隠岐島前・石英閃長岩のアルカリ長石

The chemistry and intemal textures of alkali feldspar in quartz syenite from Oki-Dozen
island, Japan, are described together with the chemical compositions of coexisting hornblende，biotite and opaque minerals．The alkali feldspar shows complicated microtextures in thin　section，consisting of two different areas，turbid and clear（non-turbid）．The turbid areas show irregular mosaics of microperthitic texture，whereas the clear areas are either featureless or show various complicated textures such as wedge−shaped and ripple-like．As a whole，the feldspar consists microscopically of at least four different phases（areas）with different compositions．The problems on their formation are briefly discussed．