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Teresa P. Silva^a*, João P. Veiga^b

^aLNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Estrada da Portela, Apartado 7586, 2610-999 Amadora, Portugal

^bCENIMAT/I3N (Centre for Materials Research), Materials Science Dept., Faculty of Sciences and Technology, NOVA University of Lisbon, 2829-516 Caparica, Portugal

Cultural heritage when involving geological mining patrimony could carry a vast scientific knowledge, namely about iron sulphates formed in abandoned sulphide-ore mines. The variety of waste materials that resulted from the mining activity through the ages usually are dispersed along the mine, resulting in the occurrence of acidic drainage (AMD).

The geochemical behaviour of secondary iron sulphates, more or less hydrated, is strongly conditioned by their crystal structure, e.g. jarosite. The knowledge of these materials is particularly important in what concerns preservation strategies of mining heritage.

Jarosites (s.l.) - with general formula AB₃(OH)₆(SO₄)₂, where A is mainly K⁺, Na⁺, plus minor Ag⁺, Tl⁺, NH₄⁺, Pb²⁺, Bi³⁺, and B is essentially Fe³⁺ (jarosite s.s.) or Al³⁺ (alunite) - have a trigonal crystal structure [1] and display Kagomé-type layers of corner-sharing B octahedra, [Fe/AlO₂(OH)₄], that give rise to unique magnetic properties [2]; the large cation A stays in pseudo-icosahedral coordination by 6 O-atoms from [SO₄] tetrahedra and 6 hydroxyls shared with A octahedra [3].

A minerochemical study by X-ray diffraction (XRD) and X-ray fluorescence spectrometry (XRF) performed on materials collected in São Domingos abandoned mine (southern Portugal, Iberian Pyrite Belt of polymetallic sulphide ores) showed preferential retention of lead in jarosite-rich wastes [4]. In this work we describe the geochemical selectivity of secondary iron sulphates focusing on the positive contribution of some minerals whose environmental connotation is usually negative.

References:

[1] B. Hendricks (1937). The crystal structure of alunite and jarosite. Amer. Inst. Min. Metallurg. Engr. Technical Publ., 22: 773-784.

[2] A.S. Wills et al. (2000). Magnetic properties of pure and diamagnetically doped jarosites: model kagomé antiferromagnets with variable coverage of the magnetic lattice. Phys. Rev B, 61: 6156-6169.

[3] S. Menchetti & C. Sabelli (1976). Crystal chemistry of alunite series: crystal structure refinement of alunite and synthetic jarosite. Neues Jahrb. Miner. Monatsch. Heft, 9: 406-417.

[4] M.O. Figueiredo, T. Pereira da Silva & J. Mirão (2006). Minerochemistry of the secondary iron sulphates: an environmental approach related to the abandoned mine of São Domingos (in Portuguese). VII Congr. Nac. Geologia, Estremoz, Abstracts Book, Edts. J. Mirão & A. Balbino, vol. II, 475-478.

Abstract presented in the Conference Materiais 2019, 14-17 Abril 2019, Lisboa, Portugal