Solubility and Diffusional Uptake of
Hydrogen in Quartz at High Water Pressures: Implications for Hydrolytic
Weakening
A.K. Kronenberg and S.H. Kirby
U.S. Geokogical Survey, Menlo Park, CA
Roger D. Aines and George R.
Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA
Abstract
Attempts to introduce molecular water into dry, natural quartz crystals
by diffusive transport and thus weaken them hydrolytically at T =
700°-900°C and PH2O = 400-1550 MPa have failed. Infrared
spectroscopy of hydrothermally annealed single crystals of natural
quartz reveals the diffusive uptake of interstitial hydrogen (resulting
in hydroxyl groups) at rates similar to those previously proposed for
intracrystalline water at high water pressures. The solubility of
interstitial hydrogen at these conditions is independent of temperature
and pressure; instead, it depends upon the initial aluminum
concentration by the local charge neutrality condition [Hi·] =
[AlSi']. The rate of interstitial hydrogen diffusion parallel to c is
given by an Arrhenius relation with D0 = 1.4 x 10-1 m2 /s and Q = 200 ±
20 kJ/mol, in close agreement with H diffusivities reported for much
lower pressures (PH2O = 2.5 MPa). Deformation experiments following
hydrothermal annealing show no mechanical weakening, and the lack of
any detectable broadband absorption associated with molecular water
shows that the diffusion rates of structural water are much lower than
those of hydrogen. These results are consistent with the available
oxygen diffusion data for quartz and with the failure to observe
weakening in previous studies of quartz deformation at pressures of
300-500 MPa; they call into question the rapid rates of diffusion
originally suggested for the hydrolytic weakening defect. It is
suggested that the observed weakening in many previous experiments was
complicated by microcracking processes in response to nonhydrostatic
stresses and low effective confining pressures. Extensive microcracking
may prm:ide a mechanism for molecular water to enter quartz and allow
local plastic deformationto occur. It does not appear that molecular
water can diffuse far enough into uncracked quartz to allow hydrolytic
weakening over annealing.times that are feasible in the laboratory.