Lotharmeyerite, Ca(Zn,Mn) ₂(AsO ₄)2-(H ₂O,OH) ₂

Lotharmeyerite, calcium bis(zinc/manganese) bis(arsenate) bis(hydroxide/hydrate), Ca(Zn,Mn ³⁺) ₂(AsO ₄) ₂(H ₂O,OH) ₂, is a member of the natrochalcite group of minerals, which are characterized by the general formula AM ₂(XO ₄) ₂(H ₂O,OH) ₂, where A may be occupied by Pb ²⁺, Ca ²⁺, Na ⁺, and Bi ³⁺, M by Fe ³⁺, Mn ³⁺, Cu ²⁺, Zn ²⁺, Co ²⁺, Ni ²⁺, Al ³⁺, and Mg ²⁺, 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 MO ₆ octahedra (site symmetry 1̄) extending along [010], which are interconnected by XO ₄ tetrahedra (site symmetry 2) and hydrogen bonds to form [M ₂(XO ₄) ₂(OH,H ₂O) 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 Mn ³⁺ and Mn ²⁺ are 2.72 and 2.94 v.u., respectively, consistent with the occupation of the M site by Mn ³⁺. Two distinct hydrogen bonds are present, one with O···O = 2.610(4) Å and the other O···O = 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.