The new mineral species brontesite , ammonium lead chloride (NH 4 ) 3 PbCl 5 , was found in a medium-temperature (~250°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, (NH 4 ) 4 NaAl 2 (SO 4 ) 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) A, V 1124.7(2) A 3 . The strongest six reflections in the X-ray powder-diffraction pattern [ d obs in A( 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, NH 4 11.3 (by difference), for a total of 100.0 wt.%, corresponding to the empirical formula: [(NH 4 ) 2.87 K 0.13 ] ∑3.00 Pb 1.09 (Cl 4.79 Br 0.21 ) ∑5.00 . The measured density is 2.72(1) g/cm 3 . The average index of refraction n is 1.70(3) (λ = 589 nm). Using single-crystal diffraction data, the structure was refined to a final R = 0.0238 for 1463 independent observed reflections [ I > 2σ( 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) A. 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 Rb 3 PbCl 5 . 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 Pb 2+ and NH 4 + ions, similar to that occurring for Pb 2+ and K + in the high-temperature modification of KPb 2 Cl 5 . 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 (Bρο′ντης), one of the three Cyclops (Kν′κλωπeς) and a son of Uranus.