Acta Cryst. (2012). E68, i9-i10  [ doi:10.1107/S1600536811054286 ]

Lotharmeyerite, Ca(Zn,Mn)₂(AsO₄)₂(H₂O,OH)₂

Y. W. Yang, S. H. Evans, R. T. Downs and H. Yang

Abstract: Lotharmeyerite, calcium bis(zinc/manganese) bis(arsenate) bis(hydroxide/hydrate), Ca(Zn,Mn³⁺)₂(AsO₄)₂(H₂O,OH)₂, is a member of the natrochalcite group of minerals, which are characterized by the general formula AM₂(XO₄)₂(H₂O,OH)₂, where A may be occupied by Pb²⁺, Ca²⁺, Na⁺, and Bi³⁺, M by Fe³⁺, Mn³⁺, Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺, Al³⁺, and Mg²⁺, and X by P^V, As^V, V^V, and S^VI. The minerals in the group display either monoclinic or triclinic symmetry, depending on the ordering of chemical components in the M site. Based on single-crystal X-ray diffraction data of a sample from the type locality, Mapimi, Durango, Mexico, this study presents the first structure determination of lotharmeyerite. Lotharmeyerite is isostructural with natrochalcite and tsumcorite. The structure is composed of rutile-type chains of edge-shared MO₆ octahedra (site symmetry ) extending along [010], which are interconnected by XO₄ tetrahedra (site symmetry 2) and hydrogen bonds to form [M₂(XO₄)₂(OH,H₂O)₂] sheets parallel to (001). These sheets are linked by the larger A cations (site symmetry 2/m), as well as by hydrogen bonds. Bond-valence sums for the M cation, calculated with the parameters for Mn³⁺ and Mn²⁺ are 2.72 and 2.94 v.u., respectively, consistent with the occupation of the M site by Mn³⁺. Two distinct hydrogen bonds are present, one with OO = 2.610 (4) Å and the other OO = 2.595 (3) Å. One of the H-atom positions is disordered over two sites with 50% occupancy, in agreement with observations for other natrochalcite-type minerals, such as natrochalcite and tsumcorite.

Online 23 December 2011

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