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Iron sulfates represent an essential sink for the toxic element arsenic in arid and semi-arid mining areas with high evaporation rates. Information about the structural incorporation of As5+ in iron sulfates, however, remains scarce. Here we present evidence for the heterogeneous substitution of S6+ by As5+ in the crystal structure of rhomboclase ((H5O2)Fe3+(SO4)(2)center dot 2H(2)O) and its dehydration product (H3O) Fe(SO4)(2). Rhomboclase (Rhc) was synthesized in the presence of As5+ with molar As/Fe ratios of 0, 0.25, 0.5, 0.75 and 1.0, resulting in As loads of 0.0, 0.93, 1.44, 1.69 and 1.87 wt.%, respectively. The unit cell parameters of Rhc increase from 9.729(6), 18.303(2), and 5.432(1) angstrom for a, b, and c, to 9.745(9),18332(5), and 5.436(8) angstrom when Rhc is crystallized at a molar As/Fe ratio of 1. Simultaneously, the crystallite size decreased from 304 to 176 nm. In situ dehydration of Rhc to (H3O)Fe(SO4)(2), investigated by powder X-ray diffraction, shows that Rhc starts to dehydrate at 76 degrees C, which is completed at 86 degrees C. The presence of As5+ does not impact the start or end temperatures of Rhc dehydration but does accelerate the dehydration. Xray absorption fine structure spectroscopy (EXAFS) reveals that S6+, in the Rhc and (H3O)Fe(SO4)(2) structure, is replaced by As5+, while the polymerization of AsO(4-)tetrahedra and FeO(6-)octahedra during the formation of (H3O)Fe(SO4)(2) results in a strong distortion of the AsO(4-)tetrahedron. (C) 2015 Elsevier Ltd. All rights reserved.