A new mineral philoxenite (K,Na,Pb)4(Na,Ca)2(Mg,Cu)3(Fe3+0.5Al0.5)(SO4)8 is found in sublimates of the Yadovitaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with euchlorine, langbeinite, hematite, tenorite, piypite, alumoklyuchevskite, dolerophanite, vergasovaite, cupromolybdite, ziesite, and yaroshevskite. Philoxenite forms isolated oblique-angled, pseudotrigonal or polygonal tabular crystals up to 0.3 × 0.6 mm. The mineral is transparent, colourless to very pale yellowish with a vitreous lustre. Philoxenite is brittle. Cleavage was not observed, the fracture is uneven. Dcalc = 3.03 g cm–3. Philoxenite is optically biaxial (–), α = 1.562(2), β = 1.572(2), γ = 1.580(2), 2Vmeas = 85(5)° (λ = 589 nm). The Raman spectrum is given. The chemical composition (wt %, electron-microprobe data) is: Na2O 4.67, K2O 13.34, Rb2O 0.13, CaO 2.84, PbO 4.54, MgO 6.37, MnO 0.20, CuO 5.40, ZnO 1.48, Al2O3 3.40, Fe2O3 3.29, SO3 54.62, total 100.28. The empirical formula calculated on the basis of 32 O apfu is (K3.30Na1.76Ca0.59Pb0.24Rb0.02)Σ5.91(Mg1.84Cu0.79Al0.78Fe3+0.48Zn0.21Mn0.03)Σ4.13S7.96O32. Philoxenite is triclinic, space group P-1, a = 8.8410(3), b = 8.9971(3), c = 16.1861(5) Å, α = = 91.927(3), β = 94.516(3), γ = 90.118(3)°, V = 1282.77(7) Å3, and Z = 2. The strongest reflections of the X-ray powder pattern [(d,Å(I)(hkl)] are: 5.70(18)(111); 4.030(24)(004); 3.146(100)(–220); 3.136(72)(220); 2.965(36)(–1–15), 2.912(35)(–115), 2.834(36)(0–32, 1–15), and 2.784(42)(115, 032). The crystal structure of philoxenite is unique. It is based on a novel-type microporous heteropolyhedral framework built by SO4 tetrahedra and MO6 octahedra (M = Mg, Cu2+, Zn, Fe3+ and Al). The name philoxenite is derived from the ancient Greek words φίλος, a friend, and ξένος, a guest, in allusion to the complex cationic composition of the mineral and the presence of significant amounts of admixtures at nine of eleven independent cationic positions in its crystal structure.
