting- a small inclusion within a feldspar crystal:

View attachment 335926

These are 1:1 crops taken using a 16x objective. Here's the inclusion taken with epi-darkfield:

View attachment 335927

I'm not sure what the inclusion is, but I don't think it's an opaque. If you look closely, you can see radiating lines, these are (I think) slip planes in the feldspar crystal. The reason I think that is because of the view with crossed polars:

View attachment 335928

What you are seeing is the "photoelastic effect", also called stress birefringence. This is caused by the relationship between the (tensor-valued) crystal strain and the (tensor-valued) refractive index. The inclusion is (somehow) exerting stress onto the feldspar crystal, straining the feldspar and creating the optical effect.

This same phenomenon is also used in acousto-optic components, which are devices that use the photoelastic effect (usually with ultrasound and Tellurium dioxide,TeO₂) to modulate either the propagation direction or the frequency of light.

I'm not sure what the origin of the stress is here, but the halo around the inclusion (visible in PP) could indicate a particular mechanism. Photoelasticity is complex to model but easy to demonstrate- usually molded plastic things: forks, protractors, etc. Next semester I can also show this!

This inclusion looks interesting. Maybe it is (or was) something like a high density modification of a crystal, e.g. coesite, which transformed into quartz when pressure dropped. The increase of volume led then to the cracks in the feldspar.

 

[DrDu]

This week, my understanding of petrology has been "2 steps forward, 1 step back"... I'll explain shortly.

Here's an example of fenite, one of about 4 I have:
View attachment 338692

I'm not exactly clear about this, but these rocks seem to be described in terms of a gradation starting from "Telemark gneiss" through "fenitized gneiss" to "fenite". I have another dozen or so examples of fenitized gneiss that have greater or lesser amounts of a characteristic texture, here from the above sample:

View attachment 338693

What you predominantly see are individual grains of (micro)perthite rimmed by albite, and this texture seems characteristic of high-temperature fenitization. At higher magnification, I'll show some perthite grains and an example, in the center, of 'chessboard albite':

View attachment 338694

Now for the step backwards. Perviously, when I switched from crossed to parallel polarizers, what I actually did is remove the illumination-side polarizer.... so it's not really 'parallel polarizers'. I suspect most published PP images I have seen are imaged the same way.

So, being an optics person, finally realized that I should be honest and use 2 parallel polarizers. Besides making pleochroism significantly easier to visualize, I am seeing minerals exhibit pleochroism that should not... leaving me more confused that I was before.

Here's an example of what I mean, using a tourmaline-bearing pegmatite from the Crabtree Emerald mine in North Carolina:

View attachment 338695

This is imaged using XP, and you can see the large tourmaline crystals (the sample was cut transverse to the crystals) embedded in what appears to be plagioclase crystals. Tourmaline has high pleochrosim, and to show that I'll present 2 PP images, between which the sample was rotated about 45 degrees:

View attachment 338696

View attachment 338697

The tourmaline crystal (likely schorl) shows high pleochroism as expected. Unfortunately, so does the plagioclase! I imaged the sample using XP, again with 2 sample orientations:

View attachment 338698

View attachment 338699

Imaging this way, the sample behaves 'normally'.

So, I'm confused. Any ideas?

With parallel polarizers you will allways see interference colours due to birefringence in addition to pleochroism. In fact, nowadays observation with parallel polarizers is not used anymore, or only for special reasons. For the observation of pleochroism, you should only use the polarizer, and remove the analyzer.
Here is a Michel Levy chart for both crossed and parallel polarizers:
https://www.quekett.org/wp-content/uploads/2023/06/michel-levy-chart-2023-polarised-700.jpg

 

[DrDu]

Concerning the biotite in your samples, I have similar Carbonatites and Phonolithes from the Kaiserstuhl volcanoe in Germany. The biotites in these stones are barium-rich phlogopites, which do not show strong pleochroism.

 

[Andy Resnick]

With parallel polarizers you will allways see interference colours due to birefringence in addition to pleochroism. In fact, nowadays observation with parallel polarizers is not used anymore, or only for special reasons. For the observation of pleochroism, you should only use the polarizer, and remove the analyzer.
Here is a Michel Levy chart for both crossed and parallel polarizers:
https://www.quekett.org/wp-content/uploads/2023/06/michel-levy-chart-2023-polarised-700.jpg

Thanks! I wondered if that was the case, so I computed the Jones matrices for both PP and XP configurations. They give different results but I confess I don't fully understand what the results actually mean. In practice, using PP sometimes works as per "normal" (no analyzer) and sometimes gives anomalous colors (burnt-orange) during sample rotation.

