In contrast to many successful previous ion-exchange studies of dry alkali feldspars, in which the aluminosilicate framework remained intact, “ordered” orthoclase from the Himalaya pegmatite-aplite dike system, San Diego County, California, invariably breaks down to sodalite spontaneously, very soon after Na-for-K exchange in a chloride medium. Our efforts to prepare a structurally-equivalent sodic feldspar in other exchange media, taking all precautions to eliminate water, also failed. Substitution of Rb for K appears very limited, and may also lead to breakdown and nucleation of other silicates. These anomalies are attributed to the mismatch between the coordination polyhedron about the alkali atom and an Al,Si framework unable to shrink or deform because of intimately balanced, very small triclinic domains set in a structure that is monoclinic to X-rays and neutrons. Our results support a proposal that this type of orthoclase is distinct, the first documented example of the “theoretical” maximum low sanidine or “theoretical” orthoclase structure type.

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