HEPHAISTOSITE, TlPb2Cl5, A NEW THALLIUM MINERAL SPECIES FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

Hephaistosite, TlPb2Cl5, a newly discovered species of thallium lead chloride, was found in a high-temperature fumarole (~400\ub0C) at the rim of La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. The mineral occurs as aggregates of pale yellow-green crystals tabular on {010} up to 0.1 mm in length on fragments of altered pyroclastic breccia, in association with bismuthinite (Bi2S3), cotunnite (PbCl2), challacolloite (KPb2Cl5) and pseudocotunnite (K2PbCl4). The mineral is monoclinic, space group P21/c. Cell parameters refined from powder-diffraction data are: a 8.9477(6), b 7.9218(7), c 12.4955(5) \uc5, b 90.092(4)\ub0, V 885.70(7) \uc53, with Z = 4. The strongest six reflections in the X-ray powder-diffraction data [d (in \uc5)(I)(hkl)] are: 3.696(100)(013), 3.971(83)(020), 2.109(45)(402,215), 2.569(42)(204), 1.848(41)(142,026), and 2.851(38)(213). Crystals of hephaistosite show the forms {010}, {101}, {110}, {100}, {001}, and {011}. Chemical analyses obtained by EDS microprobe gave, on average, in wt.%, Tl 23.78, K 0.01, Pb 51.78, Cl 21.40, Br 1.34, F 0.17, for a total of 98.48, corresponding to an empirical formula (based on 8 apfu) of Tl0.94Pb2.01(Cl4.85Br0.14F0.07)S5.06. Hephaistosite is isostructural with challacolloite; intermediate members of the series between the two species occur in the same fumarole. The calculated density for the synthetic compound (TlPb2Cl5) is 5.932 g/cm3. The mean index of refraction n, obtained from a natural sample using Brewster angle measurement, is 2.0(1). The structure was refined using two different crystals, with Tl:K atom ratios of 86:14 and 64:36; the final R index is 0.028 and 0.038, respectively. The mineral, approved by the IMA Commission on New Minerals, Nomenclature and Classification (No. 2006\u2013043), is named in honor of the ancient Greek god of fire, Hephaistos (Hephaestus, Hfa\ue069sto\ue0a7), whose workshops were alleged to be located at Vulcano