The role of jarosite and copiapite in the chemical evolution of acid drainage waters, Richmond Mine, Iron Mountain, California

The Richmond Mine at Iron Mountain, California is host to the most extreme acid mine drainage ever reported. Prior to treatment, pH values of mine water as low as -3.6 with dissolved solid concentrations near 1000 g/L have been recorded. A significant contributing factor to the water quality at this site is the cyclic precipitation and dissolution of secondary sulphate minerals. The exact influence of many of these minerals is not completely understood. Copiapite [FeIIFe4III(SO4) 6(OH)2.20H2O] and jarosite [KFe3 III(SO4)2(OH)6] minerals, with co-existing waters, have been collected and studied to obtain a better understanding of mineral-water interaction within the Richmond Mine.;Electron microprobe, scanning electron microscopy and x-ray diffraction techniques identified two phases in the copiapite samples: (1) a predominant magnesiocopiapite phase of semi-rectangular, platy crystals (10--50mum) and (2) a minor aluminum-rich ferricopiapite phase existing as smaller platy crystals (≤5 mum) of spheroidal aggregates.;Copiapite is a relatively soluble sulphate and has very acidic pore-water. Dissolution of significant quantities of this mineral during wet seasons likely contributes to a short-term influx of iron and sulphate in the Richmond Mine effluent. Given the relative low solubility of jarosite, it will attenuate certain elements more effectively than more soluble sulphates (i.e. copiapite). (Abstract shortened by UMI.)*.;*Originally published in MAI Vol. 39, No. 3. Reprinted here with corrected author name.