Abstract

Studying the reason for the formation of two structural sub-types, seven arsenate and vanadate compounds of descloizite and adelite groups [specifically (1) CdCo(OH)(AsO4), (2) CdCu(OH)(AsO4), (3) SrCo(OH)(AsO4), (4) SrZn(OH)(AsO4), (5) SrCu(OH)(VO4), (6) CdCo(OH)(VO4), and (7) CdCu(OH)(VO4) (bold numbers throughout paper refer to these compounds)] were synthesized under low-temperature hydrothermal conditions. 12 and 67 are isostructural with descloizite, and 35 are with adelite-group minerals and several synthetic compounds. Together with a sample of conichalcite, (8) CaCu(OH)(AsO4), they were investigated using single-crystal X-ray diffraction [R(F) = 0.0153–0.0283 for 15 and 8; for 6 and 7, R(F) = 0.0603 and 0.0444, respectively] and Raman spectroscopy. Although crystallizing in different orthorhombic space groups, the atomic arrangements of descloizite-(Pnam) and adelite-(P212121) type compounds adopt the same topology: the atomic arrangement is characterized by M2O6 octahedrons (M2 = Mg2+, Al3+, Mn2+,3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) edge-linked into chains. These chains are interconnected by XO4 tetrahedrons (X = Si4+, P5+, V5+, As5+, Mo6+) into a three-dimensional framework. Cavities host M1 atoms (M1 = Na+, Ca2+, Cd2+, Hg2+, Pb2+); their coordination varies from 7 for descloizite-type representatives to 8 for adelite-type structures. The OH stretching frequencies in the Raman spectra are in good agreement with the observed O–H···O donor-acceptor distances. A detailed discussion of the crystal chemistry of these compounds and their influence on the space-group symmetry indicate a distinct dependence of the structural changes on the average ionic radii (rM1 + rX)/2.

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