EXPERIMENTAL STUDIES OF ALUNITE .1. O-18-O-16 AND D-H FRACTIONATION FACTORS BETWEEN ALUNITE AND WATER AT 250-450-DEGREES-C

We have determined oxygen and hydrogen isotope fractionation factors b etween alunite and water over a temperature range of 250-450 degrees C by reacting synthetic natroalunite with 0.7 m K2SO4 - 0.1 to 0.65 m H 2SO4 solutions to produce K-rich alunite. From 88 to 95% alkali and is otope exchange were observed in most of these experiments, and the par tial equilibrium method was used to compute equilibrium fractionation factors. Least-squares fits of the data give 10(3) ln alpha(alunite(SO 4)-H2O) = 3.09 (10(6)/T-2 (K)) - 2.94 and 10(3) ln alpha(alunite(OH)-H 2O) = 2.28 (10(6)/T-2 (K)) - 3.90. The intramineral O-18-O-16 fraction ation factor 10(3) ln alpha(alunite(SO4-OH site)) is given by the expr ession 0.8(10(6)/ T-2 (K)) + 0.96. The alunite-water D-H fractionation factor ranges from -19 at 450 degrees C to -6 at 250 degrees C and do es not appear to be strongly dependent on temperature. Runs with alkal i exchange in the opposite direction were used to obtain O-18-O-16 and D-H fractionation. factors between natroalunite (mol% Na = 70-75) and water at 350-450 degrees C. These indicate that mol% Na has negligibl e effect on the fractionation factors over this temperature range. Mea sured O-18-O-16 and D-H fractionation factors between alunite and 1.0 m I(CI - 0.5 m H2SO4 fluids also agree within 2 sigma with the values obtained from the K2SO4-H2SO4 fluids. However, experiments with alunit e and distilled water at 400 degrees C gave a value of 10(3) ln alpha( alunite(SO4)-H2O) of 0.0, compared with a value of 3.9 obtained at thi s temperature with K2SO4- and H2SO4-bearing fluids. This suggests that changes in fluid composition can affect alunite-water O-18-O-16 fract ionation factors. Reconnaissance experiments with fine-grained natural natroalunite demonstrate that alunite-water D-H exchange can occur by hydrogen diffusion, although this process is generally not significan t in the experiments with coarser grained synthetic alunites.