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.

First Page Preview

First page PDF preview
You do not currently have access to this article.