Cr(III) solubility in aqueous fluids at high pressures and temperatures

Abstract

Trivalent chromium is generally considered relatively insoluble in aqueous fluids and melts. However, numerous counterexamples in nature indicate Cr(III) mobilization by aqueous fluids during metamorphism or hydrothermal alteration of chromite-bearing rocks, or by pegmatite melts. So far, very little is known about the chromium concentrations and speciation in such fluids.In this study, the solubility of eskolaite (Cr2O3) in 1.6–4.2m aqueous HCl solutions was determined in situ at elevated pressures up to 1GPa and temperatures ranging between 400 and 700°C using synchrotron micro–X-ray fluorescence spectroscopy (μ-XRF). Determined concentrations of dissolved Cr ranged between about 900–18,000ppm, with the highest concentrations found at 500°C and 861MPa. The Cr(III) solubility in aqueous HCl fluids is retrograde in the studied temperature range and increases with pressure.In addition, Cr(III) complexation in these fluids was explored by Raman spectroscopy on a 12.3mass% HCl fluid in equilibrium with eskolaite at 400 and 600°C, 0.3–1.6GPa. All spectra show two prominent Cr–Cl stretching bands at about 275 and 325cm−1, which display some fine structure, and in some spectra weak bands in the region between 380 and 500cm−1. The sum of the integrated intensities of the two dominant bands reveals qualitatively the same changes with temperature along an isochore, with pressure at constant temperature, and with the time required for equilibration as the Cr(III) concentrations in the fluid determined by μ-XRF. Complementary ab initio molecular dynamics simulations of a 4m HCl solution at two different densities (0.8 and 0.97g/cm3) and temperatures (427 and 727°C) were performed to investigate the vibrational properties of variousCrClx(H2O)y3-x and CrClx(H2O)y(OH)z3-x-z complexes with 3⩽x+z⩽4 and 0⩽y⩽2. Quasi-normal mode analysis reveals that both the tetrahedral symmetric and antisymmetric Cr–Cl stretching vibrations of CrCl4(H2O)0-2- have characteristic frequencies in the range of the two strongest experimentally observed Raman bands, whereas Cr–O stretching vibrations of hydroxy-chloride complexes occur at wavenumbers above 400cm−1.Solubility and complexation of Cr(III) depend strongly on the activities of Cl− and H+. At high H+ and Cl− activity, the results are consistent with CrCl3-4(H2O)0-2-1-0 complexes as major Cr(III) species, the Cr coordination number of which increases with pressure by becoming more aquated. At low Cl− activity, i.e. in our study at high-temperature low-pressure conditions, the data indicate mixed CrClx(H2O)y(OH)z3-x-z complexes with Cl–Cr ratios less than three.In situ μ-XRF solubility experiments conducted with eskolaite+(H2O+29mass% Na2CO3) and kosmochlor+(H2O+44mass% Na2Si3O7) resulted in dissolved Cr concentrations at or below the detection limit of 500ppm. Thus, acidic chloridic fluids seem to be more efficient agents for Cr(III) mobilization and transport at crustal conditions than aqueous alkali carbonate or silicate solutions.