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American Mineralogist; May 2008; v. 93; no. 5-6; p. 928-939; DOI: 10.2138/am.2008.2750
© 2008 Mineralogical Society of America
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Interaction of gypsum with As(V)-bearing aqueous solutions: Surface precipitation of guerinite, sainfeldite, and Ca2NaH(AsO4)2·6H2O, a synthetic arsenate

Juan D. Rodríguez1, Amalia Jiménez1,*, Manuel Prieto1, Laura Torre2 and Santiago García-Granda2

1 Departamento de Geología, University of Oviedo, Jesús Arias de Velasco, Oviedo, Asturias 33005, Spain
2 Departamento de Química Física y Analítica, University of Oviedo, Julian Clavería s/n Oviedo, Asturias 33006, Spain

Correspondence: * E-mail: amalia.jimenez{at}geol.uniovi.es

The interaction of arsenate-bearing aqueous solutions with gypsum at a starting pH of 9 and 25 °C results in surface precipitation of guerinite, Ca5(HAsO4)2(AsO4)2·9H2O, sainfeldite, Ca5(HAsO4)2(AsO4)2·4H2O, and occasionally Ca2Na(HAsO4)(AsO4)·6H2O, a new arsenate. These three solid phases are characterized by the simultaneous presence of HAsO42– and AsO43– groups in their structure, which is explainable since crystallization occurs within a pH range in which both HAsO42– and AsO43– are available in the aqueous solution. The interaction leads to a decrease in the As(V) concentration in the aqueous phase to reach values controlled by the solubility of these solid phases. The study combines several macroscopic experiments, in which changes in the solution chemistry are monitored as a function of time, with the characterization of solid phases by SEM-EDS and XRD. The crystal morphologies of the precipitating phases are interpreted on the basis of their respective structures. The thermodynamic solubility products of both guerinite and the new arsenate have been determined, being 10–31.17±0.05 and 10–13.83±0.03, respectively. The reaction paths followed by the system and the equilibrium endpoints have been modeled using the geochemical code PHREEQC.

Key Words: Crystal growth • calcium arsenate • analysis chemical • sainfeldite • phase equilibrium • guerinite • gypsum






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