The Effect of Ca-Fe Substitution on the Clinopyroxene Crystal Structure

The major structural change result ing from Ca-Fe substi tut ion in four cl inopyroxenes of in-termediate composition between hedenbergite and ferrosilite, measured at room temperature, is in the size and shape of the M2 polyhedron, whereas the Ml polyhedron remains essentially unchanged. The decrease in the average size of the M2 polyhedron associated with a com-posit ional change from hedenbergite to ferrosi l i te causes kinking of the tetrahedral chain and concomitant increases in the out-of-plane t i l t ing of the basal face of the tetrahedron ' As the M2 polyhedron decreases further in size, the space group changes from C2 / c to P2r / c at about FsroWoro composit ion. When the composit ion changes from FserWoru to FsrooWoo, the A chain extends and finally reverses its kink direction. The anisotropic temperature factors for these intermediate composit ions may be explained as a result of the effects of posit ional dis-order