A study of the potassium feldspare in some igneous and metamorphic rocks from the Moak-Thompson map area, Manitoba

Abstract

Sodium and potassium contents of 35 igneous and metamorphic rock specimens from the Moak-Thompson area were determined by flame photometer. Potassium feldspars were separated from the rocks and obliquities (triclinicities) of the mineral estimated from X-ray powder photographs. For a number of specimens an increase in obliquity may be correlated with an increase in the ratio K felspar:total alkali feldspar with a monoclinic K feldspar occurring in a rock with ~ 23 mole % K feldspar of the total alkali feldspar and a maximum microcline in a rock with ~ 67 mole % K feldspar of the total alkali feldspar. It is suggested that there is a microcline series extending over the composition range ~ 23 - 67 mole % K feldspar of the alkali feldspar diagram, and a phase diagram is proposed. A modification of the phase diagram for very high water pressures is proposed to account for the occurrence of microclines with a given obliquity in igneous rocks of differing alkali compositions. An orthoclase-low albite phase diagram is proposed to account for the orthoclase-cryptoperthite series... The study suggests a possible way of distinguishing between igneous and metamorphic rocks.