Major and trace element composition of copiapite-group minerals and coexisting water from the Richmond mine, Iron Mountain, California

Copiapite-group minerals of the general formula AR₄(SO₄)₆(OH)₂·nH₂O, where A is predominantly Mg, Fe²⁺, or 0.67Al³⁺, R is predominantly Fe³⁺, and n is typically 20, are among several secondary hydrous Fe sulfates occurring in the inactive mine workings of the massive sulfide deposit at Iron Mountain, CA, a USEPA Superfund site that produces extremely acidic drainage. Samples of copiapite-group minerals, some with coexisting water, were collected from the Richmond mine. Approximately 200 mL of brownish pore water with a pH of −0.9 were extracted through centrifugation from a 10-L sample of moist copiapite-group minerals taken from pyritic muck piles. The pore water is extremely rich in ferric iron (Fe³⁺=149 g L⁻¹, Fe_T=162 g L⁻¹) and has a density of 1.52 g mL⁻¹. The composition of the pore water is interpreted in the context of published phase relations in the Fe₂O₃–SO₃–H₂O system and previous work on the chemistry of extremely acid mine waters and associated minerals in the Richmond mine. Two distinct members of the copiapite mineral group were identified in the samples with coexisting water: (1) abundant magnesiocopiapite consisting of platy crystals 10 to 50 μm and (2) minor aluminocopiapite present as smaller platy crystals that form spheroidal aggregates. The average composition (n=5) of the magnesiocopiapite is (Mg_0.90Fe²⁺_0.17Zn_0.02Cu_0.01)_∑1.10(Fe³⁺_3.83Al_0.09)_∑3.92(SO₄)_6.00(OH)_1.96·20H₂O. Bulk compositions determined by digestion and wet-chemical analysis are consistent with the microanalytical results. These results suggest that magnesiocopiapite is the least soluble member of the copiapite group under the prevailing conditions. Micro-PIXE analysis indicates that the copiapite-group minerals in this sample sequester Zn (average 1420 ppm), with lesser amounts of Cu (average 270 ppm) and As (average 64 ppm).