Elizabeth A. Johnson and George R. Rossman
Division of Geological and Planetary Sciences, California Institute of Technology
Pasadena, CA 91125-2500, USA
American Mineralogist 88, 901-911.
A universal absorption coefficient was determined for quantitative analysis of OH and H2O in feldspars using infrared spectroscopy. 1H MAS (magic-angle spinning) NMR spectroscopy was used to determine the H concentration in three alkali feldspars, and for the first time, eight plagioclase samples. To accurately measure structural H concentration in samples with low H (<1000 ppm H2O) it was necessary to eliminate the signal due to adsorbed water in the powdered NMR sample. The pegmatitic and metamorphic albite samples are transparent, but contain variable (40–280 ppm H2O) concentrations of microscopic to sub-microscopic fluid inclusions. The pegmatitic albites also have sharp bands in the mid-IR similar to the OH bands found in quartz. The other plagioclase samples used in the IR calibration have broad anisotropic bands around 3200 cm–1 in the mid-IR and weak combination stretch-bend bands near 4550 cm–1 in the near-IR, indicative of structural OH. The OH vector in plagioclase is preferentially aligned parallel to the crystallographic a axis. The concentration of structural OH in the plagioclase samples ranges from 210–510 ppm H2O by weight. The microcline samples contain structural H2O molecules (1000–1400 ppm H2O) and the sanidine sample contains structural OH (170 ppm H2O). An approximately linear trend is produced when the total integrated mid-IR absorbance is plotted vs. the concentration of structural H determined from NMR (OH and H2O) for plagioclase and alkali feldspars. The integral absorption coefficient for the total mid-IR peak area is 15.3 ± 0.7 ppm–1·cm–1 [107000 ± 5000 L/(mol H2O·cm2)] for natural feldspar samples that contain structural OH or H2O. Measurements of band areas of unpolarized IR spectra on (001) cleavage fragments provide an estimate of H concentration for alkali feldspars, but this method does not work for most plagioclase samples.