Imaging Fossil Thin Sections: A Journey into Microscopic Worlds

[Andy Resnick]

TL;DR Summary

Post your petrographic images!

Sometimes I win at eBay:

On the right is a Ehringhaus quartz compensator in perfect condition- I believe the seller didn't know what they had, I got it for $200. Now I need to learn how to use it :)

It seems that whatever I decide to learn how to photograph, @davenn has been there already. With his encouragement, I am getting some thin sections fabricated, hopefully I'll have them by this summer. In the meantime, I scored a set of fossil thin sections (at about $5 per section), my suspicion is that they were made by a (grad?) student as part of a course.

Here's a macro-view, taken with a zoom Luminar, of a sample from the Graford Formation (Upper Pennsylvanian) show some fossils in cross-section:

And a closer view at 4X:

One thing I don't understand are the presence of small flakes of "something", I think part of the sample prep- these are uncovered and non polished sections. The flakes are uniform size, square, and highly birefringent. Two views, one with parallel polarizers and the other with crossed polars:

continued....

 

[Andy Resnick]

The next sample I worked with is from the Dneister Formation (Devonian)- here's when I start to move beyond 'hey, cool pics!'. Here are a couple grains of something, in two different rotation orientations with respect to the crossed polars at 16X:

The colors of one grain changes, but not the other....

And using the compensator on a cemented sandstone sample from the Crigglestone Rock Formation (Carboniferous) gives the following sequence as the compensator retardation changes from 0 to (I think) +1λ, based on a rotation angle of 30 degrees:

Not sure what these minerals are... any guesses?

I tried to get some interference figures, but nothing was 'clean' enough.

 

[Baluncore]

One thing I don't understand are the presence of small flakes of "something", I think part of the sample prep- these are uncovered and non polished sections.

Maybe cubes of crystal grit, (embedded in the Canada balsam cement which has the same RI as the glass slide).

 

[Andy Resnick]

Harzburgite xenolith, from Dutoitspan, Kimberley RSA, crossed polars and 100mm Luminar:

 

[Andy Resnick]

Agate, crossed polarizers, 100mm Luminar:

 

[Baluncore]

I remember the 1970s, when petrology could always be bright and colourful, no need for hallucinogenics then. At the time, I felt the rocks knew more about me, than I did of them. Handheld XRF has ruined that game.

 

[DrDu]

The next sample I worked with is from the Dneister Formation (Devonian)- here's when I start to move beyond 'hey, cool pics!'. Here are a couple grains of something, in two different rotation orientations with respect to the crossed polars at 16X:Not sure what these minerals are... any guesses?

I tried to get some interference figures, but nothing was 'clean' enough.

As far as I understand it, these sections are fossiliferous. These sections are usually thicker than petrographic sections, which makes identification of the mineral content far more difficult.
You should try to determine the thickness of your sample. To this end, use a rather highly magnifying objective (40 or 50) and focus on the upper and lower side of your sample. The fine drive should be gauged in mum on your microscope. Take the difference and multiply by the approximate index of refraction of your sample (usually 1.5). You can then determine the maximal interference colours from a Michel Levy chart (https://www.researchgate.net/publication/242341440_extended_chart).
Are these slides covered? Petrographic analysis requires the slides to be grinded (but not polished) and covered, to be able to judge e.g. the "chagrin", which allows for estimation of the index of refraction.

 

[DrDu]

Andy, maybe you want to describe your setup more in detail. I would be very interested. These luminars are rather for macrophotography than for microscopy. How did you adapt them to your mic?

 

[DrDu]

Here some pictures:
1. Prehnite, Norway
2. Essexite, Kaiserstuhl Volcanoe, Germany
3. Nummulith, Bad Adelholzen, Germany
4. Nummulith, Glauconitic filling after treatment with HCl. Bad Adelholzen
5. Limburgite, Limberg, Germany
6. Chondrite, North Africa

I just realized that the photos are not always shown in the same order. So consider this to be a quiz!

 

[Andy Resnick]

Andy, maybe you want to describe your setup more in detail. I would be very interested. These luminars are rather for macrophotography than for microscopy. How did you adapt them to your mic?

Very lo-tech. I'm using my Ultraphot in 'macro mode'... there don't seem to be any pics I can grab online to show.... here's a couple of images using a 'dummy camera':

One polarizer (small, about 40mm diameter) sits below the illuminator, there's a shallow well that can accommodate it. The analyzer is a 6" x 6" sheet of polaroid, placed directly in front of the camera (no lens on the camera)- in the images above, you can see reflections off that polaroid sheet. I first orient the polars by rotating the lower polarizer until extinction is achieved, I can't really rotate the analyzer. Once they are perpendicular, I place the illuminator in position and rotate the sample until I get good colors.

 

[DrDu]

And the Ehringhaus? It seems to live in quite a complete pol mic, doesn't it?

 

[Andy Resnick]

Here some pictures:
1. Prehnite, Norway
2. Essexite, Kaiserstuhl Volcanoe, Germany
3. Nummulith, Bad Adelholzen, Germany
4. Nummulith, Glauconitic filling after treatment with HCl. Bad Adelholzen
5. Limburgite, Limberg, Germany
6. Chondrite, North Africa

I just realized that the photos are not always shown in the same order. So consider this to be a quiz!

Nice! What lens did you use for these?

 

[Andy Resnick]

And the Ehringhaus? It seems to live in quite a complete pol mic, doesn't it?

Yes, when I want to use the Ultraphot as a microscope, I switch out all the luminar/macro stuff and install 'normal' microscope stuff.

 

[Andy Resnick]

I received a bunch of thin sections originating from the Fen Complex, I believe this one is a carbonatite, specifically søvite:

I also found crystals showing cross-hatched twinning but couldn't get a decent image (for now).

 

[berkeman]

originating from the Fen Complex,

Pretty cool place!

 

[Andy Resnick]

Another image from a sample of søvite, it's pretty wild:

I think some of the other samples are of 'damtjernite', but I haven't gotten a compelling image yet.

 

[DrDu]

The cross-hatched mineral is probably microcline. In the left corner maybe some plagioclase.

 

[Andy Resnick]

I agree. In other locations in this sample, there are micaceous brucite crystals.... lots of cool stuff to look at!

 

[Andy Resnick]

Here's a few images of a partially serpentinized olivine phenocryst, from a different sample (not sure what rock type):

A closer view shows some of the reaction-diffusion kinetics resulting in a spatial distribution of magnetite crystals:

The black stripe long the bottom is a magnetite vein where (I think) the reaction initiated, and the upper corner shows some unaltered olivine. The small black dots are magnetite crystals. Here's another example:

Enjoy!

 

[Andy Resnick]

Some samples I sent out for thin sectioning came back- I'm really pleased with how they came out. Here's agate at 4x (crossed polars and then crossed polars + 1λ waveplate)

 

[Andy Resnick]

And here's opal, first at 4x: no polarizer, and then crossed polars:

The colors come from diffraction, not birefringence. Using a 1λ plate turns the field a uniform purple-red color.

To show how opal is a diffractive material, I imaged at 40X, no polarizers, to show the microstructural spheres- opal is a naturally ocurring photonic bandgap material:

 

[Andy Resnick]

These are examples of granophyre: the first two are through crossed polars, while the third adds a full-wave plate for color.

Granophyres are examples of eutectic mixtures; in this case alkali feldspar and quartz.

 

[DrDu]

Nice myrmekitic intergrowths!