Alpersite (Mg,Cu)SO4·7H2O, a new mineral of the melanterite group, and cuprian pentahydrite: Their occurrence within mine waste

Alpersite, Mg_0.58Cu_0.37Zn_0.02Mn_0.02Fe_0.01SO₄·7H₂O, a new mineral species with direct relevance to reactions in mine waste, occurs in a mineralogically zoned assemblage in sheltered areas at the abandoned Big Mike mine in central Nevada at a relative humidity of 65% and T = 4 °C. Blue alpersite, which is isostructural with melanterite (FeSO₄·7H₂O), is overlain by a light blue to white layer dominated by pickeringite, alunogen, and epsomite. X-ray diffraction data (MoKα radiation) from a single crystal of alpersite were refined in P2₁/c, resulting in wR = 0.05 and cell dimensions a = 14.166(4), b = 6.534(2), c = 10.838(3) Å, β = 105.922(6)°, Z = 4. Site-occupancy refinement, constrained to be consistent with the compositional data, showed Mg to occupy the M1 site and Cu the M2 site. The octahedral distortion of M2 is consistent with 72% Cu occupancy when compared with the site-distortion data of substituted melanterite.

Cuprian pentahydrite, with the formula (Mg_0.49Cu_0.41Mn_0.08Zn_0.02)SO₄·5H₂O, was collected from an efflorescent rim on a depression that had held water in a large waste-rock area near Miami, Arizona. After dissolution of the efflorescence in de-ionized water, and evaporation of the supernatant liquid, alpersite precipitated and quickly dehydrated to cuprian pentahydrite. These observations are consistent with previous experimental studies of the system MgSO₄-CuSO₄-H₂O. It is suspected that alpersite and cuprian pentahydrite are widespread in mine wastes that contain Cu-bearing sulfides, but in which solubilized Fe2+ is not available for melanterite crystallization because of oxidation to Fe3+ in surface waters of near-neutral pH. Alpersite has likely been overlooked in the past because of the close similarity of its physical properties to those of melanterite and chalcanthite. Alpersite is named after Charles N. Alpers, geochemist with the United States Geological Survey, who has made significant contributions to our understanding of the mineralogical controls of mine-water geochemistry.