Amygdaloidal texture. Kimberlite. UNC sample. H-64 Rock type. amygdaloidal basalt Locality. unknown

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1 Amygdaloidal texture H-64 amygdaloidal basalt unknown The oval feature in this photomicrograph is an amygdule: a formerly open vesicle which has been filled with a secondary mineral(s) precipitated from low-t ground waters which have penetrated into the rock. In this case, the amygdule is probably filled with a zeolite mineral. Kimberlite ARK-35 kimberlite Arkansas This is a severely altered sample from the only diamondproducing kimberlite in the United States. Sorry, no diamonds in this slide, though! The polygonal shapes are relict crystals of olivine which have completed decomposed to serpentine and other alteration products.

2 Volcanic breccia WYO-19 volcanic breccia Wyoming This breccia is composed of fragments of a variety of volcanic materials. Volcanic breccia WYO-19 volcanic breccia Wyoming This breccia is composed of fragments of a variety of volcanic materials.

3 Komatiite CAN-? komatiite Canada Komatiites are rare ultramafic volcanic rocks. This sample displays the characteristic "spinifex texture" defined by extremely acicular olivine phenocrysts--probably a sign of rapid crystallization from a significantly-undercooled magma. Nepheline basalt H-99 nepheline basalt unknown Many of the phenocrysts in this basalt are nepheline.

4 Rhyolite R-15 ("rhyolite" card" rhyolite unknown This crystal-rich rhyolite contains phenocrysts of quartz, K- feldspar (sanidine), plagioclase, and biotite in a fine-grained groundmass. Note the interesting shape of the (partially resorbed? skeletal?) quartz grain in the center of the photograph. Scoria HAW-? scoriaceous basalt Hawaii Scoria is another name for a highly-vesicular (almost "frothy") basalt. The black, ovals features in this photomicrograph are vesicles. Note the acicular, white plagioclase laths throughout and the euhedral, white olivine phenocryst at the lower right.

5 Spherulites in rhyolite WYO-14 rhyolite Wyoming Spherulites are radiating masses of fibrous crystals in a glassy matrix. These spherulites are probably composed of alkali feldspars and some polymorph of SiO2, and in this crosspolarized shot, appear as round objects with dark crosses. Note the large phenocryst which forms the nucleus of one of the spherulites at center-left. Spinifex texture CAN-? komatiite Canada This komatiite sample displays the characteristic "spinifex texture" defined by extremely acicular olivine phenocrysts--probably a sign of rapid crystallization from a significantly-undercooled magma. The texture is named after a type of grass which grows in South Africa in the region where komatiites were first discovered.

6 Lightly-compacted tuff NEV-8 rhyolitic tuff Nevada In this tuff, the irregularly-shaped glass shards are still relatively undeformed. Also note the phenocryst of quartz (clear) and biotite (dark red) in this slide. Contrast with poorly-welded tuff and welded tuff. Poorly-welded tuff NEV-9 rhyolitic tuff Nevada In this sample, the glass shards are starting to get deformed. Note the phenocrysts of quartz (clear) and biotite (dark red) in this rock. Contrast with lightly-compacted tuff and welded tuff.

7 Welded tuff NEV-11 rhyolitic tuff Nevada In this sample, the glass shards are fused together in a swirly mass, and the large pumice fragment at center right is flattened. In contrast, quartz phenocrysts are relatively undeformed. Contrast with lightly-compacted tuff and poorly-welded tuff. Vesicles HAW-? scoriaceous basalt Hawaii The black, ovals features in this scoriaceous basalt are vesicles. Note the acicular, white plagioclase laths throughout and the euhedral, white olivine phenocryst at the lower right. Want to see what happens to vesicles after some hydrothermal alteration?

8 Vitrophyre BB-16 Obsidian vitrophyre Elloston, MT A vitrophyre is another name for a phenocryst-bearing obsidian. The phenocrysts in the above photomicrograph are mostly plagioclase. The groundmass is obsidian glass. Can you think of some possible explanations to account for the extremely large difference in grain size in this rock?