EXPERIMENTAL STUDY OF ALUMINUM SPECIATION IN FLUORIDE-RICH SUPERCRITICAL FLUIDS

Abstract

The solubility of the albite-paragonite-quartz mineral assemblage was measured as a function of NaCl and fluorine concentration at 400°C, 500 bars and at 450°C, 500 and 1000 bars. Decreasing Al concentrations with increasing NaCl molality in F-free fluids of low salinity (mNaCl < 0.01) demonstrates that Al(OH)4− dominates Al speciation and is formed according to the reaction 0.5 NaAl3Si3O12H2(cr)+2 H2O = 0.5 NaAlSi3O8(cr)+Al(OH)4−+H+. Log K results for this reaction are −11.28 ± 0.10 and −10.59 ± 0.10 at 400°C, 500 bars and 450°C, 1000 bars, respectively. Upon further salinity increase, Al concentration becomes constant (at 400°C, 500 bars) or even rises (at 450°C, 1000 bars). The observed Al behavior can be explained by the formation of NaAl(OH)40(aq) or NaAl(OH)3Cl(aq)0. The calculated constant for the reaction Al(OH)4−+Na+=NaAl(OH)40(aq) expressed in log units is equal to 2.46 and 2.04 at 400°C, 500 bars and 450°C, 1000 bars, respectively. These values are in good agreement with the predictions given in Diakonov et al. (1996). Addition of fluoride at m(NaCl) = const = 0.5 caused a sharp increase in Al concentration in equilibrium with the albite-paragonite-quartz mineral assemblage. As fluid pH was also constant, this solubility increase indicates strong aluminum-fluoride complexation with the formation of NaAl(OH)3F(aq)0 and NaAl(OH)2F20(aq), according to 0.5 NaAl3Si3O12H2(cr)+Na++HF(aq)0+H2O = 0.5 NaAlSi3O8(cr)+ NaAl(OH)3F(aq)0+H+, log K = −5.17 and −5.23 at 400°C and 450°C, 500 bars, respectively, and 0.5 NaAl3Si3O12H2(cr)+Na++2 HF(aq)0 = 0.5 NaAlSi3O8(cr)+NaAl(OH)2F20(aq)+H+, log K = −2.19 and −1.64 at the same P-T conditions. It was found that temperature increase and pressure decrease promote the formation of Na-Al-OH-F species. Stability of NaAl(OH)2F20(aq) in low-density fluids also increases relative to NaAl(OH)3F(aq)0. These complexes, together with Al(OH)2F(aq)0 and AlOHF20(aq), whose stability constants were calculated from the corundum solubility measured by Soboleva and Zaraisky (1990) and Zaraisky (1994), are likely to dominate Al speciation in metamorphic fluids containing several ppm of fluorine.