Argesite, (NH4)7Bi3Cl16, a new mineral from La Fossa Crater, Vulcano, Aeolian Islands, Italy : a first example of the [Bi2Cl10]4− anion

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: R3c (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) (2 2 20), 2.742 (78) (2 4 21), 6.14 (16) (1 2 6), 1.906 (16) (0 0 54), 1.686 (13) (5 6 34). The mineral is uniaxial (–), with ω = 1.731(2), ε = 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]Σ7.26Bi2.93(Cl13.33Br2.37I0.11)Σ15.81. 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.