Abstract
Abstract The crystal structure of natural krautite, MnHAsO4(H2O), was re-evaluated from a cotype specimen, confirming the previously reported monoclinic symmetry for this mineral (space group P21, Z = 8, a = 8.0093(5), b = 15.9372(10), c = 6.8065(4) Å, β = 96.534(2)° at room temperature, 5662 structure factors, 302 parameters, R1 = 0.0295, wR2 = 0.0770). Although hydrogen atoms could not be located from the single crystal X-ray diffraction study, the higher accuracy and precision of the results allowed to derive the hydrogen-bonding scheme (O⋯O = 2.55–2.90 Å) in the crystal structure of krautite. Crystals of synthetic MnHAsO4(H2O) were grown by mixing aqueous solutions of NH4H2AsO4 and MnSO4 and keeping the formed gel at 105 °C for several days. The obtained triclinic crystals were systematically and polysynthetically twinned by contact on (010). Separation of reflections from two individual domains made it possible to determine and refine the crystal structure (space group P 1 ‾ $P\overline{1}$ , Z = 8, a = 8.0105(16), b = 15.991(4), c = 6.8029(12) Å, α = 92.635(8), β = 96.534(2), γ = 90.151(8)° at room temperature, 7226 structure factors, 255 parameters, R1 = 0.0445, wR2 = 0.1381). The triclinic polymorph of MnHAsO4(H2O) does not show a direct group-subgroup relation with monoclinic krautite. Triclinic MnHAsO4(H2O) is closely related with other triclinic M IIHAsO4(H2O) (M = Co, Cu, Zn, Mg) mineral phases. Quantitative structural comparisons between the five M IIHAsO4(H2O) compounds revealed a high similarity between the Mn and Co members, and between the Zn and Mg members, respectively. Subtle distinctions between the two pairs are ascribed to a different hydrogen-bonding scheme. Although the Cu member has a similar hydrogen-bonding scheme as the Mn and Co pair, its structural similarity with triclinic MnHAsO4(H2O) is low due to the strain of the crystal structure caused by the Jahn-Teller distortions of the [CuO6] octahedra.
Highlights
A multitude of anhydrous or hydrous phases exist in the system Mn/As/O/(H), both in the form of minerals or as synthetic compounds
Due to the lack of suitable natural material at that time, the crystal structure of MnHAsO4(H2O) was subsequently solved from singlecrystal X-ray data of a synthetic sample in space group P21 [25], confirming the monoclinic symmetry reported in the original description [38]
It should be noted that crystals prepared in the same way at 90 °C have monoclinic symmetry, just like single crystals grown from a gel at room temperature [25, 45] or polycrystalline material obtained by evaporation of the solvent at 60 °C [46]
Summary
A multitude of anhydrous or hydrous phases exist in the system Mn/As/O/(H), both in the form of minerals or as synthetic compounds. Arsenic remains one of the naturally most abundant hazardous substances [3], with particular risks for poisoning drinking water sources in some areas of the world like in Bangladesh [4] For identification of these inorganic phases in soils, detailed knowledge of their formation conditions and crystal structures is required. The great number of naturally occurring arsenic compounds can be explained by the ability of arsenic acid to form different types of anions that can be present as isolated tetrahedral species like AsO43−, HAsO42−, H2AsO4− or as condensed anions with arsenic in a tetrahedral and/or an octahedral coordination by oxygen [5] Another reason for the diversity within the system Mn/As/O/(H) pertains to the different stable oxidation states that both Mn and As can exhibit, viz. A more accurate crystal-structure refinement of both natural and synthetic krautite from singlecrystal data deemed appropriate The results of these studies are presented
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