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

Abstract

The new mineral argesite, ammonium bismuth chloride (NH 4 ) 7 Bi 3 Cl 16 , 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: R 3 c (no. 167) with Z = 18; the unit-cell parameters are (single-crystal data): a = 13.093(1), c = 102.682(1) A, and V = 15245(2) A 3 . The six strongest reflections in the X-ray powder diffraction pattern are: [ d obs (A) ( 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), 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, NH 4 7.82 (by difference) total 100.00, corresponding to the empirical formula: [(NH 4 ) 6.29 K 0.91 Tl 0.06 ] ∑7.26 Bi 2.93 (Cl 13.33 Br 2.37 I 0.11 ) ∑15.81 . The measured density is 2.88(1) g/cm 3 . 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 Bi 2 Cl 10 4− and BiCl 6 3− anions where the Bi atoms are octahedrally coordinated, and NH 4 + cations are partially replaced by K + and Tl + ions.