Observations on mantled potash feldspars from the Vr├ądal granitic pluton, Telemark, Norway

Forelesninger holdt ved Advanced Study Institute of Feldspar - Oslo 1962
Lectures held at Advanced Study Institute of Feldspar - Oslo 1962

Large crystals of potash feldspar mantled with oligoclase were analysed for the mole per cent Or content. Mantled individuals had a higher content of Or in the core than in the zone nearer the mantle; unmantled crystals did not show such a distribution gradient.

The tendency for such feldspars to occur in basic rocks and their textural appearance under the microscope suggest that the plagioclase mantle is a reaction phenomenon. The material for the mantle is supplied in part by exsolution of Ah from the potash feldspar toward its margins, thereby leaving the core enriched in Or.