Electron microscopic study of dehydration transformations. II. The formation of “superstructures” on the dehydration of goethite and diaspore

It is shown that the satellites observed in the diffraction pattern of the dehydrated single crystal of the mineral goethite, which were previously attributed to a “superstructure,” are better interpreted in terms of small angle scattering by a “texture” of the dehydration product hematite. This texture consists of rows of cavities parallel to the H(003) = G(100) planes with an average distance of 30–50 Å, depending on the stage of the transformation, in a highly twinned hematite matrix. In corundum resulting from the dehydration of diaspore a more isotropic texture is formed and the diffraction pattern exhibits “halos” instead of rows of satellite spots.