BRONTESITE, (NH4)3PbCl5, A NEW PRODUCT OF FUMAROLIC ACTIVITY FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

The new mineral species brontesite, ammonium lead chloride (NH4)(3)PbCl5, was found in a medium-temperature (similar to 250 degrees C) active fumarole at La Fossa crater, Vulcano, Aeolian Islands, Sicily, Italy. The mineral occurs on a pyroclastic breccia as colorless to white tabular crystals up to 0.1 mm in length, in association with bismuthinite, godovikovite, demicheleite-(Cl), demicheleite-(Br), alunite, IMA 2008-057, (NH4)(4)NaAl2(SO4)(4)Cl(OH)(2), and IMA 2009-049, BiSI. The mineral is orthorhombic, space group Pnma (no. 62), with Z = 4; the unit-cell parameters are: a 8.434(1), b 15.759(2), c 8.462(1) angstrom, V 1124.7(2) angstrom(3). The strongest six reflections in the X-ray powder-diffraction pattern [d(obs) in angstrom(I)(hkl)] are: 3.067(100)(132), 2.020(80)(342), 2.710(78)(241), 1.910(78)(134), 2.421(75)(152) and 1.491(75)(373). Chemical analyses made with an electron microprobe in EDS mode gave, on average, Pb 47.8, Cl 36.1, Br 3.7, K 1.1, NH4 11.3 (by difference), for a total of 100.0 wt.%, corresponding to the empirical formula: [(NH4)(2.87)K-0.13](Sigma 3.00)Pb-1.09(Cl4.79Br0.21)(Sigma 5.00). The measured density is 2.72(1) g/cm(3). The average index of refraction n is 1.70(3) (lambda = 589 nm). Using single-crystal diffraction data, the structure was refined to a final R = 0.0238 for 1463 independent observed reflections [I > 2 sigma(I)]. The coordination polyhedron of the independent eight-coordinated Pb atom is a bicapped trigonal prism with Pb-Cl distances ranging from 2.777(1) to 3.724(1) angstrom. The Pb polyhedra are connected by sharing edges to form chains running along [100]. There are also two independent sites occupied by ammonium ions whose environment corresponds nearly exactly to that of the Rb atoms in Rb3PbCl5. It is essentially similar to that of the lead atom, i.e., the chlorine atoms are also arranged at the corners of bicapped trigonal prisms. There is some substitutional disorder involving the Pb2+ and NH4+ ions, similar to that occurring for Pb2+ and K+ in the high-temperature modification of KPb2Cl5. The mineral was approved as a new species by the IMA Commission on New Minerals, Nomenclature and Classification (IMA # 2008-014); it is named after Brontes (B rho o nu tau eta zeta), one of the three Cyclops (K nu kappa lambda omega pi epsilon zeta) and a son of Uranus.