Description and crystal structure of cabalzarite Ca(Mg,Al,Fe)(2)(AsO4)(2)(H2O,OH)(2), a new mineral of the tsumcorite group

Cabalzarite, Ca-M1(M2)(Mg,Al,Fe3+),((AsO4)-As-X)(2)(H2O,OH)(2), is a new mi neral of the tsumcorite group occurring in altered Mn ore at the abandoned Falotta mine (Swiss Alps). Together with other arsenates, cabalzarite docum ents the mobility of As during the retrograde stage of the Tertiary Alpine metamorphism under lowest to sub-greenschist facies conditions. Cabalzarite crystals vary in morphology from hatchet-like to fibrous and tabular. The color is light-brownish to salmon pink or orange brown, and the average ref ractive index is around 1.7. Cabalzarite is chemically inhomogeneous, with the main variations occurring on the octahedral M2 site occupied by Mg, Al, Fe3+, and Mn3+.Al and Mg are the dominant M2 cations, with Al/(Al + Mg) ra tios varying between 0.31 and 0.59 (92 analyses). Cabalzarite is the first member of the tsumcorite group with octahedral Mg and Al as major constitue nts. Single-crystal X-ray structure refinements were performed on two diffe r ent crystals. Cabalzarite is monoclinic, space group C2/m, Z = 2. The cel l parameters for the type crystal CABA11.5, of composition (Ca1.00Sr0.02)(A l0.80Mg0.77Fe0.23Mn0.03) (Sigma 1.83)(AsO4)(2)(H2O1.26OH0.74)(2), and are a = 8.925(2) Angstrom, b = 6.143(1) Angstrom, c = 7.352(1) Angstrom, beta = 115.25(3)degrees rho(calc) = 3.73 g/cm(3). The structure of cabalzarite is isomorphic with that of tsumcorite. The site M1 (Ca) is eightfold-coordinat ed (6 + 2) with average M1-O = 2.549 Angstrom; M2 is octahedral with an ave rage M2-O = 2.010 Angstrom; and the average As-O distance of the arsenate g roup is 1.689 Angstrom. Charge balance for the simultaneous occupation of M 2 by two- and three-valent cations is achieved by H2O as well as OH contrib uting to the M2 coordination.