Abstract

When plagioclase feldspar is held in H2O at 500°C to 800°C at 2 kbar the albite components are removed in solution whereas the anorthite components remain as a relatively insoluble residue. Solution is so selective that the anorthite residuum typically forms a pseudomorph after the original plagioclase, preserving details of grain outline, cleavage and twinning. The volume deficiency of a pseudomorph is manifested by an intricate network of interconnecting channelways. Although albite is removed without apparent disruption of its parent plagioclase crystal, several lines of evidence indicate that the anorthite component also enters into solution but is immediately reprecipitated.

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