Ermakovite (NH4)(As2O3)2Br, a new exhalative arsenite bromide mineral from the Fan-Yagnob coal deposit, Tajikistan

In terrestrial rocks, Br minerals are extremely rare with only nine minerals known where Br is a dominant component. A new arsenite bromide mineral ermakovite, (NH4)(As2O3)2Br, was discovered at the tract of Kukhi-Malik, Fan-Yagnob coal deposit, ca. 75 km N of Dushanbe, Tajikistan. Ermakovite is a fumarolic mineral formed directly from gas from a natural underground coal fire. Associated minerals are sulfur, realgar, amorphous As-sulfides, salammoniac, alacránite, bonazziite and thermessaite-(NH4). In addition, there are amorphous As2S3 intergrowths associated with ermakovite. The mineral typically occurs as tabular or prismatic hexagonal crystals up to 200 μm with the following forms: c (001), m (010) and p (014). Spherulites and multi-twinned intergrowths are very common. The mineral is optically uniaxial (–), ω = 1.960 (5) and ɛ = 1.716(3) (589 nm). The measured density is 3.64(2) g/cm3. The mineral is insoluble in water, HCl, HNO3 and organic solvents. The empirical formula calculated on the basis of (As+Sb) = 4 atoms per formula unit is [(NH4)0.92Na0.01]0.93(As3.94Sb0.06)4.00O6.02(Br0.97Cl0.08I0.01)1.06. The strongest lines in the powder X-ray diffraction pattern are [d, Å (I, %) (hkl)]: 9.160 (80)(001); 4.560(90)(002); 3.228(100) (102); 2.629(80)(110); and 2.522(60)(103). Ermakovite is hexagonal, P6/mmm, a = 5.271(3), c = 9.157(6) Å, V = 220.3(3) Å3 and Z = 1. The sandwich-type structure of ermakovite is based on three types of layers: (1) a honeycomb [As2O3] arsenite layer; (2) an NH4+ layer; and (3) a Br layer. The layer stacking sequence is ⋅⋅⋅NH4–As2O3–Br–As2O3–NH4⋅⋅⋅. Ermakovite has a synthetic analogue. Infrared and Raman spectra are also reported.An overview of the processes that give rise to high concentrations of Br, leading to the formation of exotic Br minerals, is given.