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

Abstract

Lotharmeyerite, calcium bis-(zinc/manganese) bis-(arsenate) bis-(hydroxide/hydrate), Ca(Zn,Mn(3+))(2)(AsO(4))(2)(H(2)O,OH)(2), is a member of the natrochalcite group of minerals, which are characterized by the general formula AM(2)(XO(4))(2)(H(2)O,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) octa-hedra (site symmetry [Formula: see text]) extending along [010], which are inter-connected by XO(4) tetra-hedra (site symmetry 2) and hydrogen bonds to form [M(2)(XO(4))(2)(OH,H(2)O)(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.