Chemical Study of Minerals from the Moine Schists of the Ardnamurchan Area, Argyllshire, Scotland

Abstract

The mineralogy of the Moine schists of the Ardnamurchan area has been studied, using twenty-two analyses of muscovites, eighteen of biotites, seven of garnets (including five partial analyses), and one analysis of an epidote. The muscovites range in composition from phengite-ferrimuscovite in the biotite grade of metamorphism to nearly pure muscovite (sensu stricto) in the staurolite grade. The amounts of aluminium in octahedral and tetrahedral co-ordination in the muscovites increase as the grade of metamorphism increases. In the associated biotites the amount of octahedral aluminium shows only slight variations which are not related to the grade of metamorphism, but the amount of tetrahedral aluminium increases with increase in the grade of metamorphism. Muscovites and biotites from epidote-bearing rocks contain smaller amounts of tetrahedral aluminium than micas from rocks without epidote at the same grade of metamorphism. The amounts of tetrahedral aluminium in micas from rocks of similar composition can probably be used as an index of the grade of metamorphism. Magnesium and ferrous iron show a consistent distribution relation between coexisting muscovites and biotites. The value of the distribution coefficient for these cations between the two micas is about 2.5 to 3.0, with magnesium preferentially concentrated in the muscovite. The value of the distribution coefficient does not vary appreciably within the medium grades of regional metamorphism. There is also a consistent pattern of distribution of magnesium, ferrous, and ferric iron between the two micas, but ferric iron is relatively concentrated in muscovite in rocks that contain epidote without garnet; it is possible that the biotites in these rocks attain a maximum possible content of Fe3+ at about 10 per cent of the total Mg+Fe2++Fe3+ content. The consistent distribution of magnesium and iron between coexisting micas suggests that it can be used as an index of the attainment of chemical equilibrium with respect to these two elements in metamorphic rocks. All but one of the analysed micas have deficiencies in the occupancy of the large cation site; there appears to be a consistent relationship in the extent of the large cation deficiency between the two coexisting micas, wjth the biotites having a greater deficiency than the muscovites. Sodium is relatively concentrated in the muscovites but does not show a consistent distribution relationship between the coexisting micas. The analysed garnets are almandine-spessartines with appreciable solid solution towards grossularite. The garnets that contain the largest amounts of grossularite are those from the epidote-bearing rocks. In the rocks without epidote the garnets show a poor correlation of decreasing calcium content with increasing grade of metamorphism.