Abstract
Recent study of the plagioclase feldspars2 has led to the conclusion that they represent a case of atomic isomorphism, one calcium atom being equivalent, not to two sodiums as often supposed, but to a single sodium atom; and SiO4 not being replaced by Si3O8, but remaining the silicate radicle throughout the series. In this connection it seemed worth while to consider whether other instances of alleged replacement of Ca by Na2 and of SiO4 by Si3O8 were really valid. The formulas ascribed to certain of the zeolites were ‘therefore investigated, and in one case the relations turned out to be so different from those usually recognized that a brief note seems desirable.
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