Calcium distribution in alkali feldspar of a quartz syenite from Cape Ashizuri, Shikoku, southwest Japan.

Abstract

Alkali feldspar grains in a quartz syenite from Cape Ashizuri, southwest Japan, consist of two parts: clear and turbid. The rims of grains are turbid and show lamellar textures of microperthites. Both clear and turbid parts are scattered irregularly inside grains. The clear parts are optically featureless and cryptoperthitic. The inner turbid parts are mosaic and lamellar microperthites. In some sections, only lamellar microperthites are developed throughout a grain. Calcium distribution maps were obtained for feldspar grains showing the above textures. The rims are always poor in calcium (nearly 0 mol% An). The inner clear parts are generally rich in calcium (5 mol% An at the maximum) around the composition of about Or33Ab64An3, but in some cases poor in calcium (-0.1mol% An). The sodium-rich phase of the microperthite of the inner turbid parts are variable in calcium content from 7 to nearly 0 mol% An. On the other hand, the potassium-rich phase of the microperthite is always poor in calcium both in the rims and inside the grains. The total calcium distribution patterns suggest a possible calcium transfer from the inside to the outside of grains under hydrother-mal conditions with microperthite formation.