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Simple Efflorescent Sulphates from Iberian Pyrite Belt (Portugal)

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Simple Efflorescent Sulphates from Iberian Pyrite Belt (Portugal) Comunicación 126 Simple Efflorescent Sulphates from Iberian Pyrite Belt (Portugal) Simple Efflorescent Sulphates from Iberian Pyrite Belt (Portugal) / NUNO DURÃES (1) / IULIU BOBOS (2) / EDUARDO FERREIRA DA SILVA (3) (1.) Centro de Geologia da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal (2.) Departamento de Geologia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal (3.) GeoBioTec - Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal Simple efflorescent sulphates were studied by X-ray diffraction, infrared spectroscopy and secondary electron microscopy. Zn, Cu - melanterite was formed after the dissolution of green melanterite. Melanterite is one of the simple sulphates which sto- red rapidly the heavy metals solubilized in acid mine waters. Rozenite and szomolnokite have resulted from melanterite dis- solution caused by humidity decreasing and temperature increasing as well. INTRODUCTION within the Volcanic Hosted Massive at the Lousal, Aljustrel and São Sulfides (VHMS) province (Oliveira et Domingo. Samples were gently ground Poorly- and well-crystallized efflorescent al., 2001). Accordingly, the Lousal mine- and mixed for X-ray diffraction (XRD) and sulphates sequences beginning with ralization is related to an exhalative Fourier-Transform Infrared spectroscopy the simple hydrated salts from divalent deposit probably formed in brine pools (FTIR). The XRD patterns of randomly to trivalent cations are described in acid within the lower VSC (Upper Devonian – oriented aggregates were obtained with mine drainage (AMD) elsewhere (Alpers Lower Carboniferous). The Aljustrel a Philips X’Pert MPD machine equipped et al., 1994; Nordstrom, 1999; mine area is a Late Devonian – Early with CuKa radiation. Infra-Red spectra in Nordstrom and Alpers, 1999; Buckby et Carboniferous polymetallic volcanogenic transmittance technique were recorded al., 2003; Romero et al., 2006; among massive sulphide deposits, which com- in the 4000 - 400 cm-1 frequency region others). Weathering of pyrite and pyrro- monly shows an overall metal zoning using a Bruker Tensor-27 Infra-Red thite produces acidity and Fe, followed from a zinc-rich zone near the hanging spectrometer equipped with a room by precipitation of the simple hydrated wall and a cooper-rich zone near the temperature TGC - detector. Morphology divalent sulfates. The Fe+2 may be footwall, separated by low grade to of simple efflorescent sulphates was substituted by other divalent cations barren massive pyrite. The massive sul- carried out with an environmental scan- (i.e., Cu, Pb, Zn, Co, Mg, Mn, Ca, etc) phide horizon in the Aljustrel mine area ning electron microscope (ESEM) FEI generating isomorphic substitutions in occurs at or near the top of the Lower Quanta 400FEG, equipped with a X-ray the crystal-chemistry structure of each Rhyolite unit in two linear belts that energy dispersive (EDAX Genesis X4M). simple hydrated sulphates groups. trend from northwest to southeast. In Efflorescent sulphates may occur in the São Domingos, a unique ore body RESULTS AMD as monomineralic or mixture pha- occur at the contact between metasse- ses, and their crystal-chemistry variety dimentary and volcanic rocks with a X-ray diffraction. Simple efflorescent suggests a short evolution in time and Upper Devonian – Lower Carboniferous sulphates identified by XRD are: melan- +2 space, depending to the environmental age. Intense wall-rock hydrothermal terite (Fe SO4.7H2O), siderotil +2 geochemistry related area and the sea- alteration occurs surrounding the ore (Fe SO4.5H2O), hexahydrite (MgSO4 sonal climatic conditions. body and a big gossan, covers the top 7H2O), chalcanthite (CuSO4 5H2O), +2 of the ore body and extends until the rozenite (Fe SO4.4H2O) and szomolno- +2 The main goal of this paper is to descri- surface. kite (Fe SO4.H2O). The d(111) peak of be the crystal-chemistry of the simple green or blue melanterite, as well as efflorescent sulphates precipitated in siderotile, occurs at about 4.90 Å, whe- the Portuguese Iberian Pyrite Belt. reas for rozenite at 4.47 Å for szomolno- kite at 3.44 Å. X-ray diffraction pattern GEOLOGY of chalcanthite (CuFeSO4 7H2O) was now acquired due to its small amounts The Iberian Pyrite belt extends about collected. 