On the anorthoclase feldspars of the alkalic rocks from the Iki and adjacent Islands

Abstract

Six anorthoclase feldspars, separated from alkali rhyolite, trachyte and anorthoclasite inclusion of alkali basalt of the Iki and adjacent Islands, northern Kyushu, have been chemically analyzed and optically determined. The anorthoclases from alkali rhyolite belong to high albite-anorthoclase series and do not have An content. On the other hand, the anorthoclases from trachyte and anorthoclasite are characterized by high Or and An molecules and their compositions are plotted on the liquidus minimum point of the system NaAlSi₃O₈-KAlSi₃O₈.