In terms of photography tho, using PP + 1/4 λ plate ("glimmer plate") often gives really psychedelic results, often more interesting than XP + 1λ plate.

 

[DrDu]

It is relatively easy: Have a look at the two colour strips at the bottom of the graphic
https://www.quekett.org/wp-content/uploads/2023/06/michel-levy-chart-2023-polarised-700.jpg
The upper one are the interference colours you observe with crossed polarizers, the lower ones the colours you observe with parallel polarizers. If birefringence is very low, the mineral will appear white between parallel polarisators, or show its natural color / pleochroism. For more birefringent crystals this is no longer the case and they will exhibit strong interference colours.

 

[Andy Resnick]

I'll change things up a little bit; this sample is from the Gardnos impact crater, a highly eroded 5km wide crater that formed when a 300m stony meteorite deposited 10¹⁹ J of energy into the earth about half-a-billion years ago:

The site was first thought to be a volcanic explosion breccia. This particular sample is a "lithic impact breccia". The fragments of this sample are primarily quartz, with some feldspar mixed in:

The darker groundmass is primarily Carbon (rather than Iron), and considered to occur from the vaporization (!!!) of an overlying shale. Notice how the fragments are sharp-edged, evidence for this sample being an impactite rather than of volcanic origin.

All the biotite crystals are full of kink bands:

The small colorful grain is likely hornblende. (A closer view also showing some amphibole needles):

I'm surprised there was enough time for any crystals to form... In any case, kink bands in biotite are not considered definitive evidence of an impact event, given the mechanical properties of biotite.

However, many of the quartz and feldspar grains contain evidence of extremely high shock pressures (> 20 GPa) called "planar dislocation features" (PDF). Note, for comparison, the hydrostatic pressure level experienced by those unfortunate Titanic explorers was only 0.02 GPa. Here's some PDFs (and a few faint fractures) in quartz:

The bubbles presumably formed during the shock, so these are "decorated PDFs". In feldspar grains, the PDFs have a characteristic 'ladder' appearance:

The PDF structures are better visualized in epi-darkfield:

Violence was done here....

 

[Andy Resnick]

#thinsectionthursday :)

This sample is (I think) a garnet-clinopyroxene-hornblende metagabbro :

The large central dark area and large dark areas in the periphery are garnet. The sample has a granoblastic amoeboid texture; major minerals are porphyroclasts of clinopyroxene and hornblende with a minor amount of plagioclase. The rock has a cataclastic structure.

Opaques are likely rutile. Many of the clinopyroxene grains have what appear to be exsolution lamella, but it's very unclear (to me) what is happening here.

There are ocassional symplectic intergrowth of clinopyroxene and plagioclase:

A close-up of plagioclase twinning (possibly cordierite?)

Deformed plagioclase?

An extreme closeup, looks kinda biomorphic:

 

[Andy Resnick]

Next up is sample, Fen 20:

The left side of this rock is magmatic, while the right side is carbonatite. This rock could be an example of "Rødberg" (Hematite-calcite-dolomite carbonatite), but I'm not sure.

One the right side- equigranular subhedral grains of calcite:

containing massed cumulophyric grains of apatite within an aphanitic matrix:

and massed phenocrysts of muscovite pseudomorphed after (probably) nepheline displaying reaction rims/coronas (possibly calcite):

Calcite grains in contact with the magmatic component (left side) show curved cleavage surfaces, possibly due to shear stress or partial melting/solidification under shear?

The magmatic portion contains minute octahedral (magnetite or hematite) dispersed throughout:

 

[Andy Resnick]

#thinsectionthursday

This sample was taken at the chilled margin of a lamprophyric dyke near Hørte (UTM coordinates
⁵073-⁶⁵873). The dyke (left side) is classified as a fine-grained, inhomogeneous, melanocratic to mesocratic calcite bearing cpx-hbl lamprophyre, while the country rock (on the right) is likely a Telemark gneiss:

The country rock is highly altered, with much of the K-feldspar is sericitized:

Also present in this image are clots of sphene (above, on the left side) and biotite with iron oxide precipitated out (middle of bottom edge and left, next to the sphene). In other places, there are anhedral grains of calcite (pastel), K-feldspar (yellow and blue), and acicular crystals of amphibole (probably hornblende):

The lamprophyre itself is an ultrapotassic rock, very Si-undersaturated (45% Si), and is very colorful and photogenic:

Also present are a lot of zoned clinopyroxene and zoned amphibole grains- this one is amphibole:

The center (nucleus?) of this crystallite appears to contain an inclusion; in another Hørte lamprophyre sample I have most of the grains show this feature so I will re-visit this phenomenon another time.

