Abstract

A variety of heulandite occurs as minute idiomorphic crystals and as crusts attached to detrital sand grains in friable marine sandstones of upper Miocene age in southern Santa Cruz County, California. It is certainly authigenic and is typical, in small amounts, of a thin stratigraphic horizon at the base of sandstones containing volcanic detritus. The mineral has been identified by optical and crystallographic properties determined with a universal stage, and although these properties do not agree exactly with those of either ordinary heulandite or the silica-rich variety, they indicate that the mineral is a type of heulandite.

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