Summary: The crystal structure of kilchoanite, Ca6(SiO4)(Si3O10), has been determined from three- dimensional X-ray methods (r = 0·089 on 348 independent reflections). It is closely related to that of γ-Ca2SiO4, which is olivine-like. In kilchoanite, slabs of γ-Ca2SiO4 structure alternate with ones composed of Ca ions and finite chains of three tetrahedra; these have the composition Si3O108− and have rarely been observed previously. A related, synthetic phase, 8CaO. 5SIO2, probably has a similar structure but with a higher proportion of γ-Ca2SiO4 regions, and should thus be formulated as Ca8(SiO4)2(Si3O10). Disordered synthetic phases also occur that appear to have similar structures with still higher proportions of γ-Ca2SiO4 regions. These phases collectively form an interesting contrast to the chondrodite minerals.