Amazonitic alkali feldspar from the Tanakami Granitic pegmatite, southwest Japan

Abstract

This is the first detailed description of amazonitic alkali feldspar in Japan. The amazonitic feldspar is from pegmatite in the Tanakami Granite, southwest Japan. Macroscopically, the feldspar is characterized by the coexistence of pale blue and white parts. The two parts of microperthite are distinctly different in terms of their microscopic texture. The pale blue part has clear crosshatched twin patterns, albite laths, and few albite patches with relatively few micropores. In contrast, the white part has unclear twin patterns, few albite laths, and many albite patches with abundant micropores. The pale blue color is due to the host microcline containing 103 ppm of PbO at the maximum, and the white color is due to both the two albite phases and turbid microcline causing strong diffuse reflection. It is deduced that plagioclase laths were first crystallized and then enclosed by pale blue microcline that crystallized later. Crosshatched twins were formed in the microcline during cooling, and albite replaced the microcline forming perthitic patches with many micropores. The composition of plagioclase laths is very close to that of pure albite, which is same as that of albite patches. The microcline part having undergone extensive albitization appear white to the naked eye; the part that did not undergo albitization has retained its pale blue color. The amazonitic microcline crystallization and albitization are estimated to have occurred at very low temperatures around 200 °C.