Thallium and co-genetic trace elements in hydrothermal Fe-Mn deposits of Central Spain.

Thallium (Tl) is a hazardous trace metal that can harm human and environmental health. Tl pollution can result from the mining and smelting of Tl-bearing minerals, but also the natural weathering of Tl-bearing sulfide minerals may induce Tl release to the environment. In this study, hydrothermal deposits hosted in dolostone rocks sited along fossil thermal springs in the Lodares region (Soria province, central Spain) were studied. In this hydrothermal mineralization zone, Tl association with primary minerals, identified Tl-bearing secondary products resulting from natural weathering of primary minerals, as well as the dispersion from its natural source along a seasonal small streambed were explored. Samples were analyzed by chemical, microscopic and spectroscopic techniques and epithermal pyrite, sphalerite, galena and barite and secondary gypsum, jarosite, scorodite, anglesite, goethite, epsomite and elemental sulfur produced by both inorganic and bacterial processes were found. The highest Tl contents were found in hydrothermal pyrite (188 mg kg-1), jarosite (142 mg kg-1), Mn-oxides (27 mg kg-1) or kerogen (13 mg kg-1). Feldspar was identified by electron probe microanalysis as potential host phase of Tl. XANES results confirmed the association of Tl(I) with metal sulfides in pyrite-rich samples and highlighted the role of jarosite-like minerals for Tl(I) sequestration upon pyrite oxidation, even in carbonate-rich samples at near-neutral pH. In addition to micaceous minerals, jarosite-group minerals and K-feldspars may contribute to the natural attenuation of Tl in soils and sediments.

Historical roasting of thallium- and arsenic-bearing pyrite: Current Tl pollution in the Riotinto mine area.

Samples of an open-air pyrite roasting heap from the 19th century in the Riotinto mine area (SW Spain) and surrounding sediments and soil along a seasonal surface runoff channel were analyzed to study thallium (Tl) phase transformations during historical roasting of Tl-bearing arsenian pyrite, secondary weathering processes, Tl dispersion and current environmental pollution. Results from Electron Probe Microanalyses (EPMA) indicate an even distribution of Tl in pyrite grains of an ore sample (22 mg kg-1 total Tl), suggesting that Tl is incorporated in the pyrite structure rather than in discrete Tl-sulfide microparticles. The roasting residue (122 mg kg-1 total Tl) consists mainly of hematite. EPMA suggested that Tl in the roasting residue and contaminated soil was co-occurring with Fe oxide particles, with a mean Tl point concentration of 0.12% in samples from the roasting residues. Total concentrations of Tl in soil samples decrease with distance from the roasting heap to 14 mg kg-1. X-ray absorption near-edge structure (XANES) spectra collected on pyrite roasting residue and a soil sample suggest that most Tl is Tl(I) substituting K in jarosite. Sequential extractions show that most Tl (85-99%) in the soil and sediment samples is concentrated in the residual fraction and, thus, is rather strongly bound. Lastly, water extracts indicate that colloidal particles (i.e. <1 µm size) may contribute to the dispersion of Tl around and away from the roasting heaps.

Chemistry and phase evolution during roasting of toxic thallium-bearing pyrite.

In the frame of a research project on microscopic distribution and speciation of geogenic thallium (Tl) from contaminated mine soils, Tl-bearing pyrite ore samples from Riotinto mining district (Huelva, SW Spain) were experimentally fired to simulate a roasting process. Concentration and volatility behavior of Tl and other toxic heavy metals was determined by quantitative ICP-MS, whereas semi-quantitative mineral phase transitions were identified by in situ thermo X-Ray Diffraction (HT-XRD) and Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) analyses after each firing temperature. Sample with initial highest amount of quartz (higher Si content), lowest quantity of pyrite and traces of jarosite (lower S content) developed hematite and concentrated Tl (from 10 up to 72 mg kg-1) after roasting at 900 °C in an oxidizing atmosphere. However, samples with lower or absent quartz content and higher pyrite amount mainly developed magnetite, accumulating Tl between 400 and 500 °C and releasing Tl from 700 up to 900 °C (from 10-29 mg kg-1 down to 4-1 mg kg-1). These results show the varied accumulative, or volatile, behaviors of one of the most toxic elements for life and environment, in which oxidation of Tl-bearing Fe sulfides produce Fe oxides wastes with or without Tl. The initial chemistry and mineralogy of pyrite ores should be taken into account in coal-fired power stations, cement or sulfuric acid production industry involving pyrite roasting processes, and steel, brick or paint industries, which use iron ore from roasted pyrite ash, where large amounts of Tl entail significant environmental pollution.

Low-magnesium uranium-calcite with high degree of crystallinity and gigantic luminescence emission.

Cabrera (Madrid) low-Mg calcites exhibit: (i) an unusual twofold elevation in X-ray diffraction pattern intensity; (ii) a 60-fold elevation of luminescence emission, compared to six common natural calcites selected for comparison purposes; (iii) a natural relatively high radiation level of circa 200 nSvh(-1) not detected in 1300 other calcites from the Natural History Museum of Madrid. Calcites were analysed by the X-ray diffraction powder method (XRD), cathodo-luminescence spectroscopy in scanning electron microscopy (CL-SEM), thermoluminescence (TL), differential thermal analysis (DTA), X-ray fluorescence spectrometry (XRF) and particle size distribution (PSD). The Cabrera calcite study shows: (i) helicoidally distributed steps along the (0001) orientation; (ii) protuberance defects onto the (0001) surface, observed by SEM; (iii) XRF chemical contents of 0.03% MgO, 0.013% of Y(2)O(3), and 0.022% of U(3)O(8), with accessory amounts of rare earth elements (REE); (iv) DTA dissociation temperature of 879 degrees C; (v) TL maxima peaks at 233 and 297 degrees C whose areas are 60 times compared to other calcites; (vi) spectra CL-SEM bands at 2.0 and 3.4 eV in the classic structure of Mn(2+) activators; (vii) a twofold XRD pattern explained given that sample is a low-Mg calcite. The huge TL and CL emissions of the Cabrera calcite sample must be linked with the uranyl group presence. This intense XRD pattern in low-Mg calcites could bring into being analytical errors.