Polyarsite, Na7CaMgCu2(AsO4)4F2Cl, a New Mineral with Unique Complex Layers in the Novel-Type Crystal Structure

The new mineral polyarsite, ideally Na7CaMgCu2(AsO4)4F2Cl, was discovered in high-temperature incrustations of the active Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with aegirine, sanidine, ferrisanidine, hematite, halite, sylvite, cassiterite, evseevite, axelite, badalovite, johillerite, arsmirandite, aphthitalite, tridymite, potassic-magnesio-fluoro-arfvedsonite and litidionite. Polyarsite forms short-prismatic, equant or tabular crystals up to 0.15 mm across, their clusters up to 0.3 mm in size or crusts up to 0.5 mm across and up to 0.03 mm thick. Polyarsite is transparent, sky-blue to light blue, with vitreous lustre. It is brittle, no cleavage is observed and the fracture is uneven. Dcalc. = 3.592 g cm−3. Polyarsite is optically biaxial (+), α = 1.624 (4), β = 1.645 (4), γ = 1.682 (4) (589 nm), 2Vmeas. = 70 (10)°. The empirical chemical formula calculated based on 19 O+F+Cl apfu is Na7.04Ca1.00Mg0.92Cu2.06Fe3+0.06(As3.96S0.05)Σ4.01O16.28F1.66Cl1.06. Polyarsite is monoclinic, space group I2/m, a = 8.4323(4), b = 10.0974(4), c = 10.7099(6) Å, β = 90.822(4)°, V = 911.79(8) Å3 and Z = 2. The crystal structure was determined based on SCXRD data, R = 0.0391. Polyarsite demonstrates a novel structure type. The structure is based on the (1 0 1) heteropolyhedral layers formed by Cu2O8Cl dimers built by CuO4Cl tetragonal pyramids sharing common Cl vertex, AsO4 tetrahedra and MgO4F2 octahedra. Adjacent layers are linked via CaO8 cubes to form a pseudo-framework which hosts octahedrally coordinated Na cations. Polyarsite was named based on the Greek words πολύς, poly, “many” and due to belonging to arsenates: this arsenate contains many chemical components ordered between different positions in crystal structure.