Abstract: New chemical analyses, electron and X-ray powder diffraction data, and comparison with gyrolite and reyerite show that tungusite has the ideal formula [Ca14(OH)8](Si8O20)(Si8O20)2[Fe92+(OH)14], symmetry P1¯ and a = 9.714(9), b = 9.721(9), c = 22.09(3), α = 90.13(1)°, β = 98.3(2)°, γ = 120.0(1)°, Z = 1. A structural model for tungusite is derived by splitting the double tetrahedral layer of reyerite and inserting a trioctahedral X sheet which is ideally occupied by Fe²⁺. Polytypism phenomena due to different relative positions between tetrahedral and X sheets are discussed. A substitutional solid solution represented by the formula [Ca₁₄(OH)₈]Si_24−yAl_yO₆₀[Na_xM_9−(x+z)□_z(OH)_{14−(x+y+2z)}·(x+y+2z)H₂O] includes tungusite (x = y = z = 0, M = Fe²⁺ and gyrolite (x = 1, y = 1, z = 6, M = Ca).