2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 3
Presentation Time: 8:35 AM


DESBOROUGH, George A.1, SMITH, Kathleen S.1, SWAYZE, Gregg A.2, DIEHL, Sharon F.2, LOWERS, Heather A.1, HAMMARSTROM, Jane M.3, DRISCOLL, Rhonda L.1 and LEINZ, Reinhard W.4, (1)U.S. Geol Survey, Box 25046, Denver Federal Center, M.S. 973, Denver, CO 80225-0046, (2)U.S. Geol Survey, Box 25046 DFC, M.S. 964, Denver, CO 80225-0046, (3)U.S. Geol Survey, 954 National Center, Reston, VA 20192, (4)U.S. Geol Survey (retired), Box 25046, Denver Federal Center, M.S. 973, Denver, CO 80225-0046, ksmith@usgs.gov

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.