Evaluation of the Stability Relations of Amphibole Asbestos in Metamorphosed Iron-Formations

Abstract

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.