A survey of hydrous species and concentrations in igneous feldspars
-
Elizabeth A. Johnson
and
George R. Rossman
Abstract
The hydrous components in 85 feldspars from various igneous environments spanning the range of naturally occurring compositions were examined with infrared spectroscopy. The 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 play 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. The amount of water stored as fluid inclusions within feldspars in the upper crust (1 × 1019 kg) is small compared to the 1.35 × 1021 kg of water in the oceans, but is roughly equivalent to the reservoir of water stored in hydrous minerals in the upper crust.
© 2015 by Walter de Gruyter Berlin/Boston
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- Unoccupied states of pyrite probed by electron energy-loss spectroscopy (EELS)
- Tugtupite: High-temperature structures obtained from in situ synchrotron diffraction and Rietveld refinements
- SIMS microanalyses for Au in silicates
- Geochemistry of oxide minerals of Nb, Ta, Sn, and Sb in the Varuträsk granitic pegmatite, Sweden: The case of an “anomalous” columbite-tantalite trend
- An X-ray and electron microprobe study of Fe, Ni, Ga, and Ge distribution and local structure in a section of the Canyon Diablo iron meteorite
- Mineralogical approaches to fundamental crystal dissolution kinetics
- Cu L3X-ray absorption spectroscopy and the electronic structure of minerals: Spectral variations in non-stoichiometric bornites, Cu5FeS4
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- The effect of TiO2on Pd, Ni, and Fe solubilities in silicate melts
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- A single-crystal study on the pressure behavior of phlogopite and petrological implications
- Quantitative electron microprobe analysis of Fe3+/ΣFe: Basic concepts and experimental protocol for glasses
