Advances and Gaps in the Knowledge of Thermodynamics and Crystallography of Acid Mine Drainage Sulfate Minerals

Acidic and metal-rich waters produced by sulfide decomposition at mining sites are termed acid mine drainage (AMD). They precipitate a number of minerals, very often sulfates. The recent advances in thermodynamic properties and crystallography of these sulfates are reviewed here. There is a reasonable amount of data for the divalent (Mg, Ni, Co, Fe2+, Cu, Zn) sulfates and these data may be combined with and optimized by temperature-relative humidity brackets available in the literature. For the sulfates with Fe3+, most data exist for jarosite; for other minerals and phases in this system, a few calorimetric studies were reported. No data whatsoever are available for the Fe2+-Fe3+ sulfates. A significant advance is the development of the Pitzer model for Fe3+-sulfate solutions and its confrontation with the available thermodynamic and solubility data. In summary, our knowledge about the thermodynamic properties of the AMD sulfates is unsatisfactory and fragmented

The effect of chemical variability and weathering on Raman spectra of enargite and fahlore

Enargite (Cu3AsS4) and tennantite (Cu12As4S13) are typical As-bearing sulfides in intermediate-and high-sulfidation epithermal deposits. Trace and major element variations in enargite and tennantite and their substitution mechanisms are widely described. However, Raman spectra of the minerals with correlative quantitative chemical information are rarely documented, especially for enargite. Therefore, comparative electron and μ-Raman microprobe analyses were performed on enargite and fahlore grains. These spectra can be used in the industrial detection and subsequent removal of As-bearing sulfides prior to ore beneficiation in order to diminish the environmental impact of the metallurgical technologies. A simple Sb5+-As5+ substitution in enargite was confirmed by Raman analyses. Similarly, a complete solid solution series from tetrahedrite to tennantite (i.e., Sb3+-As3+ substitution) can be correlated with a gradual evolution in their Raman spectra. In turn, Te4+ occupies the As3+ and Sb3+ sites in fahlore by the coupled substitution Te4+gCu+g(As, Sb)3+g(Cu, Fe, Zn)2+. Accordingly, Raman bands of goldfieldite (Te-rich member) are strongly broadened compared with those of tetrahedrite and tennantite. A secondary phase with high porosity and a fibrous or wormlike texture was found in enargite in a weathered sample. The chemical composition, Raman spectrum, and X-ray diffraction signature of the secondary phase resemble tennantite. A gradual transformation of the primary enargite into this secondary phase was visualized by comparative electron and Raman microprobe mapping

Isotope diffusion and re-equilibration of copper and evaporation of mercury during weathering of tetrahedrite in an oxidation zone

To understand the mobility of heavy metals during oxidative weathering of sulfides, we investigated weathering processes of tetrahedrite [(Cu,Fe,Zn,Hg)12(Sb,As)4S13] in an oxidation zone with abundant siderite (FeCO3) and baryte (BaSO4) at Rudňany (Slovakia). The focus of this work lied in the isotopic (δ65Cu, δ202Hg, δ34S) variations of the minerals during weathering and the interpretation of such changes. In the studied oxidation zone, Hg-rich tetrahedrite converts in situ to pockets of powdery cinnabar (HgS) and an X-ray amorphous mixture rich in Sb, Fe, and Cu that slowly re-crystallizes to Cu-rich tripuhyite (FeSbO4). Copper is mobile and precipitates as malachite [Cu2(OH)2(CO3)], azurite [Cu3(OH)2(CO3)2], or less abundant clinoclase [Cu3(AsO4)(OH)3]. The isotopic composition (δ65Cu) of tetrahedrite correlates well with the degree of weathering and varies between 0.0 ‰ and −4.0 ‰. This correlation is caused by isotopic changes during dissolution and subsequent rapid equilibration of δ65Cu values in the tetrahedrite relics. Simple diffusion models showed that equilibration of Cu isotopic values in the tetrahedrite relics proceeds rapidly, on the order of hundreds or thousands of years. Abundant secondary iron oxides draw light Cu isotopes from the aqueous solutions and shift the isotopic composition of malachite and azurite to higher δ65Cu values as the distance to the primary tetrahedrite increases. Clinoclase and tripuhyite have lower δ65Cu values and are spatially restricted near to the weathering tetrahedrite. The Hg and S isotopic composition of tetrahedrite is δ202Hg = −1.27 ‰, δ34S = −1.89 ‰, that of the powdery secondary cinnabar is δ202Hg = +0.07 ‰, δ34S = −5.50 ‰. The Hg isotopic difference can be explained by partial reduction of Hg(II) to Hg(0) by siderite and the following evaporation of Hg(0). The S isotopic changes indicate no involvement of biotic reactions in the oxidation zone, probably because of its hostility owing to high concentrations of toxic elements. This work shows that the Cu isotopic composition of the primary sulfides minerals changes during weathering through self-diffusion of Cu in those minerals. This finding is important for the use of Cu isotopes as tracers of geochemical cycling of metals in the environment. Another important finding is the Hg in the oxidation zones evaporates and contributes to the global cycling of this element through atmospheric emission

