Billwiseite, ideally Sb 3+ 5(Nb,Ta) 3WO 18, a new oxide mineral species from the Stak Nala Pegmatite, Nanga Parbat - Haramosh Massif, Pakistan : description and crystal structure

Billwiseite, ideally Sb 3+ 5(Nb,Ta) 3WO 18, is an oxide mineral from a granitic pegmatite on the eastern margin of the Nanga Parbat - Haramosh massif at Stak Nala, 70 km east of Gilgit, Pakistan. It is transparent, pale yellow (with a tinge of green), has a colorless to very pale-yellow streak, a vitreous luster, and is inert to ultraviolet radiation. Crystals are euhedral with a maximum size of 3c 0.5 7 0.25 7 0.15 mm and show the following forms: 100 pinacoid 48 011 pinacoid 48 410 prism; contact twins on (100) are common. Cleavage is 100 indistinct, Mohs hardness is 5, and billwiseite is brittle with a hackly fracture. The calculated density is 6.330 g/cm 3. The indices of refraction were not measured; the calculated index of refraction is 2.3, 2V(obs) = 76(2)\ub0. Billwiseite is colorless in transmitted light, non-pleochroic, and the optic orientation is as follows: X || b, Y 27 c = 72.8\ub0 (in \u3b2 acute). It occurs scattered across the surface of a large ( 3c5 7 2.5 7 1.3 cm) crystal of lepidolite from a miarolitic cavity. The most abundant minerals in the cavities at Stak Nala are albite, quartz, K-feldspar, tourmaline, muscovite or lepidolite, topaz and fluorite, and billwiseite can be partly mantled by B-rich muscovite. Billwiseite is monoclinic, space group C2/c, a 54.116(5), b 4.9143(5), c 5.5482(5) \uc5, b 90.425(2)\ub0, V 1475.5(2) \uc5 3, Z = 4, a: b: c = 11.012: 1: 1.131. The strongest seven lines in the X-ray powder-diffraction pattern [d in \uc5(I)hkl] are as follows: 3.147(100)(911, 911), 3.500(55) (511, 511), 1.662(53)(1422), 3.017(48)(1800), 1.906(47)(1820), 1.735(30)(113, 113), 1.762(25)(2711, 2711). Chemical analysis by electron microprobe gave Nb 2O 5 12.03, Ta 2O 5 19.31, Sb 2O 3 48.34, TiO 2 0.99, WO 3 19.96, sum 100.63 wt.% where the valence state of Sb was determined by crystal-structure analysis. The resulting empirical formula on the basis of 18 O anions is Sb 3+ 4.87(Nb 1.33Ta 1.28Ti 0.18W 1.26) \u3c34.05O 18. The crystal structure of billwiseite was solved by direct methods and refined to an R 1 index of 4.71% based on 2122 observed reflections collected on a three-circle diffractometer with MoK\u3b1 X-radiation. The structure consists of two distinct sheets of M (= Ta,Nb,W) octahedra and three distinct sheets of Sb 3+ polyhedra parallel to (100). These sheets alternate in the a direction to form a continuous structure

Rubies and sapphires from Winza, Central Tanzania

Since late 2007, rubies and sapphires have been mined by hand methods from both eluvial and primary deposits at Winza in central Tanzania. The gem corundum is related to "dikes" of amphibolitic rocks that belong to the Paleoproterozoic Usagaran Belt. Based on crystal morphology, Winza corundum is subdivided into two types: prismatic-tabular-rhombohedral and dipyramidal. In general, medium red and dark (orangy) red top-quality rubies are rhombohedral. Pinkish red and purplish red rubies, as well as pink, purple, and blue (often strongly color zoned) sapphires are, for the most part, dipyramidal. The top-quality rubies are characterized by a distinct assemblage of long tube-, fiber-, needle-, or hair-like inclusions containing an orange-brown material (most likely limonite). The lower-quality material generally contains a larger amount of solid inclusions (mostly amphibole crystals), fissures, and growth features. Unique to corundum from this locality are bluish violet color zones oriented parallel to the prism and basal pinacoid, and occasionally also parallel to rhombohedral and dipyramidal faces. The relatively high Fe content of Winza rubies separates them from most other natural and almost all synthetic counterparts