Abstract: It is suggested that the minerals of the palygorskite-sepiolite group occupy the region of discontinuity between dioctahedral and trioctahedral minerals. There is thus a series of minerals, the two extremes having a planar lattice, and the intermediate members a fibrous lattice. The lamellar-fibrous change in structure takes place as the number of vacancies increases progressively.
Among the minerals of fibrous structure, the structural change from sepiolite to attapulgite occurs in such a way as to allow no more than one vacancy per structural fiber, and per half cell, as if this vacancy were not distributed at random, but symmetrically among the octahedral positions. The greater the number of vacant octahedral positions, the smaller the extension of the octahedral layer. The margin of variation in the number of vacant octahedral positions per structural fiber is greater in sepiolite than in attapulgite, probably because of the greater width of the fiber in the former mineral.