Boletín de la Sociedad Geológica Mexicana

Volumen 67, núm. 3, 2015, p. 479-491

Identification of diagenetic calcium arsenates using synchrotron-based micro X-ray diffraction

Francisco Castillo1,2,3, Miguel Avalos-Borja4,5, Heather Jamieson6, Gerardo Hernández-Bárcenas1, Nadia Martínez-Villegas1,*

1 IPICyT, Instituto Potosino de Investigación Científica y Tecnológica, División de Geociencias Aplicadas, Camino a la Presa San José No. 2055, Col. Lomas 4a Sec. 78216, San Luis Potosí, SLP, México.
2 IPICyT, Instituto Potosino de Investigación Científica y Tecnológica, División de Materiales Avanzados, San Luis Potosí, SLP, México.
3 CONACYT Research Fellow, Instituto de Geología, Universidad Autónoma de San Luis Potosí.
4 Centro de Nanociencias y Nanotecnología-UNAM, Ensenada, BC, México.
5 On leave at IPICyT, Instituto Potosino de Investigación Cientifica y Tecnológica, División de Materiales Avanzados, San Luis Potosí, SLP, México.
6 Department of Geological Engineering, Queen´s University, Kingston, ON, Canada.

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In this work, we identify the type of calcium arsenates found in sediment samples from an aquifer located in Matehuala, San Luis Potosí, México. Sediments in contact with levels up to 158 mg/L of arsenic in neutral pH water were studied by X-ray diffraction, scanning electron microscopy coupled to energy dispersive X-ray analyses (SEM-EDS), and synchrotron based X-ray diffraction. Identification of these calcium arsenates by X-ray analysis has proved to be very difficult to achieve because the precipitates of interest are on the microscale and immerse in a matrix of calcite, gypsum, and quartz comprising nearly 100 % of the samples. Needle-like specimens composed of calcium, arsenic, and oxygen were, however, commonly observed in sediment samples during SEM-EDS analyses in backscattered mode. Synchrotron based X-ray analyses revealed some peaks that were compared with published data for guerinite, haindingerite, and pharmacolite suggesting that these were the calcium arsenates present in sediments, the calcium arsenates that control the solubility of arsenic in the contaminated aquifer in Matehuala, and the calcium arsenates that prevail in the long-term in the environment after cycles of dissolution and precipitation. The identification of these calcium arsenates is consistent with the environmental conditions prevailing at the study area and the SEM-EDS observations. However, its identification is not unequivocal as the comparison of experimental data collected in single crystal specimens against X-ray diffraction references collected in powders prevents a strictly proper identification of the specimens analyzed. In this way, scorodite was also identified by synchrotron based X-ray analyses however its presence is inconsistent with the environmental conditions and the calcium arsenate associations found in this study. Scorodite identification was therefore considered tentative. A thorough examination, with additional and/or improved analytical techniques, should be undertaken to find an environmentally sound explanation for the diffraction peaks assigned to scorodite, which might be from a clay a mineral, probably with no arsenic.

Keywords: calcium arsenates, arsenic contamination, guerinite, haindingerite, pharmacolite, diagenetic calcium arsenates.