Minerals group, jarosite

Alunite-jarosite minerals (and the other secondary sulfates) form, in part, because of evaporative concentration of pore and capillary waters in pyritic materials, and also at depth in saturated tailings (Dubrovsky et al. 1985). They are found in soils beneath acid sulfate evaporation ponds (Peterson et al. 1983), and are also precipitated directly from acid mine waters (cf. Filipek et al. 1987 Alpers et al. 1989). The alunite-jarosite mineral group also occurs in the weathered zones on top of metal sulhde deposits (Scott 1987) and in the sediments of acid hypersaline lakes (Alpers et al. 1992). 

HUP and many other natural or synthetic compounds belong to the torbernite mineral group of general formula M, y(U02X04)(H30)y. 3H2O where X is P or As and M" " is a cation such as K, NH4", Na, Li, Ca, Pb, Ag or AP. The role of interlayer cations and water molecules in torbernite is similar to that in zeolites, clay minerals, some feldspars and micas, jarosites, autunites and clathrate compounds. In all these materials solid solutions between protonic and ionic derivatives are readily observed ion exchange and protonic conductivity can thus be expected. 

The mineral alunite, and its iron equivalent jarosite, is a special case. Alunite (KAl2(S02)2(OH)2) contains four sites where elements containing stable isotopes are found and both the sulfate and hydroxyl anionic groups may provide information on fluid source and condition of formation. 

Another group of As-bearing minerals contains arsenic in the 5+ valence state as arsenate commonly substituting for the phosphate group. An unidentified As-bearing iron phosphate, usually associated with banded iron oxyhydroxide as veins or masses, has a P/As atomic ratio on the order of 4 (Belkin et al., 1997). Jarosite... 

Twelvefold coordination of lower valence actinides is inferred only for the orthophosphate mineral eylettersite, (Th,Pb)i.xAl3(P04,Si04)2(0H)6 [77], which is a member of the alunite-jarosite supergroup that includes the isotypic florencite group of phosphates and arsenates. Charge balance in eylettersite is maintained through partial occupancy of the Th site, and by substitution of Si forP. 

The minerals found in United States coals continue to be studied with the availability of improved instrumental procedures such as x-ray diffraction, infrared absorption, and scanning electron microscopy beyond the traditional optical and chemical mineralogical techniques as applied to thin sections, polished pellets, and isolated particles. The minerals may be grouped into the silicates (kaolinite, illite montmorillonite, and chlorite), the oxides (quartz, chalcedony, hematite) the sulfides (pyrite, marcasite, and sphalerite) the sulfates (jarosite, gypsum, barite, and numerous iron sulfate minerals) the carbonates (ankerite, calcite, dolomite, and siderite) and numerous accessory minerals (apatite, phosphorite, zircon, rutile, chlorides, nitrates, and trace minerals). 

Sulfate Minerals (Figure 6.6) Fifteen sulfate minerals are present in the thermodynamic database. The calculations show that many are supersaturated in our water samples. Several members of the alunite-jarosite group are grossly supersaturated, but alunite and jarosite are known to be supersaturated without precipitating in some acidic surface waters (Alpers et al., 1994). Aluminum hydroxy sulfates, Al(OH)SC>4 (jurbanite), and Al4(OH)ioSQ4 (basaluminite), are also supersaturated at some points... 

Minerals of alunite-jarosite groups have the general formula MAsCSOJjCOH) where M is H3O+, NH4+, Na+, K+, Rb+, Ag+, T1+, O.SPb " and A is AP" (alunite) or Fe (jarosite). Related synthetic materials MFe(S04)2(0H)6 (M = H3O+, NH4+), MGa3(S04)2(0H)6 also form solid solutions Hydronium alunite can be prepared by hydrothermal reaction from Al2(S04)3.16H2O at 250 °C while only the mixed derivative is obtained by refluxing at 105 °C. Sulphate... 

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