Secondary sulphate minerals from Bhanine Valley coals (South Lebanon) – a crystallochemical and geochemical study

Abstract

Rich efflorescences of various Fe and Al sulphate mineral mixtures on coal seams of  Bhanine, South Lebanon, were examined using (1) Powder X-Ray Diffraction (with the Rietveld method and unit cell parameters calculation), (2) Scanning Electron Microscopy with standardized Electron-Dispersive Spectroscopy system, and (3) Inductively-Coupled Plasma Mass Spectroscopy. The sulphates most likely originated from coal-contained pyrite to form Fe(II) sulphates (melanterite, rozenite, and the most common szomolnokite), followed by Fe3+-rich sulphates (coquimbite group, copiapite group) and Al sulphates (alunogen, tamarugite). The halotrichite group and minor voltaite, metavoltine, and possibly secondary rozenite and szomolnokite were the last species to be formed. Strong enrichment in Al in copiapites and coquimbites, common occurrence of aluminocoquimbite, and Al likely entering the structure of Fe(II) sulphates makefurther phenomena, during which the initial ferrous copiapites were oxidized in the presence of Al-rich solutions, not out of the question. The obtained unit cell parameters sometimes stand for threshold values in the literature-based ranges drawn, but the values are usually below the 2% discrepancy. The Bhanine sulphates bear relatively large amounts of Tl, Hg, and Co when compared to Coal Clarke and mean crustal abundancies, being also moderately enriched in Ni and As.