Steropesite, Tl3BiCl6, a new thallium bismuth chloride, was found in an active high-temperature fumarole (similar to 450 degrees 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) angstrom, beta 108.11(2)degrees, and V 4993(2) angstrom(3). The strongest six reflections in the X-ray powder-diffraction pattern [d(obs) in angstrom(I)(hkl)] are: 3.773(100)(040), 2.113(93)( (12) over bar 00), 3.846( 40)((3) over bar 32), 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)(Sigma 2.90)Bi-1.21(Cl5.32Br0.56)(Sigma 5.88). The calculated density for the compound with the above empirical formula is 5.737 g/cm(3). The average index of refraction n is 1.97( 2) (lambda = 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 > 2 sigma( 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 angstrom. Considering a maximum value of 3.6 angstrom for the Tl-Cl distances, five of the twelve independent Tl atoms are six-coordinated ( Tl-Cl in the range 3.023-3.596 angstrom), six are seven-coordinated (Tl-Cl between 3.106 and 3.558 angstrom), and one is eight-coordinated ( Tl-Cl in the range 3.229-3.511 angstrom). The structure can be compared with those of other hexachlorobismuthates[III], such as (NH4)(3)BiCl6, Cs3BiCl6 and CsK2BiCl6.
