Summary: The classification of framework structures built from chains of four-membered rings of type UUDD is incomplete: there are two basic ways in which the chains can be cross-linked to form eight-membered rings. The first type of frame-work (already described) is extremely flexible, permitting the cavities to be either large or small; the second type is inflexible and the cavities are small. Feldspar belongs to the second type and had been incorrectly ascribed to the first type: paracelsian, harmotome, and gismondine had been correctly placed in the first group.