Do Jarosite Minerals Generate Acid in Legacy Mining Wastes?

1. K 20 mm 10 mm 20 mm Na 20 mm Do Jarosite Minerals Generate Acid in Legacy Mining Wastes? Kathleen S. Smith, George A. Desborough, Gregg A. Swayze, Heather A. Lowers, Sharon F. Diehl, Jane M. Hammarstrom, and Rhonda L. Driscoll U.S. Geological Survey, M.S. 964D, Denver Federal Center, Denver, CO 80225-0046, ksmith@usgs.gov Jarosite in Mining Waste Jarosite is an iron-sulfate mineral that can form in legacy mining waste as a result of sulfide weathering, and is an indicator of acidic conditions. Jarosite’s role as a potential source of acid generation in mining waste is not completely understood. The traditional view is that acid can be generated due to the oxidation of sulfide minerals, the dissolution of highly soluble sulfate salts, and the dissolution of less-soluble sulfate minerals (e.g., jarosite). In mining wastes where all of these materials are mixed together, it is difficult to apportion the contribution from the various components to acid generation. As a further complication, jarosite minerals can accommodate numerous substitutions in their structure, which results in a wide variety of jarosite minerals with variable compositions and reactivities. It is likely that most jarosite minerals found in legacy mining wastes are relatively stable, and that these jarosite minerals make only a limited contribution to acid generation. However, freshly formed natural jarosite minerals may generate significant acidity due to selective dissolution of relatively unstable components. In acidic environments where remnant jarosite minerals are found, acid most likely is generated by other potential contributors, such as soluble salts. Background • Jarosite-group minerals commonly form as weathering products from sulfide-bearing materials and are associated with acid-generating mining wastes and acid-sulfate soils. • The role of jarosite minerals as potential sources of acid generation in legacy mining wastes is a controversial subject. • Jarosite can generate acid by dissolution according to the following reaction: • KFe3(SO4)2(OH)6 → 3FeO(OH) + K+ + 2SO42- + 3H+ • Chemical composition and particle size are recognized as important factors in determining the solubility of jarositeminerals. • A complicating factor is that natural jarosite crystals cannot be individually studied due to their small size (generally <2 µm, often <0.5 µm). • It has been common practice to use synthetically prepared jarosite minerals of known composition to serve as analogs to determine the solubility and acid-generating potential of naturally occurring jarositeminerals. • The widespread practice of heating synthetic jarositesat least appears to drive off structural water, which results in synthetic samples with stable crystal structures that are not representative of natural jarosites formed in low-temperature environments. Natural Jarosite Study • We conducted a detailed study of the chemical and mineralogical characteristics of 23 natural jarosite minerals from diverse environments (Desborough et al., 2010). Findings from this study include: • Most natural remnant K- and Na-jarosite minerals are stoichiometric and relatively stable in the surficial environment. • There is little evidence for significant substitution (> 5 mol%) of hydronium in the structure of most natural remnant K- and Na-jarositeminerals, which would contribute to acid generation upon weathering. • Natural jarosite does not appear to have significant solid solution between the K and Na end members under conditions in the surficial environment; instead, they exist singly or as physical mixtures of both end members. • Desborough, G.A., Smith, K.S., Lowers, H.A., Swayze, G.A., Hammarstrom, J.M., Diehl, S.F., Leinz, R.W., and Driscoll, R.L., 2010, Mineralogical and chemical characteristics of some natural jarosites: Geochim. Cosmochim. Acta, v. 74, p. 1041-1056. Locations, sources, associated minerals, and cell parameters of natural stoichiometric supergene or secondary jarosites and natrojarosites used in this study. Scanning electron micrograph of potassium jarosite (identified by X-ray diffraction) from a mine-waste pile showing the typical small grain size of supergene jarosite minerals observed in mining wastes. The circle represents the diameter of the electron beam defocused to 10-mm for electron microprobe analysis of jarosite, and illustrates the difficulty of obtaining an elemental analysis of individual grains. The unit cell of a mineral is the smallest unit that possesses the symmetry and properties of the mineral. Differences in the dimensions of the three-dimensional unit cell can be diagnostic of element substitutions or vacancies within the unit cell. Element substitutions or vacancies can lead to decreased stability of the mineral phase. We examined the a- and c-cell dimensions for 19 natural jarosites and 12 natural natrojarosites by X-ray diffraction. Electron probe micro-analysis X-ray intensity maps of ~5-10 mm jarosite crystals from the Richmond Mine, California. The top map shows the occurrence of K and the bottom map shows the occurrence of Na in brighter colors. Note the Na and K zoning, which is less than 2 mm. These images suggest that K- and Na-jarosite grew at different times and have limited solid solutions. No Solid Solution George A. Desborough 1937 – 2010 Cell dimensions of 19 natural jarosites (K end member) and 12 natrojarosites (Na end member) measured by X-ray diffraction. Ten wt % alumina (Al2O3) was used as an internal standard for each sample. [“+/-” values are the mean least-square errors for a and c, respectively.]

