Abstract

Clinohypersthene, Fe1.02 Mg0.88 Ca0.04Al0.05 (Si1.93 Al0.06)O6, space group P21/c, and hypersthene occur in two separate, partly glassy rocks a few feet apart in a buchite “chimney” associated with a burned coal seam. The clinohypersthene occurs as phenocrysts and is accompanied by plagioclase (An80), cordierite [Mg/(Mg + Fe2+) = 0.65], spinel (Mt62Uv26Hc22), and brown glass. The hypersthene has the same Wo content but is more magnesian [Mg/(Mg + Fe2+) = 0.50-0.52] than the nearby clinohypersthene [Mg/ (Mg + Fe2+) = 0.30-0.46]. The data suggest that the two-phase region marking the ortho-pyroxene-to-pigeonite transition in the system Ca0-Mg0-Fe0-Si02 intersects the enstatite-ferrosilite join. The clinohypersthene probably crystallized at a temperature above 1060°C.

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