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The formation-dissolution-precipitation cycle of melanterite at the abandoned pyrite mine of Genna Luas in Sardinia, Italy: environmental implications

Published online by Cambridge University Press:  05 July 2018

F. Frau
F. Frau*
Affiliation:
Dipartimento di Scienze della Terra, Università, di Cagliari, Via Trentino 51, I-09127 Cagliari, Italy
*
*E-mail: frauf@unica.it
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Abstract

At the abandoned pyrite mine at Genna Luas (Sardinia, Italy), melanterite is the most environmentally important secondary phase associated with pyrite oxidation. A complete cycle, including formation on pyrite surface, dissolution in water, precipitation from water and subsequent alteration, can be observed. The oxidation of pyrite to melanterite is related mainly to the ‘intrinsic’ features of the Genna Luas pyrite, such as its cryptocrystalline pseudo-colloidal texture. During melanterite formation on pyrite there is transient storage of Fe2+, SO42- and acidity. This is controlled mainly by wetting-drying seasonal cycles. Laboratory experiments have shown that the simple dissolution of melanterite is an acidity-generating process which affects the geochemistry of water and, indirectly, contributes to the supply of the Fe3+ required for the aqueous oxidation of pyrite. According to this geochemical model, the interaction between rainwaters and ground pyritic materials accumulated in heaps generates hyperacidic saline waters (pH = 0.6; TDS = 300 g/l) with very high SO42+ contents (203 g/l), Fe2+ contents (73 g/l), Zn contents (11 g/l), and numerous potentially toxic elements. Melanterite precipitates from these waters (SIMelanterite = 0.15, calculated using PHRQPITZ software), giving rise to attractive emerald green crystal aggregates which, when exposed to air, alter to a fine white powder composed mainly of melanterite, and minor amounts of rozenite and szomolnokite.

Keywords

melanteritepyrite oxidationhyperacidic watersenvironmental geochemistryGenna Luas minesiteSardiniaItaly
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 6 , December 2000 , pp. 995 - 1006
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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