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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