Symmetry relations in alkali feldspars of some amphibolite-facies rocks from the southern Norwegian Precambrian

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

320 alkali feldspars from Precambrian granites and gneisses were examined for obliquity (Δ) by means of the X-ray powder method. The gneisses that surround the granites belong to the middle amphibolite facies, and the level of exposure is catazonal. K-feldspars from the granites and pegmatites exhibit maximum obliquity, those from ordinary gneisses generally have maximum obliquity or variable obliquity, and those from augen gneisscs show lower and variable obliquity. The obliquities of K-feldspar crystals from a single hand specimen of augen gneiss may be highly variable. These augen gneisses exhibit apparent replacement features. Because microcline appears to be the stable K-feldspar phase throughout the entire area, the writer suggests that the K-feldspars of intermediate and variable obliquity are metastable phases caused by the interplay of bulk composition changes, growth rate, deformation, and temperature.