Some sillimanite-kyanite quartzites, which may grade into sillimanite-kyanite topazfels, from NE. Tanzania are described. In view of the suggestion that a solid-solution series might exist between topaz and sillimanite, the cell dimensions and some optical properties of both minerals were investigated. No abnormal results, suggesting solid solution, were obtained and the coexistence of topaz, sillimanite, and kyanite is ascribed to particular physico-chemical conditions.