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Understanting Jarosite -- from Mine Waste to Mars.

U.S. Geological Survey Circular 1328, p. 8 - 13, 2008.

by Gregg A. Swayze, George A. Desborough, Kathleen S. Smith, Heather A. Lowers, Jane M. Hammarstrom, Sharon F. Diehl, Reinhard W. Leinz, and Rhonda L. Driscoll

Reference:
Swayze, G.A., G.A. Desborough, K.S. Smith, H.A. Lowers, J.M. Hammarstrom, S.F. Diehl, R.W. Leinz, and R.L. Driscoll, 2008, Understanding jarosite -- from mine waste to Mars, in Understanding Contaminants Associated with Mineral Deposits, P.L. Verplanck, (ed.), U.S. Geological Survey Circular 1328, p. 8 - 13.

Abstract

The presence of jarosite, an iron-sulfate mineral, in soil or in mining waste is an indicator of acidic, sulfate-rich conditions. Physical and chemical properties of synthetically prepared jarosites are commonly used as analogs in laboratory studies to determine solubility and acid-generation potential of naturally occurring jarosites. In our work we have mineralogically and chemically characterized both natural and synthetic jarosites. Analyses of natural potassium (K)- and sodium (Na)-jarosites from hydrothermal and weathering environments indicate that there is little solid solution (chemical mixing); instead, they revealed that samples consist of discrete mixtures of potassium- and sodium-endmembers. Hydronium (H3O+)-bearing jarosite was detected in only one relatively young natural sample, suggesting that terrestrial H3O-bearing jarosites are unstable over geologic timescales. Although the presence of H3O+ in jarosite is very difficult to measure directly with traditional analytical methods, we found that heating H3O-bearing samples at high temperatures (greater than 200 to 300 degrees Celsius) produces an Fe(OH)SO4 compound. Fe(OH)SO4 is easily detected by X-ray diffraction and reflectance spectroscopy; hence, it can be used as a post-mortem indicator of the presence of H3O+ in jarosite. Results from our synthetic jarosite studies indicate that natural H3O-bearing jarosite, alkali site deficient, and iron-deficient forms of natural jarosite are metastable and likely are significant factors in acid generation of some mining wastes, but that these natural phases are not accurately represented by synthetic jarosite prepared by commonly used methods. The widespread practice of heating synthetic jarosites to at least 110 degrees Celsius (C) after synthesis appears to drive off structural water, resulting in samples with crystal structures similar to those of the hydrothermal jarosites, which are a much more stable form of jarosite. Therefore, synthetic jarosite should not be heated above 60C if it is to be used as an analog for low-temperature natural jarosite found in mine waste. An extension of our work on jarosite reveals that reflectance spectroscopy can be used in remote sensing studies to identify H3O-bearing and iron-deficient jarosites on the surface of the Earth and Mars.

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This page is maintained by: Dr. Gregg A. Swayze gswayze at speclab.cr.usgs.gov
Last modified May 28, 2009.