Abstract
It has been accepted that the crystal structure of sepiolite consists of talc-like ribbons, each of which is linked to four identical ribbons to construct a chain structure. In this structural outline, all possible structural types can be obtained by the combination of three factors controlling the sepiolite structure, (1) the positions where tetrahedral sheet inverts apical directions, (2) mode of stacking of tetrahedral sheets in a talclike ribbon and (3) changes of stacking direction in successive ribbons. These possibilities are reduced to eight distinct structures; two orthorhombic, two monoclinic and four triclinic structure. They can be regarded as polytypes of sepiolite. Two structural models proposed by Nagy and Bradley (1955) and Brauner and Preisinger (1956) are involved in these eight polytypes.
The fact that the calculated X-ray powder patterns for eight structures resemble one another suggests the difficulty in identifying the kinds of polytypes by means of X-ray powder diffraction.
