MACLA 6 XXVI REUNiÓN (SEM) / XX REUNiÓN (SEA) - 2006 THE STABILITY OF SCHWERTMANNITE AND ITS INFLUENCE ON THE CHEMISTRY OF ACID ROCK DRAINAGE IN THE IBER IAN PYRITE BELT C. AYORA (1), P ACERO (1), C. TORRENTÓ (1) y J.M. NIETO ( 2 ) (1) Institut de Ciencies de la Terra Jaume Almera, CSIC, Lluis Solé Sabarís sin, 08028 Barcelona, Spain. E-mail: cayora@ija .csic.es (2) Departamento de Geología, Universidad de Huelva, Campus El Carmen, Av. Fuerzas Armadas sin, 21071 Hue/va The behavior of trace elements in Acid Rock Drainage (ARD) is controlled by several factors, such as mineralogy of the primary ores and enc10sing rocks, weathering conditions and hydrological variability. Moreover, precipitates newly formed from ARD, such as jarosite, schwertmannite and goethite may play a key role in the removal of trace elements from solution. In particular, schwertmannite is a ubiquitous mineral formed from ARD in the Iberian Pyrite Belt, SW Spain, and plays a major role in control ling the water chemistry of many streams. It has been reported, however, that schwertmannite is metastable and transforms into goethite in months (Bigham et al., 1996). The influence of this transformation in the retention of trapped contaminants is a process that is not sufficiently understood. The stability of schwertmannite was investigated in the acid discharge of the Monte Romero abandoned mine, Iberian Pyrite Belt, SW Spain. The mineral precipita tes from supersaturated solutions mainly caused by the oxidation of Fe(II) to Fe(II I), and forms the characteristic terraces (Figure 1). During the transit of the water along the schwertmannite terraces (around 4 hours of residence time), only As and V showed a systematic decrease in concentration, which is in agreement with earlier studies. The remaining contaminants, such as Zn, Al, Cu, Pb, Cd, Ni and Co do not experience any significant variation downstream. This is attributed to the fact that, at acidic pH, only As and V form anions as predominant species in solution, and are expected to be electrostatically attached to the positively charged schwertmannite surface (zero charge pH= 7.2) . Samples of precipitates were taken from the surface and from soe depths down to 20 cm deep from the terraces (Figure 1). Surface samples consisted of a yellow-orange powder and a highly porous and unconsolidated brown solido The samples gained consistency and turned dark brown with depth. Examination of powdered solid samples with XRD revealed that a l l the surface fresh precipitates corresponded to monomineral lic schwertmannite, whereas goethite was the most abundant phase detected in the precipitates underneath. Unlike previous reports, jarosite was always found together with goethite. The inspection of the samples under SEM showed different crystal morpho logy depending on whether they were from the surface or collected at depth. The typical aggregates of spheres with pin-cushion morphology were observed in the surface fresh precipitates, whereas the deeper samples showed similar spherulitic aggregates but made up of elongated and planar crystals (Fig. 2) . In the laboratory, natural schwertmannite was kept in contact with its coexisting acid water in a flask with a solidliquid mass ratio of 1:5 for 353 days. During this time, the pH of the solution dropped from 3.07 to 1 . 74 whereas the concentrations of S04 ' Fe increased, and schwertmannite transformed into goethite plus H 3 0- jarosite during the first five months, and then into only goethite. This evolution is qualitatively simi lar to the results presented by Bigham et al. (1996). Nevertheless, the evolution observed in our experiments is faster than that described by these authors. The fact that in our experiments the transformation was almost complete after only 323 days (vs. 543 days in the work by Bigham et al., 1996) can be due to the use in our study of a lower Fresh preCipitates (Scertmannlte) Aged precipitates (Goethite, Jarosite) Figure 1: Terraces of precipita tes from the acid stream of Monte Romero mine, and sketch of the sampling points. MACLA 6 Página 75