Partial replacement of plagioclase by smectite within the Takakumayama Granite, Kagoshima, Japan

Abstract

Plagioclase crystals up to 5 mm long partially replaced by pink-colored smectite, from the Takakumayama Granite, Kagoshima, Japan, were examined using optical microscopy, X-ray diffraction, electron probe microanalysis, and X-ray analytical microscopy. A map of Ca concentration reveals distinct zoning that is probably due to abrupt changes in growth conditions. In terms of zoning, there are three distinct regions within the plagioclase crystal: (1) an unzoned core, (2) an intermediate region between the core and the rim that corresponds to the pink alteration mineral visible in hand specimens, and (3) oscillatory zoning in the rim. The pink alteration mineral was identified as smectite, with a characteristic basal spacing of 1.58 nm under air-dried and untreated conditions. The alteration is associated with defects and dendritic textures that are abundant in the intermediate regions of the crystals. It is postulated that the alteration resulted from the action of externally derived hydrothermal fluids that accessed the defects and dendritic texture in the plagioclase crystals via microfractures that subsequently sealed during cooling of the granite.