The kinetics of hydrogen mobility in grossular with a chemically homogeneous composition of Gr83.2 And14.3 Py2.2 were studied by sequential annealing experiments monitored by Fourier transform infrared spectroscopy. Slices of single crystals <0.5 mm thick were annealed at temperatures in the range 1073–1323 K at ambient pressure in air and in gas mixtures of Ar(90%)/D2(10%) and Ar(90%)/H2(10%). The change of total infrared (IR) absorbance in the OH-stretching region (3700–3500 cm−1) and the OD-stretching region (2750–2580 cm−1) was used to calculate the diffusion coefficients. The law for diffusion of deuterium is given by D = D0 exp[−102±45 kJ mol−1/RT] with log D0 (m2/s) = −7.6. For hydrogen extraction in air the diffusion law is expressed by D = D0 exp[−323±46 kJ mol−1/RT] with log D0 (m2/s) = 1.0. This activation energy agrees with the values found for Dora Maira pyrope and for other pyropes from mantle xenoliths, but the diffusivity is slower for the grossular. A detailed investigation of the decrease in individual OH bands during hydrogen extraction in air revealed two different kinds of kinetics behaviour, suggesting that at least two different types of OH defects are present in this grossular.

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