The crystal structure of a new mineral britvinite Pb_7.1Mg_4.5(Si_4.8Al_0.2O₁₄)(BO₃)(CO₃)[(BO₃)_0.7(SiO₄)_0.3](OH, F)_6.7 from the Lángban iron-manganese skarn deposit (Värmland, Sweden) is determined at T = 173 K using X-ray diffraction (Stoe IPDS diffractometer, λMoKα, graphite monochromator, 2θ_max = 58.43°, R = 0.052 for 6262 reflections). The main crystal data are as follows: a = 9.3409(8) Å, b = 9.3579(7) Å, c = 18.8333(14) Å, α = 80.365(6)°, β = 75.816(6)°, γ = 59.870(5)°, V = 1378.7(2) ų, space group P1, Z = 2, and ρ_calcd = 5.42 g/cm³. The idealized structural formula of the mineral is represented as [Pb₇(OH)₃F(BO₃)₂(CO₃)][Mg_4.5(OH)₃(Si₅O₁₄)]. It is demonstrated that the mineral britvinite is a new representative of the group of mica-like layered silicates with structures in which three-layer (2: 1) “sandwiches” composed of tetrahedra and octahedra alternate with blocks of other compositions, such as oxide, oxide-carbonate, oxide-carbonate-sulfate, and other blocks. The tetrahedral networks (Si₅O₁₄)_∞∞ consisting of twelve-membered rings are fragments of the britvinite structure. Similar networks also form crystal structures of the mineral zeophyllite and the synthetic phase Rb₆Si₁₀O₂₃. In the crystal structures under consideration, the tetrahedral networks differ in the rotation of tetrahedra with respect to the layer plane.