2. 7.36 SYNTHETIC JAROSITE INDIRECT IDENTIFICATION OF HYDRONIUM-BEARING JAROSITE 7.35 synthetic natrojarosites 90-100C 7.34 The range of a- and c-cell dimensions for 10 jarosites and four natrojarosites we synthesized at 90-100C are shown below. Also shown is the range of a- and c-cell dimensions of our 19 "mature" natural jarosites (K end member) and 12 "mature" natural natrojarosites (Na end member). Note that there are small but significant differences between the cell dimensions of mature natural samples and synthetic samples. For example, synthetic samples tend to have larger a-cell dimensions than their natural counterparts, and synthetic K jarosites tend to have smaller c-cell dimensions. The reasons that synthetic jarosites might have larger a-cell and smaller c-cell dimensions include (1) hydronium substitution in the alkali site, (2) vacancies in the alkali site, or (3) vacancies in the Fe3+ site and their associated protonated hydroxyl sites. synthetic jarosites 90-100C 7.33 mature natural natrojarosites Heating of synthetic K-jarosites, including hydronium-bearing jarosites, at 240C resulted in a measurable reduction in the a-cell dimension and an increase in the c-cell dimension. Ten of the heated samples contain XRD-detectable Fe(OH)SO4; chemical analysis of these samples show them to contain 71-88 mole% K. The loss of hydronium from the jarosite crystal structure is accompanied by a small, but significant, change in cell dimensions, which may coincide with the development of end-member K-jarosite when hydronium is lost. The formation of crystalline Fe(OH)SO4 from hydronium jarosite may require the loss of water and formation of amorphous iron hydroxide, as shown in the equation: (H3O)Fe3(SO4)2(OH)6(s)2Fe(OH)SO4(s) + 2H2O + Fe(OH)3(s) with end-member K-jarosite also being produced. We did not detect Fe(OH)SO4 by X-ray diffraction when hydronium was absent from jarosites. Also, by using thermogravimetric analysis, we did not observe a peak between 170-260ºC when hydronium was absent. Therefore, we conclude that hydronium loss from jarosite is the cause of Fe(OH)SO4 production. Consequently, the easily XRD-detectable Fe(OH)SO4 phase can be used as a post-mortem indicator of the presence of hydronium-bearing jarosite. We did not detect Fe(OH)SO4 upon heating the natural jarosites, which is in agreement with our quantitative electron probe microanalysis results showing that there is no detectable alkali-site deficiency, or hydronium substitution, in the natural jarosites we studied. The exception to this is samples from the Richmond Mine in California. These samples yield Fe(OH)SO4 after thermal treatment at 240ºC, which indicates that they contain some hydronium-bearing jarosite. 7.32 a cell dimension (Å) 7.31 mature natural jarosites 7.30 mean least-square errors 7.29 7.28 16.5 16.6 16.7 16.8 16.9 17.0 17.1 17.2 17.3 c cell dimension (Å) HEATING AT 110ºC Range of cell dimensions for “mature” natural jarosites and natrojarosites versus metastable synthetic jarosites and natrojarosites formed at low temperature (90-100C). Triangles are for natural jarosites from five Colorado mine-waste samples. Circles are for “modern” jarosite stalactite samples from the Richmond Mine, California. The mean least-square error is shown for both the a-cell and c-cell dimension. We synthesized a series of eight hydronium-bearing jarosites, using from 0.1 g to 5.6 g of KOH, at 95ºC and dried them at 60ºC for 1 hour. We compared their cell dimensions with those of the same samples dried at 110ºC, which is a common practice to remove “excess” water. We found that that “drying” at 110ºC changes the cell dimensions and apparently drives off structural water from protonated hydroxyl sites. Cell dimensions of jarosite concentrates from five historical mine-waste piles in Colorado are plotted as triangles on the above figure and reveal mixtures of two or more jarosites. These include two samples with natrojarosite and three samples with end-member K-jarosite. Note that the cell dimensions for some of these concentrates resemble those for synthetic samples, and others resemble cell dimensions for mature samples. Samples from abandoned underground mine workings at the Richmond Mine, California, are plotted as circles and are the only natural samples we have studied that show intermediate cell dimensions. Note that the cell dimensions for these samples most closely resemble those of synthetic jarosites. Cell dimensions for synthetic hydronium-bearing jarosites prepared at 95ºC and dried at 60ºC. End-member jarosite (J), natrojarosite (N), and hydronium jarosite (H) are also shown. ACKNOWLEDGMENTS Funding for this work was provided by the U.S. Geological Survey Mineral Resources Program. We thank all the persons who provided us with jarosite samples for this study. CONSIDERATIONS FOR THE USE OF SYNTHETIC JAROSITE AS AN ANALOG FOR NATURAL JAROSITE The chemical composition, cell dimensions, and other properties of synthetic jarosites formed at low temperature ( 95ºC) may simulate jarosites found in mining wastes. Most of the low-temperature synthetic jarosites seem to be metastable due to substitution of hydronium in the alkali site, or deficiency of Fe3+ and associated H2O in the hydroxyl site, or alkali-site deficiencies. Heating synthetic jarosite samples drives off structural waters and changes the properties of the jarosites. Metastable or “modern” supergene jarosites and low-temperature synthetic jarosites have larger a- and smaller c-cell dimensions than do “mature” natural jarosites. In addition, mature natural jarosites appear to have none of the alkali-site vacancies, iron deficiencies, and significant hydronium substitutions that are observed in some synthetic and modern natural jarosites. Recognition of metastable jarosite phases is important because they will tend to have different solubility and acid-generation properties than mature or high-temperature synthetic jarosites. In order for synthetic jarosites to be representative of modern jarosites in mining wastes, they should not be heated above 95oC after preparation. High-magnification BSE image of supergene sample 81196u showing hollow cores and the highly porous nature of jarosite in crystals smaller than 5 mm.