The new, non-centrosymmetric chain silicate, K2.9Rb0.1ErSi3O9, was prepared by a high-temperature flux-growth technique, and its crystal structure was determined from single-crystal X-ray intensity data (Mo-Kα, 293 K) in space P1, with a = 6.672(1), b = 6.719(1), c = 6.725(1) Å, α = 108.87(3), β = 106.72(3), γ = 107.61(3)°, V = 245.82(6) Å3, Z = 1, R(F) = 2.81%. The compound represents a novel structure type. K2.9Rb0.1ErSi3O9 is characterized by a mixed octahedral-tetrahedral framework, in which each corner of the isolated ErO6 octahedron (<Er—O> = 2.26 Å) is linked to infinite [Si3O9] chains extending approximately along . This connectivity results in a microporous character with two different, narrow channels that extend parallel to  and  and host K+ cations. The atomic arrangement is strongly pseudorhombohedral. A single-crystal Raman spectrum of K2.9Rb0.1ErSi3O9 is in agreement with the low space-group symmetry. Relations to minerals and synthetic compounds based on [Si3O9] chains are discussed, revealing that the geometry of the chain in K2.9Rb0.1ErSi3O9 is similar to that in pectolite, NaCa2[HSi3O9].