It is now well known that the ordering, unmixing, and optical properties of the alkali feldspars are closely dependent on the conditions and rate of cooling. As yet, however, little attention has been given to the properties and composition of sanidine discharged in ash clouds, which should have a thermal history rather similar to sanidine formed synthetically. This paper describes the pyroclastic soda-rich sanidine crystals from a tuff bed in the John Day formation of Oregon.

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