Iskandarovite, Sb ₆ O ₇ (SO ₄ ) ₂ , a new antimony oxysulfate mineral from sublimates of an underground coal fire at the Fan-Yagnob coal deposit, Tajikistan

A new mineral iskandarovite, Sb 6 O 7 (SO 4 ) 2 was discovered at the tract of Kukhi-Malik, Fan-Yagnob coal deposit, ca.75 km N of Dushanbe, Tajikistan.Iskandarovite is an exhalative mineral formed directly from a gas of the natural underground coal fire.The mineral forms colorless prismatic crystals (50-120 m).Mohs hardness is ~ 2.0.Refractive indices: = 1.985(3), = 2.052(5), (calc.)= 2.069(9) (590 nm).The mineral is non-soluble in H 2 O and ethanol at room temperature.Raman spectra show the absence of water, hydroxyl, carbonate, or other anions.The chemical composition determined by electron-microprobe analysis is (wt.%):Sb 2 O 3 -82.57;Bi 2 O 3 -1.28;As 2 O 3 -0.05;SO 3 -15.17;total -99.07.The empirical formula (based on 15 O apfu) is: Sb 5.95 Bi 0.06 As 0.01 S 1.99 O 15 .The strongest lines of the X-ray powder diffraction pattern are (d-I-hkl): 10. 1-40-110; 5.08-50-220; 3.26-25-311; 3.146-100-241; 2.933-73-331.Iskandarovite is orthorhombic, Ccc2, unit-cell parameters a = 12.0402(3), b = 18.9599(5), c = 5.8638(2) , V = 1338.59(7) 3 , Z = 4, and the crystal structure was refined to R 1 = 0.0236 on the basis of 2053 unique reflections with F o > 4F o .The crystal strucure of iskandarovite is based on sulfate tetrahedra and [O 7 Sb 6 ] 4+ rod-like chains that are elongated along the c axis.Synthetic Sb 6 O 7 (SO 4 ) 2 analogue is known.The crystal chemistry of 13 known Sb(III) sulfates and oxysulfates is reviewed.