Elizabeth A. Johnson, George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125-2500, U.S.A.
The hydrous components in eighty-five feldspars from various igneous environments spanning the range of naturally occurring compositions were examined with infrared spectroscopy. Feldspars contain structural OH (0-512 ppm H2O), H2O (0-1350 ppm H2O), and NH4+ (0-1500 ppm NH4+) groups as well as fluid inclusions and alteration products. Although composition and structure do influence the type of hydrous species that can be incorporated into a particular feldspar mineral, the concentration of these species is not controlled by major element composition. Coarse perthitic microclines have a heterogeneous distribution of hydrous species, and contain H2O or NH4+ in K-rich lamellae and fluid inclusions in Na-rich areas. The structural OH in plagioclase feldspars is not associated with twin boundaries or exsolution lamellae. All of the possible structural hydrous species are found in pegmatite feldspars, whereas volcanic feldspars contain only structural OH. The variation in OH concentration within a given feldspar composition suggests that fluids in the geologic environment plays a role in determining the hydrogen concentration of each sample. The vast majority of plutonic feldspars have undergone partial or total equilibration with meteoric fluids during low-temperature (400-150°C) hydrothermal exchange, obliterating any structural hydrogen and creating substantial concentrations of fluid inclusions (up to 4000 ppm H2O) in the exchanged regions.