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 ~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)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 (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 pseudoisotypic 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 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)6[Sb(OH)6]2.

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