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Published April 2004 | public
Journal Article

A survey of hydrous species and concentrations in igneous feldspars

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 H_2O), H_2O (0–1350 ppm H_2O), and NH_4^+ (0–1500 ppm NH_4^+) 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 H_2O or NH_4^+ 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 H_2O) in the exchanged regions. The amount of water stored as fluid inclusions within feldspars in the upper crust (1 × 10^(19) kg) is small compared to the 1.35 × 10^(21) kg of water in the oceans, but is roughly equivalent to the reservoir of water stored in hydrous minerals in the upper crust.

Additional Information

© 2004 American Mineralogist. Received March 30, 2003; Accepted November 28, 2003; Manuscript handled by Darby Dyar. Many individuals generously contributed the feldspar specimens that were part of this study. Our thanks go to A. Beran, D. Bird, A. Boettcher, J. Bryce, D. Burnett, D. Cole, A. Colville, K. Cooper, J. Dilles, C. Douthitt, A. Eggers, J. Eiler, C. Francis, B. Gray, T. Giordano, B. Gregory, R. Hill, W. Hildreth, C. Johnston, B. Kamb, P. Keller, W. Larson, T. Latorette, R. Reed, D. Rimstidt, R. Shannon, L. Silver, D. Stewart, E. Struhsacker, H. Taylor, Jr., J. Valley, J. Vasquez, G.J. Wasserburg, and W. Wise in this regard. H.P. Taylor, Jr., kindly provided access to many of his samples and made the original suggestion to study the relationship between interstitial water in feldspar and oxygen isotopic exchange. The authors acknowledge and thank G. Cleve Solomon for the preliminary work that set the stage for this project. This study was supported by NSF grants EAR-0125767 and EAR-9218980. This is contribution number 8938 of the Division of Geological and Planetary Sciences of the California Institute of Technology.

Additional details

Created:
August 22, 2023
Modified:
October 23, 2023