Stationary and mobile hydrogen defects in potassium feldspar
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Hydrogen defects in adularia from Kristallina, Switzerland (Or90.2 Ab8.7 An0.0 Cs1.1) have been investigated by examining their vibrational modes in the infrared and near-infrared, and by measuring rates of hydrogen loss and hydrogen gain at elevated temperatures. Principal absorption bands exhibited by adularia at wavenumbers of 362 and 345.5 mm-1 (corresponding to O-H stretching modes) are strongly dichroic, with maximum and minimum absorptivities measured for vibrations a (E at 5 to a) and (E at 5 to c*), respectively, whereas bands at 328 and 309 mm-1 are more nearly isotropic. Similarly, near-infrared bands at 525 and 513 mm-1 (associated with combination H-OH bend, O-H stretch modes) exhibit maximum peak heights for a while a lesser band at 475 mm-1 appears to be nearly isotropic. Comparison of fundamental and combination band intensities reveal that molecular water is the predominant hydrogen-bearing species, consistent with previous results for microcline and orthoclase crystals in which H2O substitutes for K. However, differences in magnitude of fundamental and combination band polarizations suggest multiple defect sites or potentially a secondary population of hydroxyl defects. Rates at which these defects can be eliminated from samples annealed in air at temperatures T from 500 to 900C are much faster than those predicted by oxygen mobilities, yielding diffusivities of D [m2/s] = 6.2 10-4 exp(-172 15 [kJ/mol]/RT), indistinguishable from those reported for proton interstitials in quartz. Dissociation of stationary molecular water defects to mobile proton interstitials which leave crystal interiors requires that oxygen defects are left behind. Hydrogen defects can be added to adularia crystals annealed at elevated water pressures (corresponding to H2O fugacities of 412 and 1710 MPa and H2 fugacities up to 174 MPa), again at rates that exceed oxygen mobilities. In addition, significant redistribution amongst sites is suggested by changes in band character and polarization. Neither Fe nor other multivalent impurities are sufficiently abundant to accommodate local charge balance upon the loss or gain of protons and other mechanisms of internal adjustment are required.