230 Km along strike-faults with a width Fig. 1. Geological sketch of the Iberian Pyrite Belt (adapted ranging from 35 to 50 km, which forms from Carvalho et al., 1976) Infrared spectroscopy. a roughly back-arc through southern Portugal and Spain (Fig. 1). The polyme- MATERIALS AND METHODS Chalcanthite, green and blue melanteri- tallic massive sulfide mines of Lousal, te and rozenite were analyzed by infra- Aljustrel and São Domingos are situa- Efflorescent sulphates samples with red spectroscopy (Fig. 2). Sulphate ted in the NW region of the Iberian Pyrite lighter colours from blue, green, pal- minerals analyzed contain loosely Belt (IPB) along an alignment of the green, yellow and white were collected adsorbed water molecules showing a Volcano – Sedimentary Complex (VSC) from several abandoned mines located strong O – H bending at 1630 cm-1 and palabras clave: sulfatos simples eflorescentes, difracción de rayos-X, key words: simple efflorescent sulphates, X-ray diffraction, espectroscopía de infrarojos, mapas de rayos-X, Faja Pirítica Ibérica Infrared spectroscopy, X-ray maps, Iberian Pyrite Belt. Resumen WORKSHOP 2008: Durães et al., Macla 10 (2008) 126-128 *corresponding author: [email protected] macla. nº 10. noviembre´08 revista de la sociedad española de mineralogía 127 Back-scatter images and X-ray maps. X-ray maps of blue melanterite corres- pond to Fe and S distributions (Fig. 4a and b). This blue melanterite incorpora- tes Zn and Cu from solid solution (Fig. 4c and d). X-ray maps of chalcanthite show a homogeneous S and Cu distribu- tion (Fig. 4e and f). DISCUSSION The more common hydrated iron-sulfate minerals that occur as simple efflores- cent salts on the surfaces of weathe- ring pyrite include melanterite, rozenite and szomolnokite. Melanterite is one of Fig. 2. Infrared spectroscopy spectra of simple efflorescent sulphates: chalcanthite (a), green melanterite (b), blue melante- the first soluble sulfate forms in acid rite (c) and rozenite (d). mine drainage areas studied. Usually, a very large O - H stretching at about tinct splitting in melanterite and rozeni- stalactite of green melanterite up to 3400 cm-1. The IR spectra exhibit featu- te. 30–40 cm could be observed in the flo- res resulting from the fundamental tation plant of the Lousal mine. -2 vibrations of the SO 4 anion. Secondary electron microscopy. Fundamental vibrational bands (some of The dehydration process of melanteri- which are split into 2 components) for Blue melanterite morphology exhibits te is accompanied by dissolution of chalcanthite occur at ~1114 - 1094 pseudo-octahedral crystals with short melanterite which is always with acid (ν3), ~ 987 (ν1), ~ 700 (ν4), and ~ 479 prismatic or tabular habit (Fig. 3a). production: -1 - 454 (ν2) cm . The ν1 vibration mode Incipient dissolution is characterized at 987 cm-1 show the sulfate sorbed on by pit-etch and holes observed on the FeSO4 7H2O(s) + 0.25 O2(g) = Fe(OH)3(s) metal (i.e, Cu). The intensity band surface and cleavage planes of melan- + SO -2(aq) + 4.5 H2O + 2H+ (aq) (1) decreases either green or blue melante- terite (Fig. 3b). Rozenite crystals occur 4 rite and disappeared in the samples of jointed with melanterite (Fig. 3c). These efflorescent salts are highly solu- rozenite. Also, the ν3 vibration corres- Chalcanthite exhibit the platy shape ble and provide an instantaneous source ponding to sulphate did not exhibit dis- morphology (Fig. 3d). of acidic water upon dissolution and hydrolysis (Nordstrom 1982). The disso- lution of melanterite is independent of the oxygen fugacity (Jerz & Rimstidt). Zinc- or Zn – (Cu) melanterite was formed after the dissolution of green melanteri- te. Isomorphic substitution of Fe+2 for Cu+2 or Zn+2 affects the melanterite dehydration producing a structural reor- ganization of the sulphate tetrahedral. The transformation is accompanied by color changes from green to blue. According to equation (1) more acid is produced and the heavy metals (i.e., Cu+2, Zn+2, etc) from solution will trap in a new structure corresponding to or Zn- (Cu) melanterite or Cu-(Zn) melanterite. Melanterite dehydrates and an incipient dissolution does occur with increasing temperature or decreasing humidity, where rozenite or szomolnokite precipi- tated jointed with green melanterite. Siderotil results either from melanterite or from chalcanthite due to Cu for Fe substitution. During the first stage of heavy metals geochemical cycle in acid mine drainage areas, melanterite is one of the simple sulphates which stored rapidly the heavy metals solubilized in acid mine Fig. 3. Secondary electron microscopy images of blue melanterite (a), incipient dissolution of melanterite (b), inicial stage of rozenite precipitation from melanterite dissolution (c) and chalchantite (d). waters. depósito legal: M-38920-2004 • ISSN: 1885-7264 Comunicación 128 Simple Efflorescent Sulphates from Iberian Pyrite Belt (Portugal) Fig. 4. X-ray maps for blue melanterite (a,b,c and d) related to Fig.3a and chalcantite (e and f) related to Fig.3.d. REFERENCES America, Spec. Publ., 10, 37-56. Alpers, C.N., Blowes, D.W., Nordstrom, D.K. _ (1999): Sulfates. in “Encyclopedia of & Jambor, J.L. (1994): Secondary minerals Environmental Science” D.E. Alexander & and acid mine-water chemistry. in “The R.W. Fairbridge, eds., Kluwer Academic Environmental Geochemistry of Sulfide Publishers, 580-585. Mine-Wastes” J.L. Jambor & D.W. Blowes, eds. Mineralogical Association of Canada., _ & Alpers, C.N. (1999): Negative pH, Short-Course Handbook, 22, 247-270. efflorescent mineralogy, and consequen- ces for environmental restoration at the Buckby, T., Black, S., Coleman, M.L. & Iron Mountain Superfund site, California. Hodson, M.E. (2003): Fe-sulfate-rich evapo- Proceedings of the National Academy of rative mineral precipitates from the Río Sciences, 96, 3455-3462.
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