BACKITE, Pb2Al(TeO6)Cl, A NEW TELLURATE MINERAL FROM THE GRAND CENTRAL MINE, TOMBSTONE HILLS, COCHISE COUNTY, ARIZONA: DESCRIPTION AND CRYSTAL STRUCTURE

Backite, Pb2Al(TeO6)Cl, is a new tellurate mineral from the Grand Central mine, Tombstone Hills, Cochise County, Arizona. It occurs as rosettes of hexagonal plates perched on microcrystalline quartz, associated with schieffelinite, oboyerite, and rodalquilarite. Backite is dark to pale blue-grey, moderately transparent with opacity increasing with intensity of color, has a very pale bluish-grey streak, an adamantine luster, and does not fluoresce under ultraviolet light. It has a perfect micaceous cleavage parallel to {001}. Mohs hardness is 2-3, and backite is brittle with a splintery fracture. The calculated density is 5.573 g/cm(3). The indices of refraction are greater than 1.80, and grains are pleochroic in shades of blue-grey. Backite is trigonal, space group P312, a 5.0441(7), c 9.4210(5) angstrom, V 205.58(8) angstrom(3), Z = 1, c: a = 1: 1.868. The six strongest lines in the X-ray powder diffraction pattern are as follows: d (angstrom), I, (h k l): 3.193, 100, (012); 4.363, 55, (010); 2.521, 55, (013, (1) over bar 20); 1.555, 35, ((1) over bar 32, 122); 1.978, 28, (022); 1.715, 20, (015, (1) over bar 24, 114). Chemical analysis by electron microprobe gave TeO3 24.05, Al2O3 7.07, PbO 63.74, S 1.15, Cl 2.28, O + Cl -1.09, sum 97.20 wt.% where the valence states of Te, Pb, and S were determined by crystal-structure analysis. The resulting empirical formula on the basis of 6 O anions is Pb2.05Al1.00Te0.98O6(Cl0.46S0.26) and the endmember formula is Pb2AlTeO6Cl. The crystal structure of backite was solved by direct methods and refined to an R-1 index of 4.20% based on 422 observed reflections collected with a three-circle rotating-anode diffractometer using MoK alpha X-radiation. In the structure of backite, Te and Al octahedra occupy the vertices of a 63 net and each octahedron shares three edges with three adjacent octahedra. These layers of octahedra repeat along the c direction and are linked by a thick slab of lone-pair stereoactive Pb2+ cations and Cl- anions. The structure is held together in the c direction by long, weak Pb-Cl bonds, accounting for the perfect cleavage parallel to {001}.