Minerals & Miscroscopes

Just an aspiring geologist and her blog.

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Pyroxene with high iron content in Yellowstone Lava Creek Tuff. In hand sample, the rock appears densely welded, but under magnification, glass and pumice fragments show varying degrees of shear and deformation. This photo is a beautiful example of deformation of the glass around the more dense crystal. This grain is a bit of a mystery. The birefringence is a bit too high for pyroxene, but the cleavage and optics check out for augite. It’s not pleochroic but it is deeply colored. Finding this little beauty along with other characteristics of the rock have prompted us to do whole-rock geochemical data to check for high alkali content...maybe we have a sodic magma source down there under the Midwestern US?

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Opal /  Art by Ira Blick

https://www.tumblr.com/blog/irablick

oil on canvas

~Thanks for the gorgeous submission Ira~

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Favorite rock and mineral?

Favorite rock has gotta be a charnockite. I spent my first summer in the field hunting them down in the middle of the woods in Virginia. Favorite mineral is probably galena or hematite!

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Lava Creek Tuff: Yellowstone National Park

Densely welded silicic tuff from the 1,000 km3 eruption of Yellowstone Supervolcano at 640 (+/-) ka. The source rock that supplied these thin sections is a porphyritic rhyolite obsidian with marked perlite texture, a pattern of cracks that occurs from the expansion of hydrated volcanic glass. The spherulites (round things) shown here are post-depositional features that indicate high gas levels in the source magma. The spherulites exhibit encapsulated quartz and feldspar grains at the center, which is a common trait of the devitrification feature.

Photo 1: spherulite with crossed polars (10x magnification). Photo 2: spherulites in plane polarized light (4x magnification).

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Cape Point, South Africa. 

While in South Africa we stumbled upon this incredible view at the end of the peninsula, and while preparing this post I stumbled upon an interesting controversy in oceanography. Cape Point is lauded as the place where the eddies from the warm-water Agulhas current of the Indian Ocean meets the eddies from the cold water Benguela current of the Atlantic Ocean: in essence, a place where two oceans collide. However, Cape Agulhas residents also claim this title for their oceanic backyard. So which is it? Is Cape Point truly the place where two oceans meet or is Cape Agulhas? 

Cape Agulhas may lay claim to the geographic divide between the Indian and Atlantic Oceans, but the dynamic systems of ocean currents and their features means that the true division between water bodies is not stationary. Interestingly enough, both capes host the oceanic divide during different times of the year. The result of this is one of the most unique marine habitats in the world. 

[mossy-rox-blog]

Rounded, calcite ooids and elongate pelecypoda in dolomite matrix. Pelecypoda are a type of ancient clams that exhibit two calcareous layers and a chitinous layer in their shells. Both are common in marine carbonates such as this limestone.

I’m interpreting the cement as dolomite due to its blocky, and in some cases euhedral, shape. The euhedral grains are likely second order replacement of calcite at low temperature. The smaller, granular grains could be preserved original calcite.

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Can I have more thin sections please?

Coming up soon! :) 

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I’m an igneous and metamorphic petrology kind of gal, but I when I looked down the microscope at this limestone I audibly gasped. These nebula-looking things are protists called foraminifera that have been replaced by calcite. The forams are identified by their chambered tests, which are preserved in the rock, showing the protist’s original shape. 

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Pitted limestone structures near Cape Point, South Africa. The pitting is a result of erosion of the soft rocks by water and algae over thousands of years. 

We found this outcrop on the edge of a cliff above the beach on the southern west coast of the peninsula. Its presence above the beach could indicate marine regression; although at the time I did not have the wherewithal to hunt for nearby shale or sandstone for reference. My attention was - unfortunately - consumed by the pitting structures and fiddling with the lighting on my camera.

If you know more about these gorgeous structures, please send me a message.

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Thin section prep. Can't wait to show you guys some new minerals!

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“The Comet”

Oxidized euhedral pyroxene crystals in ashy groundmass of volcanic tuff. Explosive volcanic eruption likely caused the two pyroxene grains to smash together, causing this comet and tail-like shape. 

[earthstory]

Maybe my eyes are playing tricks on me but the angles in that crystal look an awful lot like 60 degree angles - I think that looks a lot like an amphibole.

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Oops, I think you’re right! I’ll edit accordingly.

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"Church Window"

Subhedral olivine grains altered to red iddingsite; surrounded by plagioclase grains and volcanic glass. Basaltic lava sample from Mount Bachelor Volcano.

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“The Comet”

Oxidized euhedral amphibole crystals in ashy groundmass of volcanic tuff. Explosive volcanic eruption likely caused the two grains to smash together, causing this comet and tail-like shape. 

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Rare photo of me outside of the lab. My partner and I are measuring stream velocity vs. depth while our male counterparts chill.

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"Pigeonite" pyroxene under 20x magnification. Pigeonite is named for the feathered texture of the crystal caused by exsolution, a reaction that alters the overall chemical composition of the grain. #crystals #rocks #geology #pyroxene

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Calcite crystal under 10x magnification. Calcite is identified by its high birefringence colors and perfect rhombohedral cleavage (x). However, under the microscope calcite is almost indistinguishable from dolomite, which also occurs in marbles such as this rock. #marble #geology #rocks #crystals #calcite

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Andalusite replaced by white mica in retrograde metamorphic reaction.