Crystal chemistry of a clinopyroxene series in ultrafemic xenoliths from North-Eastern Brazil

A series of clinopyroxenes (Cpx) in peridotitic spinel nodules from Rio Grande do Norte, North-Eastern Brazil, was investigated by X-ray diffraction and electron microprobe analyses and compared with an analogous series from spinel peridotite nodules from Mt. Leura, Victoria, Australia. The examined Cpx series is mainly characterized by increasing Mg/(Mg+Fe²⁺) ratio (i.e. increase of refractory character of the host nodules), by the substitutions of Al^vi vs Mg in the M1 site and Na vs (Ca+Mg) in the M2 site, whereas Al^iv in the T site remains substantially constant. The strong Na depletion in the Brazilian Cpx causes a different charge balance from that in the Australian Cpx, in which Na remains quite constant. This feature, associated with the differences in polyhedral and cell volumes which are significantly higher in the Brazilian Cpx than in those of the Australian Cpx, suggests that the Brazilian Cpx crystal chemistry may be related to a lower pressure regime, compared with that of the Australian Cpx series.