SOLUBILITY OF CORUNDUM IN SUPERCRITICAL 0.1 MOLAL CACL2

Corundum, a-Al₂O₃, solubility was measured in 0.1 m CaCl₂ solutions from 400 to 600 ^oC between 0.6 and 2.0 kbar. The Al molality at 2 kbar increases from 3.1x10^-4 at 400 ^oC to 12.7x10^-4 at 600 ^oC. At 1 kbar the solubility increases from 1.5x10^-4 m at 400 ^oC to 3.4x10^-4 m at 600 ^oC. These molalities are somewhat less than corundum solubility in pure H₂O (Walther, 1997) at 400 ^oC but somewhat greater at 600 ^oC. They are significantly lower than corundum solubility in 0.1 and 0.5 m NaCl as shown in the figure. A distribution of species calculation was done for the CaCl₂ solutions considering the Al species Al(OH)₃^o and Al(OH)₄^- consistent with the solubility of corundum in pure H₂O and association constants reported in the literature. The calculated solubility was greater than that measured except at 600 ^oC and 2.0 kbar. This suggests neutral-charged species interactions are important. A Setchénow model for neutral species was considered. However, Setchénow model effects on charged species resulted in poor fitting of the model to measured values at 1.0 kbar. This suggests that Al(OH)₃^o has a greater stability relative to Al(OH)₄^- than given by the models of Pokrovskii and Helgeson (1995) or Diakonov, et al (1996). The significantly lower Al molalities in CaCl₂ relative to those in NaCl solutions at the same concentration confirm that a Na-Al rather than a Cl-Al complex must be significant in supercritical NaCl solutions to give the observed increase in corundum solubility with increasing NaCl concentrations. These observations help distinguish the nature of the fluid phase in environments where Al is mobile from those where it appears to be conserved in reactions between minerals.