Hemimorphite, Zn4Si2O7(OH)2·H2O, is one of the most common minerals in non-sulfide Zn deposits, together with hydrozincite and smithsonite. Precipitation of hemimorphite during the supergene evolution of non-sulfide Zn deposits is a debated issue (Choulet et al., 2014; McPhail et al., 2003) in which the scientific community has given till now a traditional inorganic geochemical explanation and, despite their importance for the development of ore genesis and metal dispersion models, the stability properties of hemimorphite are poorly known. Our research is right addressed to understanding the hemimorphite stability performing solubility experiments in free-drift batch reactors on inorganic hemimorphite, from a supergene non-sulfide Zn deposit, and biological hemimorphite precipitated by bacterial activity. Solid samples were characterized, before and after the solubility experiments, exploiting laboratory and synchrotron radiation based multi-technique approach that combines X-ray powder diffraction (XRPD), Scanning Electron Microscopy (SEM), and X-ray Absorption Spectroscopy (XAS). The calculated solubility product constants (logKs) are similar for both “geological” (logKs = 30.3±0.4) and biogenic hemimorphite (logKs = 30.5±0.1). We demonstrate that the solubility experiments trigger an amorphous to crystalline phase transition of biological hemimorphite, while mineralized bacterial sheaths and organic filaments, that allowed us to demonstrate its biological origin, were dissolved and no longer recognizable by Scanning Electron Microscopy. This study provides new thermodynamic data on hemimorphite allowing us to present improved predominance diagrams (PCO2 vs. pH) for the chemical system Zn – O – H – C – Si, which may be useful for developing genetic models of ore deposits, for predicting mineral dissolution/precipitation reactions and for investigating metal dispersion processes. The amorphous to crystalline state transition of bio-hemimorphite associated to the obliteration of its biomorphological features, open up the question if bacteria could have played a role in the formation of hemimorphite deposits. Such interaction between biosphere and geosphere raises an important question about the possibility of using endemic bacteria to develop more sustainable technologies for Zn abatement and recovery in abandoned mine systems.
Supergene non-sulfide Zn deposits: could hemimorphite have a biological origin?
Medas D.
;Podda F.;Meneghini C.;De Giudici G.
2017-01-01
Abstract
Hemimorphite, Zn4Si2O7(OH)2·H2O, is one of the most common minerals in non-sulfide Zn deposits, together with hydrozincite and smithsonite. Precipitation of hemimorphite during the supergene evolution of non-sulfide Zn deposits is a debated issue (Choulet et al., 2014; McPhail et al., 2003) in which the scientific community has given till now a traditional inorganic geochemical explanation and, despite their importance for the development of ore genesis and metal dispersion models, the stability properties of hemimorphite are poorly known. Our research is right addressed to understanding the hemimorphite stability performing solubility experiments in free-drift batch reactors on inorganic hemimorphite, from a supergene non-sulfide Zn deposit, and biological hemimorphite precipitated by bacterial activity. Solid samples were characterized, before and after the solubility experiments, exploiting laboratory and synchrotron radiation based multi-technique approach that combines X-ray powder diffraction (XRPD), Scanning Electron Microscopy (SEM), and X-ray Absorption Spectroscopy (XAS). The calculated solubility product constants (logKs) are similar for both “geological” (logKs = 30.3±0.4) and biogenic hemimorphite (logKs = 30.5±0.1). We demonstrate that the solubility experiments trigger an amorphous to crystalline phase transition of biological hemimorphite, while mineralized bacterial sheaths and organic filaments, that allowed us to demonstrate its biological origin, were dissolved and no longer recognizable by Scanning Electron Microscopy. This study provides new thermodynamic data on hemimorphite allowing us to present improved predominance diagrams (PCO2 vs. pH) for the chemical system Zn – O – H – C – Si, which may be useful for developing genetic models of ore deposits, for predicting mineral dissolution/precipitation reactions and for investigating metal dispersion processes. The amorphous to crystalline state transition of bio-hemimorphite associated to the obliteration of its biomorphological features, open up the question if bacteria could have played a role in the formation of hemimorphite deposits. Such interaction between biosphere and geosphere raises an important question about the possibility of using endemic bacteria to develop more sustainable technologies for Zn abatement and recovery in abandoned mine systems.
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