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

The major structural change resulting from Ca-Fe substitution in four clinopyroxenes of intermediate 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 compositional change from hedenbergite to ferrosilite causes kinking of the tetrahedral chain and concomitant increases in the out-of-plane tilting of the basal face of the tetrahedron. As the M2 polyhedron decreases further in size, the space group changes from C2/c to P2₁/c at about Fs₈₀Wo₂₀ composition. When the composition changes from Fs₈₅Wo₁₅ to Fs₁₀₀Wo₀, the A chain extends and finally reverses its kink direction. The anisotropic temperature factors for these intermediate compositions may be explained as a result of the effects of positional disorder.