A new mineral, ammonioalunite [ideal composition NH4Al3(SO4)2(OH)6], has been found in a sample from the hot-springs setting at The Geysers (Sonoma County), California. Chemical analysis yields 33.0% Al2O3, 36.0% SO3, 12.24% H2O, 5.39% (NH4)2O, 0.19% K2O, 0.17% Na2O, and 11.6% SiO2. After subtraction of the silica as an amorphous impurity, the mineral has the following chemical formula: {(NH4)0.92K0.02Na0.02[Al2.88-(SO4)2.00(OH)6.05]}0.45-, assuming a stoichiometric sulfate content. Within the analytical uncertainties, this analysis approximates a stoichiometric composition. Synthetic ammonioalunite has high H2O and low Al and NH4 contents and yields the following chemical formula: (NH4)0.84(H3O)0.16[Al2.80(SO4)2.00(H2O)0.83(OH)5.40], assuming 2.00 SO42− anions, electrostatic neutrality, and that excess H2O exists as H3O+ and as H2O in Al02(OH)4 octahedra. X-ray, thermogravimetric, and mid-infrared data all support the presence of H3O+ in synthetic ammonioalunite. Structural H2O was not directly observed in the synthetic ammonioalunite. Ammonioalunite is rhombohedral, space group R3m or R3m, a = 7.013(1) Å, and c = 17.885(5) Å. The five strongest X-ray diffraction lines are [d (Å)(Irel(hkl)] 3.023(100)(113); 5.04(93)(012); 2.996(50)(021); 1.917(32)(303); 2.353(31)(107). Based on mid-infrared data, the NH4+ molecule appears to have undistorted Td symmetry in the alunite structure. Diagnostic near-infrared bands associated with NH4+ make possible the remote detection of NH4-bearing alunite.

Three NH4-rich alunite samples (30-50 mol% NH4) were also found near the fossil hotspring locality with mercury-gold mineralization in the Ivanhoe district, Elko County, Nevada. These NH4-rich alunites occur with opal and quartz, and one occurs with low-ammonium alunite. NH4-rich alunite is indicative of the following restricted chemical environment: acidic solutions less than 100 °C with abundant NH4+ and SO42−, and little K+.

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