Instability of Perovskite in a CO2-RICH Environment : Examples from Carbonatite and Kimberlite (original) (raw)
1998, Canadian Mineralogist
Intricate multiphase pseudomorphs after perovskite (Nb-, LREE-poor) from calcite carbonatite (Sebljavr complex, Kola Peninsula, Russia) and serpentine calcite kimberlite (Iron Mountain, Wyoming) are described. In the kimberlite, the major products of perovskite replacement are (in order of crystallization): kassite, anatase and titanite plus calcite, ilmenite, LREE-Ti oxide [? lucasite-(Ce)]. In the carbonatite, perovskite is initially replaced by anatase plus calcite and, subsequently, ilmenite and ancylite-(Ce). In both instances, the development of calcite and Ti-bearing phases after perovskite involved initial progressive leaching of Ca 2+ from the structure followed by crystallization of ilmenite and LREE minerals in the final stages, after the precipitation of groundmass calcite. The formation of kassite and titanite in the pseudomorphs in kimberlite was controlled by a lower Ca leach-rate and higher activity of SiO 2 in this system, compared with the carbonatite. The similarity between the two types of pseudomorphs results from the instability of Nb-LREE-poor perovskite in a CO 2-rich fluid at low temperatures. Perovskite is considered an unsuitable host for radioactive 90 Sr and REE isotopes due to the low resistance of CaTiO 3 to leaching and incompatibility of Sr and the rare earths with the products of perovskite replacement.
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