The dissolution rate of rutile in hydrofluoric acid was measured to examine the importance of line and point defects on the reaction rate. Well-annealed rutile was shocked with an explosive charge to induce a high density of dislocations (4 x 1011/cm2), paramagnetic point defects (≈2 x 1019/cm3), and lattice strain as measured by X-ray line broadening (≈ 2 x 10-3). Sub-samples of the shocked material were then thermally annealed in order to prepare samples with a wide variation in dislocation density and point defect concentration. The unshocked starting material had a dislocation density of about 106/cm2, no detectable lattice strain (<10-5), and no detectable point defects as measured by electron spin resonance (≈1016/cm3). In spite of this large variation in dislocations and point defect concentrations, only a factor-of-two variation was observed in the reaction rate for shocked and annealed material. Existing theories suggested that etch pits nucleated and grew at the outcrop of a dislocation on the crystal surface. Assuming that the measured dislocation densities could be used to estimate the density of surface outcrops, and that each outcrop dissolved to form an etch pit, the relationship between dissolution rates and dislocations could be interpreted in terms of a mean rate of etch pit growth. The mean rate of etch pit growth in shocked samples was less than 9 x 10-27cm3/s. In experiments with single rutile crystals, however, observable etch pits grew at rates of the order of 0-19cm3/s. This discrepancy suggested either that: (i) the observed etch pits grew much more rapidly than the mean rate; or (ii) bulk dislocation density was not a useful measure of potential sites for etch pit growth on rutile crystal surfaces.

Crystal Defects and the Dissolution Kinetics of Rutile. Casey, W.H., Carr, M.J., Graham, R.A.: Geochimica et Cosmochimica Acta, 1988, 52[6], 1545-56