EXPERIMENTAL STUDIES OF ALUNITE .2. RATES OF ALUNITE-WATER ALKALI ANDISOTOPE-EXCHANGE

Rates of alkali exchange between alunite and water have been measured in hydrothermal experiments of 1 hour to 259 days duration at 150 to 4 00 degrees C. Examination of run products by scanning electron microsc ope indicates that the reaction takes place by dissolution-reprecipita tion. This exchange is modeled with an empirical rate equation which a ssumes a linear decrease in mineral surface area with percent exchange (f) and a linear dependence of the rate on the square root of the aff inity for the alkali exchange reaction. This equation provides a good fit of the experimental data for f = 17% to 90% and yields log rate co nstants which range from -6.25 moles alkali m(-2)s(-1) at 400 degrees C to -11.7 moles alkali m(-2)s(-1) at 200 degrees C. The variation in these rates with temperature is given by the equation log k = -8.17(1 000/T(K)) + 5.54 (r(2) = 0.987) which yields an activation energy of 3 7.4 +/- 1.5 kcal/mol. For comparison, data from O'NEIL and TAYLOR(1967 ) and MERIGOUX (1968) modeled with a pseudo-second-order rate expressi on give an activation energy of 36.1 +/- 2.9 kcal/mol for alkali-felds par water Na-K exchange. In the absence of coupled alkali exchange, ox ygen isotope exchange between alunite and water also occurs by dissolu tion-reprecipitation but rates are one to three orders of magnitude lo wer than those for alkali exchange. In fine-grained alunites, signific ant D-H exchange occurs by hydrogen diffusion at temperatures as low a s 100 degrees C. Computed hydrogen diffusion coefficients range from - 15.7 to -17.3 cm(2)s(-1) and suggest that the activation energy for hy drogen diffusion may be as low as 6 kcal/mol. These experiments indica te that rates of alkali exchange in the relatively coarse-grained alun ites typical of hydrothermal ore deposits are insignificant, and suppo rt the reliability of K-Ar age data from such samples. However, the fi ne-grained alunites typical of low temperature settings may be suscept ible to limited alkali exchange at surficial conditions which could ca use alteration of their radiometric ages. Furthermore, the rapid rate of hydrogen diffusion observed at 100-150 degrees C suggests that fine -grained alunites are susceptible to rapid D-H re-equilibration even a t surficial conditions.