Hydrogen diffusion in Dora Maira pyrope

Abstract

Kinetics of hydrogen extraction were investigated by FTIR spectroscopy on a Dora Maira pyrope single crystal. Annealing experiments were performed at ambient pressure, between 1073 and 1323 K, and under two different redox conditions (pO2=0.21 atm and pO2 ≈ 10−16 atm). During hydrogen extraction the two principal OH absorption bands behave independently. The OHa triplet, centred on 3651 cm−1, decreases at least five times faster than the OHb band at 3602 cm−1. This suggests the presence of two distinct H defects, each with its own kinetics, which are slightly dependent on oxygen partial pressure: Both kinetics are slower than previous data (Wang et al. 1996), but activation energies are similar. At the same time as the OHa bands decrease, an exchange of hydrogen from OHa to OHb occurs. The extent of this transfer can be modelled by assuming it is directly proportional to the fraction of OHa band which has been removed, independent of temperature. This suggests that OHb defects could be produced by partial dehydrogenation of OHa defects. Activation energies and values of diffusion coefficients are very close to those found by Kohlstedt and Mackwell (1998) for the diffusion of metal vacancies in olivine. Thus it is very likely that cation vacancies control the kinetics of dehydrogenation reactions in pyrope.