Oxygen exchange experiments were carried out by using single crystals of sanidine feldspar, and water which was enriched with 17O and 18O, at temperatures ranging from 550 to 850C under a total pressure of 1kbar. The O isotope diffusion profiles in directions perpendicular to (001) were determined by using an ion microprobe. It was found that the diffusion data for both isotopes obeyed the same Arrhenius relationship:

D (m2/s) = 3.95 x 10-12exp[-109.7(kJ/mol)/RT]

By combining the concentration versus distance data for 17O and 18O for each profile, using a double inverse error function plot, it was possible to obtain the ratio, 17O/18O, of the diffusion coefficients directly. At 550 and 650C, the values of the isotope effect ranged from 0.83 to 1.03. This suggested an interstitial mechanism for O diffusion in this feldspar, although the uncertainties did not preclude the operation of a vacancy mechanism.

R.Freer, K.Wright, H.Kroll, J.Göttlicher: Philosophical Magazine A, 1997, 75[2], 485-503