Crystal chemistry of the new mineral brandholzite, Mg(H2O)6Sb(OH)62, and of the synthetic analogues M2+(H2O)6Sb(OH)62(M2+= Mg, Co)

Brandholzite, a new magnesium antimony hydroxide hydrate mineral, Mg (H2O) 6Sb (OH) 62, 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 ~1 mm in size. Natural as well as synthetic samples obtained by slow evaporation of an aqueous solution exhibit 101̅0 twinning, leading to a pronounced ̅3̅1m pseudo-symmetry. The crystal structures of brandholzite and its synthetic analogue were investigated using single crystal X-ray CCD data: trigonal, space group P3, Z = 6, a = 16. 119 (1) Å, c = 9. 868 (1) Å, R1 = 0. 034 for 14788 Fo > 4σ (Fo) (brandholzite), and a = 16. 113 (1) Å, c = 9. 868 (1) Å, R1 = 0. 029 for 16624 Fo > 4σ (Fo) (synthetic analogue) and 525 variable parameters each. The structures are isotypic with bottinoite, Ni (H2O) 6Sb (OH) 62, 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 (Å), 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 nω = 1. 570 (2) and nε = 1. 569 (2). The crystal structure of the pseudo-isotypic synthetic compound Co (H2O) 6Sb (OH) 62 was also investigated: trigonal, space group P3, a = 16. 105 (1), c = 9. 851 (1), Z = 6, R1 = 0. 051 for 13516 reflections with Fo > 4σ (Fo) and 525 parameters. Compared to the Mg-antimonates and bottinoite, a significant rotation of some Sb (OH) 6 octahedra is observed in Co (H2O) 6Sb (OH) 62.