STATIONARY AND MOBILE HYDROGEN DEFECTS IN POTASSIUM-FELDSPAR

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 hyd rogen loss and hydrogen gain at elevated temperatures. Principal absor ption 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 alpha (E a t 5 degrees to a) and beta (E at 5 degrees to c), respectively, where as bands at 328 and 309 mm(-1) are more nearly isotropic. Similarly, n ear-infrared bands at 525 and 513 mm(-1) (associated with combination ii-OH bend, O-H stretch modes) exhibit maximum peak heights for or whi le a lesser band at 475 mm(-1) appears to be nearly isotropic. Compari son of fundamental and combination band intensities reveal that molecu lar 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 an d combination band polarizations suggest multiple defect sites or pote ntially a secondary population of hydroxyl defects. Rates at which the se defects can be eliminated from samples annealed in air at temperatu res T from 500 degrees to 900 degrees C:are much faster than those pre dicted by oxygen mobilities, yielding diffusivities of D[m(2)/s] = 6.2 x 10(-4) exp(-172 +/- 15 [kJ/mol]/RT), indistinguishable from those r eported for proton interstitials in quartz. Dissociation of stationary molecular water defects to mobile proton interstitials which leave cr ystal interiors requires that oxygen defects are left behind. Hydrogen defects cain be added to adularia crystals annealed at elevated water pressures (corresponding to H2O fugacities Of 412 and 1710 MPa and H- 2 fugacities up to 174 MPa), again at rates that exceed oxygen mobilit ies. In addition, significant redistribution amongst sites is suggeste d 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 o f internal adjustment are required.