Abstract

Electron microprobe and bulk chemical analyses of a spherulitic andesite indicate that notassium is concentrated in the spherulites, whereas sodium and magnesium are depleted. Calcium, silicon and titanium are concentrated, and aluminum and potassium are depleted in a l0μm-wide band of brittle mica(?) which is peripheral to the spherulites. The spherulites appear to consist of an open aggregate of 10μm-wide, radially-oriented plagioclase fibers with abundant potassium feldspar and minor biotite in the interfibrillar areas. The groundmass contains unoriented plagioclase fibers, some potassium feldspar, hornblende(P) and hematite. Both the groundmass and spherulite plagioclase have a composition of approximately An40 and have potassium-rich rims. This distribution of elements conforms to that predicted from observations of spherulite formation in polymer melts. The growth rate of the spherulites was approximately 10-6 cm/sec.

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