Abstract
The crystallization behaviour of jarosite and schwertmannite has been studied by precipitation-aging experiments performed using different parent-solution concentrations at acidic conditions and ambient temperature. Schwertmannite exhibits low crystallinity and is the only mineral identified during low-concentration (LC) experiments. However, in high-concentration (HC) experiments, a relatively rapid Ostwald ripening process leads to the transformation of schwertmannite into natrojarosite. The presence of sodium modifies the morphology and stability of the obtained phases. TEM observations reveal that schwertmannite particles consist of disoriented nanodomains (~6 nm) spread in an amorphous mass. In contrast, natrojarosite particles exhibit a single-domain, highly crystalline core, with the crystallinity decreasing from core to rim. The thermal behaviour of these phases depends on both their composition and their degree of crystallinity. TG and DTG analyses show that, below 500 °C, the amount of structural water is clearly higher in schwertmannite than in natrojarosite. The present results highlight the role of the ripening processes in epigenetic conditions and could be important in interpreting the formation of jarosite in Earth and Martian surface environments.
Highlights
Fe-bearing minerals such as oxides, hydroxides, oxyhydroxides, and sulphates are common in the Earth‘s lithosphere, hydrosphere, atmosphere and biosphere, where they participate in diverse biogeochemical processes [1]
The most pronounced decrease takes place during the first day of aging and the ion concentration and pH values fall slowly until the end of the reaction period. These results indicate that the evolution of the physic-chemical properties of aqueous phases is related to the different concentration of the parent solutions and the nature of the precipitates obtained during aging at various reaction times
The thermogravimetric results confirm the X-ray diffraction (XRD) results pure natural natrojarosites. These results indicate that jarosite can be considered the most abundant showing similitudes between natrojarosite obtained by precipitation and pure natural natrojarosites
Summary
Fe-bearing minerals such as oxides, hydroxides, oxyhydroxides, and sulphates are common in the Earth‘s lithosphere, hydrosphere, atmosphere and biosphere, where they participate in diverse biogeochemical processes [1]. The weathering of ferrous sulphide minerals (pyrite, chalcopyrite, pyrrhotite, mackinawite, marcasite and arsenopyrite) is of particular interest because it controls the formation of large amounts of insoluble and fairly soluble Fe-bearing secondary phases (mainly ferric hydroxides and hydroxysulphates) through processes mediated by bacterial activity in both natural systems and industrial wastewaters [2]. These secondary minerals are formed by Fe(III), are poorly soluble [3] and (with the exception of schwertmannite and ferrihydrite) have a well-developed crystalline structure. The implication of iron-bearing minerals in both early weathering processes and hydrothermal mineral genesis is one of the reasons of their prospection and mining
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