Effects of Ca-Mg substitution in C2/c pyroxene structure on natural clinopyroxenes from spinel peridotite nodules (Pico Cabugi, Brazil)

X-ray crystal structure refinements and microprobe analyses of clinopyroxenes from two suites of spinel-peridotite nodules are compared for an understanding of the structural response to a decrease in Ca content. The kinking of the tetrahedral chain (O3-O3-O3 angle) of the C2/c structure associated with decreased Ca content shows behavior similar to that observed for the A-tetrahedral chain of the P21/c structure. Contemporaneously the occupancy of the M2 split site increases. In addition, as a consequence of the low Ca content, the M2 polyhedron dominates the structural rearrangement of the clinopyroxenes considered here