THE CRYSTAL STRUCTURE OF VLADIMIRITE, WITH A REVISED CHEMICAL FORMULA, Ca4(AsO4)2(AsO3OH)•4H2O

Two samples of vladimirite, one from Bou Azzer, Morocco, and the other from a new occurrence in Copiapo, 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 P2(1)/c and unit-cell parameters a 5.8279(2), b 10.1802(4), c 22.8944(10) angstrom, beta 96.943(2)degrees, and V 1348.35(9) angstrom(3) for R100075 and a 5.8220(1), b 10.1750(2), c 22.8816(6) angstrom, beta 96.902(1)degrees, and V 1345.66(5) angstrom(3) for R080001. The structure determinations, with R-1 = 0.022 and 0.023 for R100075 and R080001, respectively, yielded an ideal chemical formula Ca-4(AsO4)(2)(AsO3OH)center dot 4H(2)O (Z = 4) for this mineral, in contrast to Ca-5(AsO4)(2)(AsO3OH)(2)center dot 5H(2)O (Z = 3) documented in the literature. The chemical compositions for R100075 and R080001 are Ca-4.03(AsO4)(2)(As0.99O3OH)center dot 4H(2)O and Ca-3.97(AsO4)(2)(As1.01O3OH)center dot 4H(2)O, with trace Zn and Mn, respectively. The structure is characterized by undulating layers formed by the four nonequivalent, rather irregular Ca polyhedra [Ca1O(6)(H2O), Ca2O(6)(H2O), Ca3O(4)(H2O)(3), and Ca4O(5)(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 center dot center dot center dot O distances (O12H-H center dot center dot center dot O3) is only 2.465(2) angstrom, 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.