A DETAILED COMPARISON OF NATURAL AND SYNTHETIC JAROSITE MINERALS

Jarosite minerals are sulfates that contain alkali elements and ferric iron. They form in both low temperature and hydrothermal acidic sulfate-rich environments, including mining wastes. Jarosites are also important in a variety of industrial applications. We mineralogically and chemically characterized jarosites collected at 6 supergene and 13 hydrothermal occurrences at 10 U.S. sites and from 8 sites in 5 other countries. Hydronium-bearing synthetic jarosites were studied for comparison. Natural jarosites have a narrow range in a and c cell dimensions, and electron microprobe analysis of hydrothermal jarosites detected no cation or anion deficiencies. Analysis of 32 hydrothermal and supergene K- and Na-jarosites indicates no (< 5 mole percent) solid solution between K and Na end members. Instead, our detailed study of cell parameters and composition reveals discrete mixtures of K and Na end members. Hydrothermal jarosites exhibit exsolution features (as small as a few micrometers) consistent with a K-Na solvus at lower temperatures. We conclude that this solvus exists for temperatures below about 95°C for K- and Na-jarosite end members. Hydronium-bearing jarosite was detected in only one supergene sample. For synthetic jarosites, we found that the common practice of heating to 110°C after synthesis results in minor changes in cell dimensions, significant weight loss (2-5%), and appears to drive off structural waters from protonated hydroxyl sites. Consequently, this methodology to prepare jarosites for thermodynamic measurements may result in errors. It appears that hydronium-bearing jarosites are metastable. When synthetic hydronium-bearing jarosites are heated at 120°C for 78 days or 240°C for 24 hours, FeOHSO₄ is formed by the reaction:

(H₃O)Fe³⁺₃(SO₄)₂(OH)₆=2FeOHSO₄ + 2H₂O + Fe(OH)₃ , with end-member jarosite also being produced.