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 ∼ 0.5 × 0.25 × 0.15 mm and show the following forms: 100 pinacoid ≈ 011 pinacoid ≈ 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)°. Billwiseite is colorless in transmitted light, non-pleochroic, and the optic orientation is as follows: X || b, Y ∧ c = 72.8° (in β acute). It occurs scattered across the surface of a large (∼5 × 2.5 × 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) Å, b 90.425(2)°, V 1475.5(2) Å 3, Z = 4, a: b: c = 11.012: 1: 1.131. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(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) σ4.05O 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 MoKα 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.