The chilled margin is very interesting- first, the size of opaques (Iron oxides) decrease as the margin is approached. Below is a montage of images taken at decreasing distances from the margin:

Top left is about 2 cm from the margin, top right is 1 cm, bottom left is 0.5 cm and bottom right shows the actual margin along the left side. Interestingly, although the opaques change size (reflecting different cooling rates), the other crystallites do not have those same size changes- or at least, any size changes are much smaller compared to the opaques.

At the margin, there are very small crystallites of (I think) calcite:

And minuscule octahedral grains of the Iron oxide:

But possibly the most interesting feature (to me) is the presence of "something" embedded in the country rock a short distance from the margin. You can see these in the very first image to the right of the margin, a scattering of black blobs. At higher magnification, these look like (PP):

Some features can be readily seen- what appears to be ocelli (the rounded blobs- those are not defects in the thin section mounting media) and porphyritic fragments against an aphanitic groundmass. In actuality, this entire grain is glassy, as shown in XP:

Just to be clear, I pushed the exposure as much as possible to see something other than black. The exposure time for this image was 30s, to be compared with the PP image exposure time of 1/40 s. The bright streaks are from striae barely visible in PP.

So, what these 2 images tell me is that these blobs are a multi-component glass. In other locations, I can find bubbles-within-bubbles (not liquid, but glass bubbles)- unfortunately, I can only attach 10 images/post and I've run out.

Until next time!

 

[DrDu]

Nice slide! But as far as the bubbles are concerned, for me, these are simply air bubbles in the mounting medium.

 

[Andy Resnick]

Thanks! Those are indeed defects in the mounting material. However, the glassy objects I am really interested in are not defects. Here's some evidence, using epi-darkfield becasue the appearances are so different:

First, a mounting defect between the sample and coverslip:

Note the bright line (meniscus) and how the interior is out of focus due to the lack of mounting medium. If the defects are on the bottom side (between the sample and slide), this is what they look like:

In brightfield, you can more easily see how I am imaging through the rock to the defects which have a textured appearance and a dark meniscus:

And lastly, a air-in mounting medium bubble taken from vesiculated pumice when there is no rock to interfere with the bubble shape:

Spheres, as expected. But notice how the cavity shows high image contrast. Reminds me of Rover (from The Prisoner)...

Now for the glassy feature:

It's easier to interpret in brightfield:

What is shown here is a small ovoid partially coated with inclusions and containing an out-of-focus striae surrounded by another glassy region, also containing inclusions, that is itself surrounded by a shell of opaques. Based on the image contrast, the central ovoid is not full of gas, and I would expect that if it was full of fluid, the fluid would have diffused out by now.

Curiouser and curiouser...

 

[Andy Resnick]

It's taken a while, but I finally positively identified one of the samples as Rødberg:

Before the higher magnification images, I wanted to quickly point out that all of my 200+ samples originate from a very small geographical region covering only a square mile or so:

By contrast, here in Cleveland/northeast Ohio where the glaciers came through, there's nothing but sandstone and shale/slate forever. There's not even any "country rock", only glacial erratics.

I've been itching to find a sample of rødberg since I started learning about the local geology:

"Rødberg ("redrock") is a carbonate rock which is red-coloured due to finely dispersed hematite. Hematite concentrations (along N W- S E striking fissures) formed by iron pneumatolysis are exploitable as iron ore (Fen iron mines). Some of the dyke-like masses show high radioactivity and an average content of 0.2% ThO₂, and 1% rare earth elements (including Ce₂O₃) . Due to the fine-grained nature of the rock the mineral containing these elements is not known.) The carbonate is mainly calcite, and mostly fine-grained. The hematite occurs as poikilitic inclusions in the carbonate crystals as well as between the grains. In thin section the rock appears full of reddish dust particles. This rock is as yet only known from the Fen area. (Bergstøl, S., and S. Svinndal. "The carbonatite and per-alkaline rocks of the Fen area." Norges geologiske undersøkelse 208 (1960): 99-110.)

