Single crystals of the previously unknown potassium calcium silicate K2Ca4Si8O21 (1) have been grown from a nonstochiometric melt as well as using, a KCl flux. The compound is triclinic with the following basic crystallographic data: space group P (1) over bar, a = 6 8052(3) angstrom, b = 7.1049(3) angstrom, c = 112132(5) angstrom, alpha = 96 680(4)degrees, beta = 105.280(4)degrees, gamma = 109.259(4)degrees, Z = 1, V = 481.28(4) angstrom(3). The crystal structure was solved by direct methods based on a single-crystal diffraction data set collected at ambient conditions. From a structural point of view, K2Ca4Si8O21 belongs to the group of single-layer silicates. The layers parallel to (001) are characterized by a complex arrangement of 6-, 8-, 10-, and 12-membered tetrahedral rings. The sheets can be built from the condensation of loop-branched funfer single chains running parallel to [100], i.e., the crystallochemical formula can be written as K2Ca4{IB,5, 1(infinity)(2)}[Si8O21], Compound I is the first example of a loop-branched layer silicate containing secondary (Q(2)) as well as tertiary (Q(3)) tetrahedra. Linkage between the layers is provided by calcium and potassium cations, which are distributed among a total of three crystallographically independent nontetrahedral sites. Alternatively, the structure can be described as a heteropolyhedral framework, based on SiO4 tetrahedra and CaO6 octahedra. The irregularly coordinated K-cations in turn are incorporated in tunnels of the network running parallel to [110] The structural investigations have been completed by Raman spectroscopy. The allocation of the bands to certain vibrational species has been aided by density functional theory (DFT) calculations.