Achyrophanite, (K,Na)3(Fe3+,Ti,Al,Mg)5O2(AsO4)5, a New Mineral with the Novel Structure Type from Fumarolic Exhalations of the Tolbachik Volcano, Kamchatka, Russia

The new mineral achyrophanite (K,Na)3(Fe3+,Ti,Al,Mg)5O2(AsO4)5 was found in high-temperature sublimates of the 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 aphthitalite-group sulfates, hematite, alluaudite-group arsenates (badalovite, calciojohillerite, johillerite, nickenichite, hatertite, and khrenovite), ozerovaite, pansnerite, arsenatrotitanite, yurmarinite, svabite, tilasite, katiarsite, yurgensonite, As-bearing sanidine, anhydrite, rutile, cassiterite, and pseudobrookite. Achyrophanite occurs as long-prismatic to acicular or, rarer, tabular crystals up to 0.02 &times: 0.2 &times: 1.5 mm, which form parallel, radiating, bush-like, or chaotic aggregates up to 3 mm across. It is transparent, straw-yellow to golden yellow, with strong vitreous luster. The mineral is brittle, with (001) perfect cleavage. Dcalc is 3.814 g cm&ndash:3. Achyrophanite is optically biaxial (+), &alpha: = 1.823(7), &beta: = 1.840(7), &gamma: = 1.895(7) (589 nm), 2V (meas.) = 60(10)&deg:. Chemical composition (wt.%, electron microprobe) is: Na2O 3.68, K2O 9.32, CaO 0.38, MgO 1.37, MnO 0.08, CuO 0.82, ZnO 0.48, Al2O3 2.09, Fe2O3 20.42, SiO2 0.12, TiO2 7.35, P2O5 0.14, V2O5 0.33, As2O5 51.88, SO3 1.04, and total 99.40. The empirical formula calculated based on 22 O apfu is Na1.29K2.15Ca0.07Mg0.34Mn0.01Cu0.11Zn0.06Al0.44Fe3+2.77Ti1.00Si0.02P0.02S0.14V0.04As4.90O22. Achyrophanite is orthorhombic, space group P2221, a = 6.5824(2), b = 13.2488(4), c = 10.7613(3) &Aring:, V = 938.48(5) &Aring:3 and Z = 2. The strongest reflections of the PXRD pattern [d,&Aring:(I)(hkl)] are 5.615(59)(101), 4.174(42)(022), 3.669(31)(130), 3.148(33)(103), 2.852(43)(141), 2.814(100)(042, 202), 2.689(29)(004), and 2.237(28)(152). The crystal structure of achyrophanite (solved from single-crystal XRD data, R = 4.47%) is unique. It is based on the octahedral-tetrahedral M-T-O pseudo-framework (M = Fe3+ with admixed Ti, Al, Mg, Na: T = As5+). Large-cation A sites (A = K, Na) are located in the channels of the pseudo-framework. The achyrophanite structure can be described as stuffed, with the defect heteropolyhedral pseudo-framework derivative of the orthorhombic Fe3+AsO4 archetype. The mineral is named from the Greek &#940:&chi:&upsilon:&rho:&omicron:&nu:, straw, and &phi:&alpha:&#943:&nu:&omicron:&mu:&alpha:&iota:, to appear, in allusion to its typical straw-yellow color and long prismatic habit of crystals.