The new mineral argesite, ammonium bismuth chloride (NH4)7Bi3Cl16, 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 pale-yellow crystals up to 0.15 mm in length, in association with bismuthinite, adranosite, brontesite, demicheleite-(Br), demicheleite-(Cl), and panichiite. Argesite is trigonal, space group: R3̄c (no. 167) with Z = 18; the unit-cell parameters are (single-crystal data): a = 13.093(1), c = 102.682(1) Å, and V = 15245(2) Å3. The six strongest reflections in the X-ray powder diffraction pattern are: [dobs(Å) (I) (hkl)] 3.164 (100) (0 3 18), 3.808 (44) (2̄ 2 20), 2.742 (78) (2̄ 4 21), 6.14 (16) (1̄ 2 6), 1.906 (16) (0 0 
), 1.686 (13) (5̄ 6 34). The mineral is uniaxial (−), with ω = 1.731(2), ɛ = 1.725(2) (589 nm). The IR spectrum shows absorptions at 3188(vs), 3060(s), and 1397(vs) cm−1, in agreement with the presence of the ammonium ion. Chemical analyses obtained by EDS electron microprobe gave (average wt%) Bi 42.26, Cl 32.57, Br 13.06, I 0.95, K 2.46, Tl 0.88, NH4 7.82 (by difference) total 100.00, corresponding to the empirical formula: [(NH4)6.29K0.91Tl0.06]∑7.26Bi2.93(Cl13.33Br2.37I0.11)∑15.81. The measured density is 2.88(1) g/cm3. The structure was refined, using single-crystal diffraction data, to a final R = 0.0345 for 1289 independent observed reflections [I > 2σ(I)]. It contains Bi2Cl104− and BiCl63− anions where the Bi atoms are octahedrally coordinated, and NH4+ cations are partially replaced by K+ and Tl+ ions.
