Determinations of the potash feldspar triclinicity ( ?) in the Vegârshei- Gjerstad augen gneisses (southern Norway) establish the large variability of ? at all scales and the presence of so-called RD-feldspars, associated with monoclinic orthoclase in the charnockitic augen gneisses (granulite facies) and maximum microcline in the normal augen gneisses (almandine-amphibolite facies). Systematic investigations of two large porphyroblasts reveal a definite tendency for the transition: orthoclase ? RD-feldspar ? microcline to take place from the core towards the periphery of the porphyroblast.
Les Gneiss oeilles de la Region de Vegårshei-Gjerstad (Norvege Meridionale) II. L'Indice de triclinisme des feldspaths potassiques