Petrography

OK, I put these images here because I think they are abstract art. They are definitely cooler if you know exactly what you are seeing, but they are aesthetically pleasing even if you don't. They are all photomicrographs of rock thin sections, so the colors are due to optical interference in the crystals. All are fom rocks I collected in the field. Most are from doubly polished thin sections, so the images are nice and clear. I put captions with each so try to explain what you are seeing and why I think they are neat. The images are links so just click for a larger image.

   
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This is a sector-zoned augite (pyroxene) from a cumulate gabbro (rock) I collected in Greenland. It shows the cumulate nature of the rock. The crystal grew in a 'rain' of the small brightly colored olivine crystals and included several in the outer rim.
 
 
 

Coesite in eclogite. This is a grain of coesite in an eclogite I collected in South Africa in 1975. It was the first time that coesite had been described in a metamorphic rock (Smyth and Hatton, 1977). The description led to the recognition of coesite in many other localities around the world.  The colored grain at the center is a hydrous clinopyroxene. The gray is coesite, and the mottled gray rim is polycrystalline quartz.
 

Inverted Pigeonite.  This is an inverted pigeonite (pyroxene) from the Bushveld Complex in South Africa. The grain initially grew as a twinned monoclinic crystal. The two twins then exsolved calcic pyroxene on (0 0 1) which formed the brightly colored chevron pattern. On cooling the pigeonite (dark) host recrystallized to orthopyroxene epitaxially and then exsolved a second fine set of (1 0 0) lamellae which form the fine vertical streaks.
 

Ruby in Kyanite. These are large grains of kyanite (Al2SiO5) with inclusions of corundum (ruby). They are formed by the release of silica from the cllinopyroxene which reacts with the corundum to form kyanite.

Kyanite and garnet exsolution from clinopyroxene. This large gray grain of omphacite (clinopyroxene) has exsolved kyanite (blue) and garnet (black) indicating that the precursor pyroxene was rich in a hydrous component, HAlSi2O6. Pyroxenes in subducting eclogites are capable of carrying sufficient water (as hydroxyl in solid solution) to recycle the entire ocean over 4.5 billion years of geologic time.
 

Inverted majorite. This grain originally crystallized as a majoritic garnet in the Transition Zone at a depth of at least 500 km. When it was transported to shallower depths it decomposed into orthopyroxene, (gray), clinopyroxene (yellow to pink) and a silica poor garnet (black).  

Migrated Twin in Augite. This grain originally crystallized as an augite (pyroxene) grain twinned on (1 0 0 ). It then exsolved the pigeonite lamellae that form the chevron pattern. Sibsequent to exsolution, stress in the rock caused the twin boundary to migrate.