CRYSTALLOGRAPHIC RELATIONSHIPS OF THE PYROXENES AND PYROXENOIDS

Abstract

The crystallographic relationships of pyroxenes and pyroxenoids are described based on the new unit cells of Narita (1973). The new unit cells clearly indicate that the pyroxenoids make a homologous series which is composed of the slabs of the pyroxenetype and wollastonite-type structures.
Ideal structures of the pyroxenoids are constructed from those of pyroxene and wollastonite assuming that large cations occupy the octahedral sites of oxygen atoms. The characteristic deviation from the ideal structure is the expansion of the c axis parallel to the silicate chain of wollastonite. In the structure of pyroxenoids a dense zone which makes an important role in transition is controlled by that of the wollastonite-type structure.