Two samples of vladimirite, one from Bou Azzer, Morocco, and the other from a new occurrence in Copiapó, Chile (designated as R100075 and R080001, respectively), were examined with an electron microprobe, single-crystal X-ray diffraction, and Raman spectroscopy. Our results show that vladimirite is monoclinic with space group P21/c and unit-cell parameters a 5.8279(2), b 10.1802(4), c 22.8944(10) Å, β 96.943(2)°, and V 1348.35(9) Å3 for R100075 and a 5.8220(1), b 10.1750(2), c 22.8816(6) Å, β 96.902(1)°, and V 1345.66(5) Å3 for R080001. The structure determinations, with R1 = 0.022 and 0.023 for R100075 and R080001, respectively, yielded an ideal chemical formula Ca4(AsO4)2(AsO3OH)·4H2O (Z = 4) for this mineral, in contrast to Ca5(AsO4)2(AsO3OH)2·5H2O (Z = 3) documented in the literature. The chemical compositions for R100075 and R080001 are Ca4.03(AsO4)2(As0.99O3OH)·4H2O and Ca3.97(AsO4)2(As1.01O3OH)·4H2O, with trace Zn and Mn, respectively. The structure is characterized by undulating layers formed by the four nonequivalent, rather irregular Ca polyhedra [Ca1O6(H2O), Ca2O6(H2O), Ca3O4(H2O)3, and Ca4O5(H2O)3] linked through the sharing of edges and vertices. These undulating layers are parallel to (010) and are interconnected by AsO4 and AsO3OH tetrahedra, as well as hydrogen bonds, along the b axis. Vladimirite is remarkable inasmuch as one of the hydrogen-bonded O–H···O distances (O12H–H···O3) is only 2.465(2) Å, which is the shortest donor–acceptor distance of all known Ca-bearing arsenate minerals, similar to the short donor–acceptor distances observed in several synthetic compounds containing AsO3OH groups.

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