STEROPESITE, Tl3BiCl6, A NEW THALLIUM BISMUTH CHLORIDE FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

Steropesite, Tl3BiCl6, a new thallium bismuth chloride, was found in an active high-temperature fumarole (~450°C) on the rim at La Fossa crater, Vulcano, Aeolian Islands, Sicily, Italy. The mineral occurs as tabular or stout lemon-yellow crystals up to 0.20 mm in length in association with bismuthinite and lafossaite. The mineral is monoclinic, space group Cc (no. 9), with Z = 16; the unit-cell parameters are a 26.686(5), b 15.127(3), c 13.014(3) Å, b 108.11(2)°, and V 4993(2) Å3. The strongest six reflections in the X-ray powder-diffraction pattern [dobs in Å(I)(hkl)] are: 3.773(100)(040), 2.113(93)(1200), 3.846(40)(332), 2.710(35)(442), 3.884(30)(402), and 2.745(30)(513). Chemical analyses obtained with an electron microprobe in wavelength dispersion gave, on average, Tl 54.57, Bi 23.40, Cl 17.42, Br 4.13, Na 0.02, K 0.01, for a total of 99.55 wt.%, corresponding to the empirical formula: (Tl2.89Na0.01)S2.90Bi1.21(Cl5.32Br0.56)S5.88. The calculated density for the compound with the above empirical formula is 5.737 g/cm3. The average index of refraction n is 1.97(2) (l = 589 nm). The structure was refined from single-crystal X-ray-diffraction data to a final R of 0.052 for 6232 independent observed reflections [I > 2s(I)]. The asymmetric unit contains 4 Bi, 12 Tl and 24 Cl atoms. The bismuth atoms display a distorted octahedral coordination, with Bi–Cl distances in the range 2.618–2.789 Å. Considering a maximum value of 3.6 Å for the Tl–Cl distances, five of the twelve independent Tl atoms are sixcoordinated (Tl–Cl in the range 3.023–3.596 Å), six are seven-coordinated (Tl–Cl between 3.106 and 3.558 Å), and one is eightcoordinated (Tl–Cl in the range 3.229–3.511 Å). The structure can be compared with those of other hexachlorobismuthates[III], such as (NH4)3BiCl6, Cs3BiCl6 and CsK2BiCl6.