Microtexture and water content of alkali feldspar by Fourier-transform infrared microspectroscopy

Alkali feldspar (Or(64.5)Ab(34.8)An(0.6)) in a granite pegmatite from Hanazono, Kosei Town, Shiga Prefecture, southwest Japan, consists of two types of regions observable with the naked eye: one is colourless and transparent, and the other is pale pink and opaque. The colourless regions are clear, cryptoperthitic and almost free of micropores, and the pale pink regions are turbid, vein microperthitic and with many micropores visible under a microscope. The latter regions may have been changed from the former by the catalytic action of water. Integrated absorbance values were calculated for both types of regions from Fourier-transform infrared microspectra in the range 2500-4000 cm(-1). These values were used as semi-quantitative parameters of water contents. The turbid regions are much more enriched in water compared to the clear regions. The chemical states of water in both regions were also evaluated. Water in the turbid regions is estimated to be present mainly as H2O inclusions in micropores and to be present to a lesser extent as structural OH groups. A small amount of water in the clear regions may be present as structural OH groups. The water distribution in the feldspar records the cooling history of hydrothermal reactions causing the coarsening of cryptoperthites to microperthites.