Plagioclase twins in some mafic gneisses from Broken Hill, Australia

Abstract

SummaryPlagioclase in mafic gneisses from Broken Hill, Australia, contains twins that can be divided into two groups on the basis of shape: firstly, lamellar twins that gradually change thickness across a grain or form lenticular terminations; and secondly, simple and lamellar twins that show angular steps in the twin interface or form abrupt, planar terminations. The former are interpreted as mechanical twins, and the latter as growth twins formed by grain growth in the solid state, arguing largely by analogy with twin shapes in experimentally produced aggregates, particularly metals. The only twin laws observed in lenticular twins are albite and pericline, these being of nearly equal abundance. Stepped twins are much less common than lenticular twins, and in rocks without lenticular twins most of the plagioclase is untwinned. Several twin laws occur among the stepped twins, but the albite law is the most common. Simple twins on the albite and albite-carlsbad laws are prominent.