Jarosite (s.s.) belongs to the alunite mineral group, having trigonal symmetry (space group R 3̄ m [4]) and general formula AB₃(TO₄)₂(OH)₆, where A represents a large cation with icosahedral coordination (coordination number CN = 12)—K⁺, Na⁺, NH₄⁺, H₃O⁺, Ag⁺, Tl⁺, Pb²⁺, Bi³⁺, plus minor Ca²⁺, Ba²⁺, Sr²⁺ and trivalent rare-earth ions, e.g., Eu³⁺, B stands for a smaller cation with octahedral coordination (CN = 6)—Fe³⁺ (jarosite s.s.) or Al³⁺ (alunite s.s.), plus minor V³⁺, Fe²⁺,Cu²⁺, Zn²⁺, Mg²⁺—and T represents a tetrahedral cation (CN = 4)—S⁶⁺, As⁵⁺, P⁵⁺. The octahedral B cations are surrounded by two oxygen anions from the tetrahedral groups (TO₄) plus four hydroxyl ions (OH) shared between two neighbour octahedra; A cations are coordinated by six OH anions shared with the octahedral cations and six oxygens from the tetrahedral groups [5].

The alunite supergroup covers more than 40 distinct mineral species [6]. There is a limited solid-solution between end-members with respect to the octahedral B cations—jarosite (Fe) and alunite (Al)—and the same holds for the large icosahedral ions filling the A site—namely, K and Na, particularly for the ferric term.

In fact, the incorporation of lead within the crystal structure of jarosite is limited [7], despite the occurrence of a specific lead mineral—plumbojarosite, Pb_0.5Fe₃(SO₄)₂(OH)₆—to which the same trigonal symmetry was attributed but with a double c-parameter of the hexagonal cell [8]. However, the need for lodging the lone electron pair of Pb²⁺ ions rises a space group ambiguity (centred R 3̄ m or non-centred R 3 m [1]), as quoted for the crystal structure of beudantite, Pb(Fe,Al)₃[(As,S)O₄]₂(OH)₆ [9] and further emphasized in a recent review on the nomenclature of the alunite supergroup [10].

The octahedral crystal field stabilizes the high-spin state ferric ions in the crystal structure of jarosite but the sharing of coordinating oxygen anions with large icosahedral cations is somehow expected to influence the electronic state of iron cations. This aspect can be suitably addressed by studying the X-ray absorption spectrum of iron at the K-edge, whose pre-edge details will reflect the potential variations in the electronic state of the metal ion.

The Portuguese sector of the Iberian Pyrite Belt (IPB) has been exploited since pre-Roman times and during the Roman occupation of Iberia, when gold and silver were extracted from extensive massive gossan caps, particularly the jarosite-rich layer at the base [11].

The modern exploitation of São Domingos mine in Alentejo (northern sector of IPB) started in 1859 for the extraction of copper along with these precious metals and since 1870 also for sulphur production until 1966, when the mine was definitely closed. Records on mine activity allow for estimating the extracted ore (mainly constituted by pyrite, chalcopyrite, sphalerite, galena, arsenopyrite and various sulphosalt minerals) at several million tons. The exploitation of the mining complex at Aljustrel started in 1847 and the mineral assemblage includes additionally tetrahedrite and tennantite. In both cases, it is nowadays envisaged to carry out a complete study of the potential sustainability of recovering scarce and valuable metals from these abandoned mine wastes, in line with EU recent efforts to promote new mineral resources through mining waste materials.

X-ray absorption experiments at the iron K-edge were carried out at the European Synchrotron Radiation Facility (ESRF, in Grenoble, France) using the instrumental set-up of beam line ID-21 [12] by irradiating a small area (∼1 μm²) of well crystallized mineral fragments of K-, Na- and Pb-jarosites, previously studied by X-ray diffraction in the laboratory.

XANES spectra were collected in fluorescence yield (FY) mode with an energy-dispersive high-purity Ge-detector mounted perpendicular to the X-ray beam in the horizontal plane. A fixed-exit Si (220) crystal monochromator assuring an energy resolution of 0.3 eV at the Fe K-edge was used in the energy scans (7,050–7,350 eV). Metallic iron was scanned for energy calibration and pre-edge details of Fe K-edge XANES spectra were deconvoluted into pseudo-Voigt components with Fityk program [13].

Two type-case materials of waste products from old mining sites in southern Portugal were comparatively analysed with the aim of ascertaining the role of jarosite as an efficient Pb-retainer: a reddish dusty residue of roasted material collected at Aljustrel (sample 2–1) and a silty greyish material from a sampled profile at Achada do Gamo, São Domingos mine (sample 3–5).

Common X-ray laboratory techniques were applied to characterize these samples: X-ray diffraction (XRD) for phase identification and X-ray fluorescence spectrometry in wavelength dispersive mode (XRF-WDS) to assess the chemical constitution and comparatively estimate elemental contents.

A powder diffractometer with Bragg-Brentano geometry operating at 50 kV and 40 mA and equipped with a large-anode copper tube and a curved graphite crystal monochromator was used to collect X-ray diffraction patterns from sampled wastes.

A Philips PW1400 automated X-ray fluorescence spectrometer equipped with a rhodium tube and various analysing crystals was used to scan the angular intervals (2θ) suitable for element detection through the scanning of selected emission lines (Table 1).

Thermo-analytical techniques—simultaneous differential thermal analysis (DTA) and thermo-gravimetry (TG)—were applied to compare the thermal evolution of the samples using a SETARAM 92-16.18 apparatus that incorporates a microbalance with a controlled gas flow of argon (inert atmosphere). About 100 mg of sample were deposited in an alumina (α-Al₂O₃) crucible. The reference material was alumina powder and the temperature ranged from ambient to 650 °C at a heating rate of 10 °C min⁻¹. After the DTA/TG test the phase constitution of the heated material was monitored by XRD and compared to the original mineralogical constitution in order to interpret the registered energy variations and mass losses.