Evaluation of the stability relations of amphibole asbestos in metamorphosed iron-formations.

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Amphibole asbestos, crocidolite (riebeckite) and amosite (grunerite) can be abundant in Precambrian iron-formations. The phase relations involving these amphiboles are evaluated in the system Na-Fe-Mg-Si-O-H on the basis of reported mineral assemblages from various rock types and available thermodynamic data. The phase relations may be divided into two types: 1) low-to medium-temperature (150°C-450°C) and 2) hightemperature at about 500 (±50)°C. The lower temperature stability relations are most applicable to the occurrence of amphibole asbestos. These relations involve three invariant points defined by assemblages riebeckite + grunerite (minnesotaite)+quartz+magnetite, riebeckite + quartz + magnetite + hematite, and riebeckite + acmite + quartz + hematite. With decreasing XRKFEof riebeckite the higher temperature assemblages approach those of low-to medium-temperature. The phase relations suggest that crocidolite may be replaced by amosite when there is no effective barrier against the possible escape of alkali-bearing solutions from the rock system. Similar replacement would also be promoted by increasing temperature. Some textural observations of the two asbestos minerals seem to support such replacement by amosite.
資源地質学会の論文