Gerstmannite, a new zinc silicate mineral and a novel cubic close-packed oxide structure

Gerstmannite, (Mn,Mg)Mg(OH)₂[ZnSiO₄], orthorhombic, a 8.185(7), b 18.65(2), c 6.256(6)Å, Z = 8, space group Bbcm, occurs as white to pale-pink mats and sprays of bunched prismatic crystals associated with calcite, manganpyrosmalite, and sphalerite in a hydrothermal vein paragenesis which cuts normal franklinite-willemite “buckshot” ore from the Sterling Hill Mine, Ogdensburg, Sussex County, New Jersey. Hardness = 4½; sp gr = 3.68(2) [3.66 g cm^-3 calculated for (Mg_0.625Mn_0.375)₂Zn(0H)₂(SiO₄)]; cleavage good parallel to {010}. The prisms are composed of {100}, {010} and {110} with no terminations. Biaxial (–), α 1.665(2), α 1.675(2), γ 1.678(2), 2V ∼ 50-60°, X|a|b, Y||c, Z||a.

R = 0.042 for 1091 independent reflections. The structure is based on oxygen cubic close-packing, the close-packed layers parallel to {130}. The structure is comprised of ²_∞[MnMg0₃(0H)₂] octahedral sheets linked to ²_∞[ZnSiO₄] tetrahedral sheets which are parallel to {010}. Average bond distances are ^|6|Mn–^|3.5|O = 2.21 Å, ^|6|Mg–^|3.7|O = 2.08Å, ^|4|Zn–^|3|O = 1.95Å and ^|4|Si–^|3.3|O = 1.64Å, where the left superscripts refer to the average coordination numbers of the atoms involved.