Abstract

Gas cavities in the interior of an andesite plug dome contain a suite of fifteen minerals. Zoned plagioclase, zoned orthopyroxene, and augite are the most abundant minerals; ilmenite, apatite, tridymite, and magnetite are common; acmite, cristobalite, hematite, hyalite, rutile, hornblende, pseudobrookite, and quartz are minor to rare. The minerals probably crystallized from a late, interstitial, residual magma which oozed into gas-formed cavities. An increase in the volatile phase, perhaps because of the crystallization of anhydrous minerals, controlled the final stages of crystallization in the cavities.

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