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-m>Km>α, 293 K) in space m>Pm>1, with m>am> = 6.672(1), m>bm> = 6.719(1), m>cm> = 6.725(1) Å, α = 108.87(3), β = 106.72(3), γ = 107.61(3)°, m>Vm> = 245.82(6) Å3, m>Zm> = 1, m>Rm>(m>Fm>) = 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 toinfinite [Si3O9] chains extending approximately along [111]. This connectivity results in a microporous character with two different, narrow channels that extend parallel to [111] and [100] 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 tothat in pectolite, NaCa2[HSi3O9].