orthopyroxene

Biotite from lava flows is invariably rimmed or totally replaced by an exceedingly fine-grained, almost isotropic intergrowth of K-feldspar, orthopyroxene, cordierite and ilmenite, probably as a result of dehydration of the magma on eruption. Cordierite and orthopyroxene (5% each) occur as euhedral phenocrysts 1 to 2 mm in diameter, but cordierite can be up to 5 mm across and occur as anhedral ovoids.