Published anomalous cell dimensions for one authigenic potassium feldspar from the Barstow Formation have been confirmed in 13 other feldspars formed by similar low temperature replacement of a zeolitic assemblage. A series of experiments on selected specimens suggest that the anomalies are caused by partial substitution of boron for aluminum. This suggestion is confirmed by synthesis of boron-bearing sanidines which duplicate all the anomalies of the natural specimens, but possess more complete Si-Al(B) disorder.

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