Abstract
Steropesite, Tl 3 BiCl 6 , 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) A, β 108.11(2)°, and V 4993(2) A 3 . The strongest six reflections in the X-ray powder-diffraction pattern [ d obs in A( 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: (Tl 2.89 Na 0.01 ) ∑2.90 Bi 1.21 (Cl 5.32 Br 0.56 ) ∑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) (λ = 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 > 2σ( 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 A. Considering a maximum value of 3.6 A for the Tl–Cl distances, five of the twelve independent Tl atoms are six-coordinated (Tl–Cl in the range 3.023–3.596 A), six are seven-coordinated (Tl–Cl between 3.106 and 3.558 A), and one is eight-coordinated (Tl–Cl in the range 3.229–3.511 A). The structure can be compared with those of other hexachlorobismuthates[III], such as (NH 4 ) 3 BiCl 6 , Cs 3 BiCl 6 and CsK 2 BiCl 6 .
Published Version
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