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EducationPOTD - rhyolite - polished section or ?

24th Apr 2024 05:51 UTCHerwig Pelckmans

See https://www.mindat.org/photo-945269.html

actual caption:

Highly silicified Rhyolite with white spherulites of finely intergrown radiating aggregates of quartz and K-feldspar and inclusions of various minerals in a dark glassy groundmass. Analysis with infrared spectroscopy has shown celadonite and bentonite/smectite being the main sources of green coloring. Angular cavities within the spherulites are in some cases filled with banded agate and druse quartz (“Thunder eggs”, unseen in this specimen), while some orbicular vesicles can be filled with common white opal. Known as ‘Rainforest Jasper’ on the market, this combination of colors and forms in Rhyolite is uncommon outside of this locality.

There is no doubt this is a most interesting rhyolite specimen, and the description is very educational.

It does leave me wondering: are we looking at a polished section of this rock, that was photographed using "reflected light", or is this a different photo of something else?

24th Apr 2024 06:03 UTCFrank K. Mazdab 🌟 Manager

It appears to be a polished block or slab. I would steer away from using "polished section" since that implies it's a polished thin section, which it isn't. I also wouldn't use "reflected light", which implies specifically reflected light petrography, which this also isn't (essentially every opaque object we view is illuminated by light "reflected" off the surface, so it would dilute the term to use it in that sense). It looks like a polished block or slab viewed under a basic picking stereoscope.

28th Apr 2024 21:35 UTCDr. Günter Grundmann 🌟 Expert

The wealth of detail on this rhyolite surface is a feast for the eyes.

This photo is a so-called "dispersed light dark-field image" of a polished thick section.

This photo type can be easily identified by zooming in on the details of the surface: traces of the polishing process can be seen along the shrinkage cracks in the form of differences in relief. There are also numerous traces of scratches on the surface caused by coarser abrasives.

29th Apr 2024 04:28 UTCFrank K. Mazdab 🌟 Manager

Hi Günter, I wouldn't call the illustrated sample a "polished thick section", because that term too has a specific meaning. "Thick sections" are still fundamentally thin sections that are purposely thicker than a standard petrographic thin section's 30 μm. "Thick sections" are typically 100 to 200 μm thick, and are usually made specifically for fluid inclusion studies or for use with laser ablation ICP-MS, where a regular thin section would be too thin (either too few captured inclusions for fluid inclusion work, or too easily drilled through for laser ablation). The pictured rhyolite is indeed an attractive sample, but it's not a "thick section" in the conventional usage... it's still just a polished block or polished slab.

29th Apr 2024 12:06 UTCDr. Günter Grundmann 🌟 Expert

Hi Frank, agreed! Thank you very much for your correction. I am improving my description as follows:

The photo (field of view 4 cm) shows details of a polished surface of a rock specimen of unknown size.

29th Apr 2024 03:10 UTCHerwig Pelckmans

Thanks, Frank and Günter, for your detailed replies. Highly appreciated!

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