Possible structure types derived from Pbca - orthopyroxene TAKEO

The possible structure types derived from the Pbca-orthopyroxene are shown. There are sixteen space groups, as subgroups of Pbca, without destroying the original unit-cell type setting. Five structure types have two kinds of tetrahedral chain, nine types have four kinds of tetrahedral chain and two types have eight kinds of tetrahedral chain, respectively. Introduction Since the first structure determination of hypersthene (Fe/(Fe +Mg)=0.30) by Warren and Modell (1930), many orthopyroxenes ranging from enstatite to orthoferrosilite, have been ascertained to have the same space group, Pbca. Ito (1950) has suggested from the theory of twinned space groups that orthopyroxene should be built by alternate twinning of clino pyroxenes with P21/c symmetry, and Morimoto (1956) found that the space group of pigeonite at low temperature is P21/c, not C2/c as of diopside. Atlas (1952) also reported that the symmetry of clinoens tatite is P21/c. Morimoto and Koto (1969) refined the structure of orthoenstatite, and concluded that Ito's hypothesis is correct within the limits of error on the unit cell scale. Ghose (1965) tried to de termine the site occupancies for cations in hypersthene (Fe/(Fe+ Mg)=0.53) from a metamorphic rock by X-ray technique and found that Fe and Mg atoms nearly ordered. Recently many precise struc ture refinements of orthopyroxenes have been performed, and de-