Preliminary observations on the feldspars from the Pilanesberg alkaline complex, Transvaal, South Africa

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

Microcline and plagioclase from a suite of alkaline rocks are described. The evidence obtained from obliquity and 2 V values, and the feldspar association point towards metastable crystallisation of orthoclase below ca. 500 °0, in some rocks, and subsequent inversion to microcline. The presence of cryptoperthite in other rocks suggests formation of orthoclase under stable conditions. The inversion (ordering) process is thought to have been accompanied by exsolution of excess Fe2O3 in the form of aegirine microlites. Microcline contains about 3-8 wt.% Ab dissolved. A variety of feldspar textures are also described.