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Mineral synthesis using the coupled dissolution-reprecipitation route

Brugger, J., Xia, F.ORCID: 0000-0002-4950-3640 and Pring, A. (2008) Mineral synthesis using the coupled dissolution-reprecipitation route. Geochimica et Cosmochimica Acta, 72 (12). A118.

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Replacement reactions (‘pseudomorphism’) are common in nature, occurring under a large range of conditions (e.g., replacement of pentlandite by violarite in the supergene zone of ore deposits; leucite by analcime under hydrothermal conditions). Replacement reactions of this kind are often assumed to proceed by solid-state diffusion of the metal ions through the mineral. However, Putnis and co-authors have recently suggested that many mineral replacements are the product of coupled dissolution and reprecipitation (CDR) reactions: the replaced mineral is fully dissolved into a fluid, and this dissolution is tightly coupled with the precipitation of the replacing phase across the reaction front. The kinetics and chemistry of the CDR route are fundamentally different from solid-state diffusion: (i) reaction kinetics is not simply temperature dependant, but also depends upon the solution chemistry (e.g., pH, fO2(g), metal and ligand concentrations). The kinetics of reaction is also dependant upon the transport properties of the fluid and the texture of the starting material, because the fluid needs to access the reaction front. (ii) As the fluid is transporting metals and other elements to and from the reaction front, the chemistry of the product can be different from that of the precursor, and the volume of reaction cannot be estimated simply by comparing the molar volumes of the product and precursor. The product usually displays high porosity, even if the product has higher molar volume than the precursor.

Two thiospinels, violarite (Ni,Fe)3S4 and linnaeite Co3S4were synthesized using pentlandite (Fe,Ni)9S8 and cobaltpentlandite Co9S8 as precursors. The Fe/Ni ratio of violarite can be adjusted by varying the reaction conditions, e.g., temperature (125-145 ºC), pH (2.90, 3.90, 5.00), and precursor stoichiometry ((FexNi1-x)9S8, x=0.4, 0.5, 0.55, 0.6). Synthesis times range from 5 to 20 days, compared to the traditional dry synthesis route for violarite that requires three months annealing to obtain a product of only 72 ± 5 wt % purity. This synthesis route is ideal for preparing compounds with low thermal stabilities (< 500 ºC).

We also studied the effect of pH on the transformation of leucite KAlSi2O6 to analcime NaAlSi2O6•H2O, showing that the reaction mechanism

Item Type: Journal Article
Publisher: Elsevier BV
Copyright: © 2008 Published by Elsevier Ltd.
Other Information: Conference: 18th Annual V M Goldschmidt Conference Location: Vancouver, CANADA
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