The palygorskite-sepiolite group1 of clay minerals includes those minerals whose structure, greatly elongated in one direction, confers on them a fibrous or lath-like crystal habit. It has been debated whether they should be classified as chain minerals or as layer minerals by those, on the one hand, who stress the preferential development of the structure in one direction, and those, on the other, who point out that the ribbons parallel to this direction are structurally similar to all layer silicates. Nevertheless, the regular interaction of the layers results in chains and channels in the direction of elongation; these chains are bound by links that are fewer than those within the sheets, or rather ribbons, themselves, and this determines the marked fibrous morphology. The structural arrangement led Zvyagin, Mishchenko, and Shitov (1963) to interchange the a and c axes used by others (e.g. Bradley, 1940; Nagy and Bradley, 1955; Caillère and Hénin, 1961), in order to conform with the orientations in all other clay minerals. This logical choice of axes will be used throughout this chapter, so that the unit-cell dimensions approximate to: a 5·2, b 18 (= 2a√3) (for palygorskite) or 27 (= 3a√3) (for sepiolite), c 13 Å. Space-group symbols are modified accordingly.