How Can Imaging Fossil Thin Sections Unlock Microscopic Wonders?

Andy Resnick · Apr 19, 2023

Sometimes I win at eBay:

Close-up photo of a black polarizing microscope fitting with a silver rotating dial scaled in degrees, a yellow label reading Pol, and a fine-adjustment micrometer knob on the left.

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:

Photomicrograph of a fossil thin section in plane light showing several cross-sections of foraminifera with concentric chambered walls against a dark matrix, and a bright white broken area along the bottom edge.

And a closer view at 4X:

Photomicrograph in plane polarised light of a fossil thin section: a coiled foraminifer with a dense ring of chambers and septa, surrounded by dark brown sediment matrix.

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:

Photomicrograph of a thin section in plane-polarised light showing dark circular bubbles or inclusions scattered across a pale grey mineral field, with a dark, textured fossil or rock region on the right.

Photomicrograph under crossed polarizers of a thin section, showing many small, square, brightly coloured birefringent crystals scattered on a dark background, with a brownish fossil or rock fragment at right.

continued....

Andy Resnick · Apr 19, 2023

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:

Photomicrograph under crossed polarized light of mineral grains with vivid interference colors in yellow, blue, magenta and orange against a dark background.

Photomicrograph under crossed polarizers of a mineral grain in thin section, showing vivid interference colors of blue, orange, pink and yellow with patchy extinction.

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:

Thin section photomicrograph under crossed polarized light of densely packed mineral grains in a sandstone, showing bright and dark interference colours in varied hues.

Photomicrograph in cross-polarised light of a sandstone thin section showing densely packed, angular mineral grains in bright interference colours of pink, yellow, blue, green and magenta against dark matrix.

Thin section photomicrograph under crossed polars showing densely packed mineral grains in vivid interference colours of pink, blue, yellow and green, with dark grain boundaries and cement between them.

Thin section photomicrograph under crossed polarized light showing densely packed mineral grains in varied interference colours, with dark extinction areas between them.

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

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

Baluncore · Apr 19, 2023

Andy Resnick said:

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 · May 12, 2023

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

$Photomicrograph of a thin section of harzburgite under crossed polarised light, showing vividly coloured mineral grains in magenta, blue, green and yellow separated by dark black fracture lines.$

Andy Resnick · May 12, 2023

Agate, crossed polarizers, 100mm Luminar:

Photomicrograph under crossed polarizers of agate, showing vivid interference colours across fibrous quartz crystals fanning out in rounded, dome-shaped bands.

Baluncore · May 12, 2023

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 · May 16, 2023

Andy Resnick said:

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 · May 16, 2023

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 · May 16, 2023

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 · May 16, 2023

DrDu said:

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':

Two side-by-side photos of a Leitz Ultraphot microscope set up for photomicrography, with a camera body mounted on top of the curved arm and a black illuminator above a stage carrying a numbered sample holder.

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 · May 16, 2023

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

Andy Resnick · May 17, 2023

DrDu said:

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 · May 17, 2023

DrDu said:

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 · Jun 14, 2023

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

Thin section photomicrograph under crossed polarised light showing a mosaic of brightly coloured mineral grains, with a central grain displaying parallel twin lamellae in orange and purple bands.

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

berkeman · Jun 15, 2023

Andy Resnick said:

originating from the Fen Complex,

Pretty cool place!

Andy Resnick · Jun 15, 2023

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

Photomicrograph of a thin rock section under cross-polarised light, showing a brown mineral grain with pale, branching veins and blue and purple interference-coloured crystals around it.

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

DrDu · Jun 20, 2023

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

Andy Resnick · Jun 20, 2023

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

Andy Resnick · Jun 20, 2023

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

Cross-polarized light photomicrograph of a thin section showing a serpentinized olivine crystal with blue and magenta interference colors, dark veins, and orange-yellow patches of altered mineral.

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

$Thin section photomicrograph under cross-polarized light showing an orange and yellow mineral grain with dark speckles and a dark fracture line, adjacent to magenta and blue-green crystal domains and a dark vein running diagonally across the lower half.$

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:

Cross-polarized thin-section micrograph of a rock with a blue elongated crystal domain containing three small pink diamond-shaped inclusions, bordered by pink-orange banded material with scattered black dots and dark cracks.

Enjoy!

Andy Resnick · Jul 17, 2023

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)

Photomicrograph of agate thin section under crossed polarized light, showing concentric banded rings of fibrous quartz with dark extinction bands and radiating crystal fibers.

Photomicrograph of agate thin section under crossed polarized light, showing vivid blue, orange and magenta interference colours in concentric banded and circular growth patterns with dark cracks.

Andy Resnick · Jul 17, 2023

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

Photomicrograph at 40X, no polarizers, of a yellowish-pink opal surface with fine scratches and a few dark specks, no clearly resolved spheres visible.

$Photomicrograph at high magnification of opal showing iridescent patches of red, green, blue and orange against a dark background, with fine diffraction-colored structures.$

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:

Photomicrograph at high magnification of opal showing a regular, grainy pattern of tiny closely packed spheres across the field, with a dark speck at left.

Andy Resnick · Aug 1, 2023

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

Photomicrograph under crossed polars of granophyre, showing intergrown quartz and alkali feldspar in a radiating, worm-like eutectic texture set among larger grains.

Photomicrograph in cross-polarised light of a granophyric intergrowth: worm-like and skeletal quartz patches set within a feldspar matrix, surrounded by angular mineral grains.

Thin section photomicrograph under crossed polars, false-coloured blue, purple and gold, showing a granophyric intergrowth of quartz and alkali feldspar in a rounded patch surrounded by other mineral grains.

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

DrDu · Aug 8, 2023

Nice myrmekitic intergrowths!

How Can Imaging Fossil Thin Sections Unlock Microscopic Wonders?

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