I haven't checked this sample with a geiger counter yet.... ?

Here are some magnified views (either XP or epi-darkfield): Starting of showing calcits, iron oxide, and aegirine needles:

I think this is pyrite, the iron oxides look more dark grey in reflection:

A lonely octahedron...

And last, a high-magnification image showing (I think) how the hematite literally stained the calcite, there's no crystalline texture at all:

 

[Andy Resnick]

This week's sample is a Tephritic Phonolite:

There are phenocrysts of (partially altered) sanidine, nepheline, and biotite with magnetite + pyroxene corrosion borders within a trachytic groundmass of acicular aegirine, platy feldspar and nepheline. Here are a few images highlighting the trachytic texture:

At higher magnification, the aegirine needles are more easily visible:

Figuring out the groundmass, to distinguish a phonolite from a trachyte, was difficult (for me) because the aegirine obscures everything. Crossed polarizers helped a little, but in the end using a full-wave plate and crossed polars more easily separated out the nepheline and feldspar by shifting the interference colors from low first-order to low second-order, shown below (reflected light, the PP, then XP + 1λ plate):

I should point out that while the appearance suggests flow, it is not clear that flow occurred during solidification. While I could find references that assert flow creates this texture, I could not find any evidence supporting that assertion. Maybe someone here can suggest a reference?

 

[DrDu]

I do not quite understand how you could differentiate the nepheline from Feldspar in the ground mass based on interference colours only.

 

[Andy Resnick]

I do not quite understand how you could differentiate the nepheline from Feldspar in the ground mass based on interference colours only.

Good point- I still struggle to distinguish between nepheline and orthoclase.

[Edit]:A better answer is that by shifting the retardance, I can more easily see grain boundaries and twin planes.

 

[Andy Resnick]

This week's sample is heterogeneous carbonatite breccia:

On the left, the carbonatite consists of subhedral grains of approximately equal size:

And the right half of the sample is a magmatic intrusion containing xenocrysts of feldspar (typically plagioclase) and phlogopite; the large object on the extreme right is too complex for me to describe easily. In between is a strip of carbonatite that melted and re-crystallized due to the magma intrusion. I know this because at the boundary, individual grains show evidence of partial melting:

There is an additional boundary layer between the re-crystallized carbonatite and breccia:

Many of the phlogopite grains show interesting deformation features:

Along the top of the sample, it's not too thin- instead, there is some sort of isotropic mineral in the groundmass, perhaps sodalite?

Another example:

There's also nepheline present:

At least, I think this is nepheline rather than orthoclase... the relief of nepheline is lower than feldspar in PP.

And the sample has a lot of opaques- mostly pyrite.

 

[Andy Resnick]

This sample is Tveitan (coordinates ⁵351- ⁶⁵422) country rock:

At this scale, it looks like an ordinary gneiss- texture: Fine-grained and medium grained, leucocratic. Major components: quartz (undulose exinction), K-feldspar (microcline and partially seritcitized orthoclase) and some plagioclase. There are minor amounts of anhedral sphene, opaques (likely magnetite/hematite), possibly chlorite, possibly diopside and possibly augite. Here are XP and epi-darkfield images of a sphene grain, some opaques, feldspar, bubbles...

On closer inspection, it's clear that a geologist selected this rock and made this sample because it has some really strange features. Fior example, this extended feature has a lot going on (PP, XP, epi-darkfield):

If I had to guess, I would say that the epi-darkfield image suggests this was originally a grain of mica (biotite?) that decomposed into.. something. The browninsh (greenish?) hues in the central grain as well as the highly undulose extinction lines are not associated with grain boundaries or twinning planes. Could it all be quartz? The second-order colored things are (I think) diopside. I have no clue, any ideas?

Now some fun stuff: myrmekite. There are multiple examples of wartlike myrmekite present in the vicinity of microcline grains, and across the sample, the maximum vermicule diameter is somewhat variable, possibly indicating calcium gradients:

You can see the myrmekite warts in a few places, surrounding the grain of (I think) microcline. Or is that cordierite?

As it happens, in most places the diopside also has a sympletic structure:

I found similar images here, which states "omphacite → kelyphitic mixture of diposide and albite." Sounds good to me! Taking photos of these was a lot of fun, all kinds of blobby saturated colors everywhere. :)

Lastly, there are a few grains like this:

My first thought was antiperthite (because of the microcline blebs?) but that's not what antiperthite is, so... any guesses?