Vitrophyre   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?		UNC sample BB-16
		Rock type Obsidian vitrophyre
		Locality Elloston, MT