Abstract

Billwiseite, ideally Sb3+5(Nb,Ta)3WO18, 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/cm3. 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 49143(5), c 5.5482(5) Å, β 90.425(2)°, V 1475.5(2) Å3, 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)(911̅, 911), 3.500(55) (511̅, 511), 1.662(53)(1422̅), 3.017(48)(1800), 1.906(47)(1820), 1.735(30)(113̅, 113), 1.762(25)(2711̅, 2711). Chemical analysis by electron microprobe gave Nb2O5 12.03, Ta2O5 19.31, Sb2O3 48.34, TiO2 0.99, WO3 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 Sb3+4.87(Nb1.33Ta1.28Ti0.18W1.26)Σ4.05O18. The crystal structure of billwiseite was solved by direct methods and refined to an R1 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 Sb3+ polyhedra parallel to (100). These sheets alternate in the a direction to form a continuous structure.

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