Crystal chemistry of the new mineral brandholzite, Mg(H2O)(6)[Sb(OH)(6)](2), and of the synthetic analogues M2+(H2O)(6)[Sb(OH)(6)](2) (M2+= Mg, Co)

Brandholzite, a new magnesium antimony hydroxide hydrate mineral, Mg(H2O)(6)[Sb(OH)(6)](2), has been discovered in Au-Sb-Quartz veins of the former mining district of Brandholz-Goldkronach, Fichtelgebirge, Germany. The new mineral is associated with stibnite and antimony-ochers and forms colorless, platelike crystals up to similar to 1 mm in size. Natural as well as synthetic samples obtained by slow evaporation of an aqueous solution exhibit {10 (1) over bar 0} twinning, leading to a pronounced (3) over bar 1m pseudo-symmetry. The crystal structures of brandholzite and its synthetic analogue were investigated using single crystal X-ray CGD data: trigonal, space group P3, Z = 6, a = 16.119(1) Angstrom, c = 9.868(1) Angstrom, R1 = 0.034 for 14788 F-o > 4 sigma(F-o) (brandholzite), and a = 16.113(1) Angstrom, c = 9.868(1) Angstrom, R1 = 0.029 for 16624 F-o > 4 sigma(F-o) (synthetic analogue) and 525 variable parameters each. The structures are isotypic with bottinoite, Ni(H2O)(6)[Sb(OH)(6)](2), and are built up by nearly regularly shaped, isolated Mg(H2O)(6) and Sb(OH)(6) octahedra which are interconnected by hydrogen bonds only. The strongest lines in the powder pattern are [d-value (Angstrom), I, hkl]: 4.636, 100, (300); 3.392, 70, (302); 4.946, 50, (002); 2.356, 40, (332). At 589 nm, the mineral is optically uniaxial negative with refractive indices n(omega) = 1.570(2) and n(epsilon) = 1.569(2). The crystal structure of the pseudo-isotypic synthetic compound Co(H2O)(6)[Sb(OH)(6)](2) was also investigated: trigonal, space group P3, a = 16.105(1), c = 9.851(1), Z = 6, R1 = 0.051 for 13516 reflections with F-o > 4 sigma(F-o) and 525 parameters. Compared to the Mg-antimonates and bottinoite, a significant rotation of some Sb(OH)(6) octahedra is observed in Co(H2O)(6)[Sb(OH)(6)](2).