Changes in antimony isotopic composition as a tracer of hydrothermal fluid evolution at the Sb deposits in Pezinok (Slovakia)

In this work, we investigated in situ isotopic compositions of antimony (Sb) minerals from two substages of the ore deposits near Pezinok (Slovakia). The δ123Sb values of the primary Sb minerals range from −0.4 and +0.8‰ and increase progressively along the precipitation sequence. In the substage II, the early-formed gudmundite (FeSbS) shows in all sections the lowest δ123Sb values, followed by berthierite (FeSb2S4), stibnite (Sb2S3), and valentinite (Sb2O3) with the heaviest δ123Sb values. A similar trend was observed for the substage III, from the initially-formed stibnite, followed by kermesite (Sb2S2O), valentinite, senarmontite (both Sb2O3), and schafarzikite (FeSb2O4). The evolution can be rationalized by a Rayleigh fractionation model with a starting δ123Sb value in the fluid of +0.3‰, applying the same mineral-fluid fractionation factor to all minerals. Thus, the texturally observed order of mineralization is confirmed by diminishing trace element contents and heavier δ123Sb values in successively crystallized Sb minerals. Antimony in substage III was likely supplied from the oxidative dissolution of stibnite that formed earlier during substage II. The data interpretation, although limited by the lack of reliable mineral-fluid fractionation factors, implies that Sb precipitation within each substage occurred from an episodic metal precipitation, likely associated with a similar Sb isotope fractionation between fluid and all investigated Sb minerals. Large isotopic variations, induced by precipitation from a fluid as a response to temperature decrease, may be an obstacle in deciphering the metal source in hydrothermal ore deposits. However, Sb isotopes appear to be an excellent instrument to enhance our understanding on how hydrothermal systems operate

INCORPORATION OF Mo6+^{6+} IN FERRIHYDRITE, GOETHITE, AND HEMATITE

Among all iron oxides, hematite (α-Fe2O3), goethite (α-FeOOH), and ferrihydrite (FeOOH⋅nH2O) are the most common mineral species. While immobilization of Mo6+ by surface adsorption on ferric oxides has been studied extensively, the mechanisms of incorporation in their structure have been researched little. The objective of this study was to investigate the relation between Mo content and its structural incorporation in hematite, goethite, and six-line ferrihydrite by a combination of X-ray absorption spectroscopy (XAS), powder X-ray diffraction (pXRD), and inductively-coupled plasma optical emission spectrometry (ICP-OES). Synthesized in the presence of Mo, the hematite, goethite, and six-line ferrihydrite phases incorporated up to 8.52, 0.03, and 17.49 wt. % Mo, respectively. For hematite and goethite, pXRD analyses did not indicate the presence of separate Mo phases. Refined unit-cell parameters correlated with increasing Mo concentration in hematite and goethite. The unit-cell parameters indicated an increase in structural disorder within both phases and, therefore, supported the structural incorporation of Mo in hematite and goethite. Analysis of pXRD measurements of Mo-bearing six-line ferrihydrites revealed small amounts of coprecipitated akaganéite. X-ray absorption near edge structure (XANES) measurements at the Mo L3-edge indicated a strong distortion of the MoO6 octahedra in all three phases. Fitting of extended X-ray absorption fine structure (EXAFS) spectra of the Mo K-edge supported the presence of such distorted octahedra in a coordination environment similar to the Fe position in the investigated specimen. Incorporation of Mo6+ at the Fe3+-position for both hematite and goethite resulted in the formation of one Fe vacancy in close proximity to the newly incorporated Mo6+ and, therefore, charge balance within the hematite and goethite structures