Hydrothermal alteration of CM carbonaceous chondrites; implications of the identification of tochilinite as one type of meteoritic PCP

摘要: Poorly characterized phases (PCP's) constitute up to 30 volume percent of some CM carbonaceous chondrites, and are therefore an important key understanding the physico-chemical conditions attending matrix evolution. An iron rich form terrestrial phase tochilinite was recently identified as a common type PCP. Tochilinite has general formula 6Fe(0.9)S.5(Mg,Fe)(OH)2 consists alternating machinawite (FeS) brucite ((Mg,Fe)(OH)2) sheets, with vacancies in sulfide sheets. In tochilinite, ferrous hydroxide, called amakinite, replaces brucite. If kchondrites have underdone hydrothermal alteration, at least, probably grew from aqueous solutions by low FO2, high FS2, pH 10 12, temperature or below 170 C.