THE CRYSTAL STRUCTURE OF VLADIMIRITE, WITH A REVISED CHEMICAL FORMULA, Ca4(AsO4)2(AsO3OH){middle dot}4H2O

Abstract

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 P 2 1 / c and unit-cell parameters a 5.8279(2), b 10.1802(4), c 22.8944(10) A, β 96.943(2)°, and V 1348.35(9) A 3 for R100075 and a 5.8220(1), b 10.1750(2), c 22.8816(6) A, β 96.902(1)°, and V 1345.66(5) A 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 (AsO 4 ) 2 (AsO 3 OH)·4H 2 O (Z = 4) for this mineral, in contrast to Ca 5 (AsO 4 ) 2 (AsO 3 OH) 2 ·5H 2 O (Z = 3) documented in the literature. The chemical compositions for R100075 and R080001 are Ca 4.03 (AsO 4 ) 2 (As 0.99 O 3 OH)·4H 2 O and Ca 3.97 (AsO 4 ) 2 (As 1.01 O 3 OH)·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 (H 2 O), Ca2O 6 (H 2 O), Ca3O 4 (H 2 O) 3 , and Ca4O 5 (H 2 O) 3 ] linked through the sharing of edges and vertices. These undulating layers are parallel to (010) and are interconnected by AsO 4 and AsO 3 OH 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) A, 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 AsO 3 OH groups.