Crystal-chemistry of high-pressure clinopyroxene from spinel lherzolite nodules: Mts. Leura and Noorat suites, Victoria, Australia

The clinopyroxene suites from lherzolite nodules from Mts. Leura and Noorat (Victoria, Australia) have been investigated by X-ray diffraction and electron probe microanalyses, (Dal Negro et al., 1984; Cundari et al., 1986). • IV 2+ The evolution of the host nodule is shown by general depletion of A1 , A1 v~, Ti 4+, FeM1 and enrichment of Si, Mg~, Fe 3÷, Cr 3+, Ca, while Na depletion occurs only in the clinopyroxenes from Mt. Noorat. Different mechanisms of cation substitution are thus involved in the two clinopyroxene suites, suggesting different total pressures of equilibration in the stability field of spinel• Modifications involving the M1 octahedron are mainly ascribed to variable amounts of trivalent cations, the volume of the M1 site increasing with decreasing content of trivalent cations in each suite. The differences in M 1 site configuration between the Mt. Leura and Mr. Noorat clinopyroxene suites are ascribed to the different amounts of A1 v~ and Fe2~. The volume of the T tetrahedral site is generally related to A1 w in each suite. An increase in T site volume from Mt. Leura to Mt. Noorat clinopyroxenes was found however, due to lengthening of the T--Obrg bond lengths, for similar AF v contents. The volume of the M2 site, generally related to Ca content, was generally higher in the Noorat clinopyroxenes for similar Ca (and Ca + Na) contents, due to the longer M2--O3C1 bond length, strongly related to Na content. Cell volume is linearly correlated to M 1 volume in each suite, but is definitely higher in the Noorat clinopyroxenes for similar M 1 volume. All the structural data show that the total pressure of equilibration in the Noorat clinopyroxenes was lower than that in the Leura clinopyroxenes, as suggested by chemical data.