Solubility of jarosite solid solutions precipitated from acid mine waters, Iron Mountain, California, U.S.A. Solubilité des solutions solides de jarosite précipitées à partir des eaux minières acides, Iron Mountain, Californie, U. S. A
Seven samples of acid mine drainage (initial pH 0.8 to 1.4) from the Hornet and Richmond massive sulfide ore deposits at Iron Mountain, West Shasta district, California, were stored for 1 1 to 13 years at 22 ± 3 °C. A yellow precipitate (Munsell color 2.5 Y 7/8) which formed in each sample is identified by powder X-ray diffraction, atomic absorption spectrophotometry, and thermogravimetric analysis as jarosite solid solution (jss) having the general formula K(x)Na(y)(H₃O)(j.x-y)Fe₃(SO₄)₂(OH)₆, where .53 < x < .83, .03 < y < . 1 5, and .10 <(1-x-y) <.32. It is assumed that at the time of collection in 1974-76, the mine waters were somewhat undersaturated with respect to jss ; subsequent oxidation of aqueous ferrous iron during storage has caused saturation and precipitation of jss. The jss crystals are 5 to 10 micrometers in diameter and rhombohedral in form. The well crystallized nature of the precipitate suggests that the oxidized mine waters have approached thermodynamic equilibrium with jss and therefore can provide useful data for the evaluation of thermodynamic properties of К-Н₃О-Na substitution in jarosite. Because of the common occurrence of 15 to 25 mole percent hydronium substitution on the alkali site in jarosites, it is necessary to consider the hydronium content of jarosites in any attempt at rigorous evaluation of jarosite solubility or of the saturation state of natural waters with respect to jarosite. A Gibbs free energy of 3293.5 ± 2. 1 kJ mol⁻¹ is recommended for a jarosite solid solution of composition К₇₇Na₀₀₃(H₃O) ₂₀Fe₃(SO₄)₂(OH)₆. Solubility determinations for a wider range of natural and synthetic jarosite solid solutions will be necessary to quantify the binary and ternary mixing parameters in the (K-Na-H₃O) system. In the absence of such studies, molar volume data for endmember minerals indicate that the K-H30 substitution in jarosite is probably closer to ideal mixing than either the Na-K or Na-H₃O substitution.