Abstract

Snowflake texture consists of more or less randomly oriented laths of alkali feldspar poikilitically enclosed by small patches of optically continuous silica, commonly alpha-quartz. The texture results from devitrification of glassy, densely welded tuff but not, apparently, from devitrification of glassy lava-flow rocks. During devitrification, alkali feldspar crystallizes first, followed by silica; iron oxide minerals become concentrated around the peripheries of devitrified regions.

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