Armbrusterite, ideally K₅Na₆Mn³⁺ Mn₁₄²⁺[Si₉O₂₂]₄ (OH)₁₀ 4H₂O, is a new silicate of potassium, sodium, and manganese found in a thin cancrinite-aegirine-microcline vein within urtite at Mt. Kukisvumchorr. The mineral occurs in intimate association with raite. Other associated minerals are lamprophyllite, mangan-neptunite, pectolite, vinogradovite, calcite, molybdenite, galena, sphalerite, and fluorite. Armbrusterite occurs as split, curved crystals and spherulites (≤2 mm diameter). The mineral is translucent (transparent in thin fragments), dark reddish-brown. It has vitreous luster and light-brown streak. Cleavage is perfect on (001) and the fracture is uneven. Mohs hardness is about 3.5. In transmitted light, the mineral is reddish-brown, with strong pleochroism: X = light yellowish-brown, Y and Z = dark reddish-brown; dispersion r > v, weak. Armbrusterite is biaxial (-): α = 1.532(2), β = 1.560(2), γ = 1.564(2) (for λ = 589 nm), 2V varies from 10° to 20°. Optical orientation: X is perpendicular to (001). The mean chemical composition determined by electron microprobe and the Penfield method (for H₂O) is (wt%): Na₂O 5.26, MgO 0.19, Al₂O₃ 0.04, SiO₂ 56.02, K₂O 6.13, CaO 0.26, TiO₂ 0.04, MnO 23.62, Mn₂O₃ 2.07, FeO 0.65, ZnO 0.20, H₂O 4.1, sum. 98.58. Empirical formula calculated on the basis of Si = 36 is K_5.03Na_6.55(Mn_12.86²⁺ Mn_1.01³⁺Fe_0.35²⁺ Mg_0.18Ca_0.18Zn_0.09Al_0.03 Ti_0.02)_∑=14.72[Si₃₆O₈₈] (OH)_10.10 · 3.75 H₂O. Armbrusterite is monoclinic, C2/m, a = 17.333(2), b = 23.539(3), c = 13.4895(17) Å, β = 115.069(9)°, V = 4985.4(11) ų, Z = 2. The strongest X-ray powder-diffraction lines are [d in Å, (I), (hkl)]: 12.28 (100) (001), 4.10 (10) (003), 3.562 (10) (113, 261), 3.260 (18) (114), 3.117 (13) (203), 3.077 (54) (004), 2.622 (10) (371). The crystal structure of armbrusterite was refined to R₁ = 0.085 on the basis of 3960 unique observed reflections. The structure is based upon double silicate [Si₉O₂₂] layers consisting of 5-, 6-, 7-, and 8-membered tetrahedra rings. The layers are linked via octahedral sheets formed by Na and Mn octahedra. The interior of the double silicate layers is occupied by K⁺ cations and H₂O molecules. The mineral is named in honor of Thomas Armbruster (b. 1950; University of Berne) for his outstanding contribution to structural mineralogy and cryst allography, especially to the study of Mn-rich minerals.