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

Gerstmannite, (Mn,Mg)Mg(OH),[ZnSiOn], orthorhombic, a 8.185(7), b 18.65(2), c 6.256n4, 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 Hil l Mine, Ogdensburg, Sussex County, New Jersey. Hardness : 4l; sp gr : 3.68(2) [3.66 g cm s calculated for (Mgo uruMno"rr)rZn(OH)r(SiO,)]; cleavage good parallel to {010}. The prisms are composed of {100}, {010} and {110} with no terminations. Biaxial (-), a 1.66s(2) , P 1.675(2) , 7 r .678(2) ,2v 50-600, x l lb , Yl lc , z l la . R = 0.042 for l09l independent reflections. The structure is based on oxygen cubic closepacking, the close-packed layers parallel to {130}. The structure is comprised of -'z[MnMgOr(OH)r] octahedral sheets l inked to -'z[ZnSiOn] tetrahedral sheets which are parallel t o { 0 t 0 } . A v e r a g e b o n d d i s t a n c e s a r e 6 r M n r 3 " t O = 2 . 2 1 A , r s r M g r a t r O : 2 . 0 8 4 , l t z n , O : 1.95A and Lr Si-r33rO : 1.644, where the left superscripts refer to the average coordination numbers of the atoms involved.