Abstract

A vitric, crystal, lithic volcanic ash near Denver, Colorado, consists predominantly of fresh, highly vesicular, rhyolitic glass shards and smaller amounts of partly altered platy glass shards. The alteration products of the platy shards are predominantly illite randomly interlayered with a small amount of montmorillonite and also containing a small amount of free montmorillonite. Among the lithic and crystal particles in the ash is biotite, probably of volcanic origin, which is partly altered to vermiculite, part of which is randomly interlayered with biotite. Some of the altered biotite crystals are long and vermicular and appear to be authigenic.

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