Aluminium replacing silicon in some silicate lattices

Extract

The purpose of this account is to review, from the literature, figures for the replacement of silicon by aluminium in biotites, calciferous amphiboles, and clinopyroxenes. Atomic ratios to be quoted have been calculated on a basis of 24(O,OH,F) in biotites, hornblendes, and pyroxenes alike. This facilitates comparison and the higher factor magnifies differences. It may be emphasized, to avoid possible confusion, that in this discussion it is the amount of silicon replaced by aluminium that will be under review. The ratio of tetrahedrally co-ordinated Al to octahedrally co-ordinated A1 will not be considered, although for the sake of completeness, figures for AI in Y groups will be furnished to show total A1 values. Machatschki's group notation is employed. Comparison between biotites, hornblendes, and clinopyroxenes, for present purposes, may best be made between minerals that crystallized under the same pressure-temperature conditions, to reduce variables. Few analyses of specimens of these three minerals, that have crystallized together in the same rock, are available however. One such paragenesis is quoted below. This association occurs as gneiss-like inclusions in calcite, taken from the Tiree marble.