Billwiseite, Sb3+5Nb3WO18, 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 x 0.25 x 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 refractive index is 2.3, 2V(obs) = 76(2)°. Billwiseite is colourless in transmitted light, nonpleochroic, and the optic orientation is as follows: X || b, Y ^ c = 72.8° (in β acute), Z ^ c = 17.3° (in β obtuse). It occurs scattered across the surface of a large (~5 x 2.5 x 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 borian muscovite. Billwiseite is monoclinic, space group C2/c, a 54.116(5), b 4.9143(5), c 5.5582(5) Å, β 90.425(2)°, V 1475.5(2) Å3, Z = 4, a:b:c = 11.012 : 1 : 1.131. The seven strongest lines in the X- ray powder diffraction pattern are as follows: d (Å), I, (h k l): 3.017, 100, (15 0 1); 3.154, 90, (-9 1 1; 1.662, 53, (26 0 2); 3.500, 51, (12 0 1); 1.906, 47, (18 2 0); 1.828, 30, (-5 0 3); 1.735, 30, (- 11 0 3). 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)Σ=4.05O18. 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 four-circle diffractometer with MoKα X-radiation. The structure consists of two distinct sheets of 2 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.
Billwiseite, Sb3+5Nb3WO18, a new oxide mineral from Thestak Nala Pegmatite, Nanga Parbat-Haramosh Massif, Pakistan: description and crystal structure
CAMARA ARTIGAS, Fernando;
2012-01-01
Abstract
Billwiseite, Sb3+5Nb3WO18, 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 x 0.25 x 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 refractive index is 2.3, 2V(obs) = 76(2)°. Billwiseite is colourless in transmitted light, nonpleochroic, and the optic orientation is as follows: X || b, Y ^ c = 72.8° (in β acute), Z ^ c = 17.3° (in β obtuse). It occurs scattered across the surface of a large (~5 x 2.5 x 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 borian muscovite. Billwiseite is monoclinic, space group C2/c, a 54.116(5), b 4.9143(5), c 5.5582(5) Å, β 90.425(2)°, V 1475.5(2) Å3, Z = 4, a:b:c = 11.012 : 1 : 1.131. The seven strongest lines in the X- ray powder diffraction pattern are as follows: d (Å), I, (h k l): 3.017, 100, (15 0 1); 3.154, 90, (-9 1 1; 1.662, 53, (26 0 2); 3.500, 51, (12 0 1); 1.906, 47, (18 2 0); 1.828, 30, (-5 0 3); 1.735, 30, (- 11 0 3). 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)Σ=4.05O18. 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 four-circle diffractometer with MoKα X-radiation. The structure consists of two distinct sheets of 2 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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