Summary: Single crystal X-ray studies of the dehydration of goethite to hematite at low temperatures show that satellite reflections occur close to the hematite diffraction spots. At higher temperature these satellites are replaced by diffuse intensity regions, which disappear on further heating. The satellites are situated on each side of hematite spots on reciprocal lattice lines parallel to the c*-axis of the hematite; their spacing corresponds to a periodicity in direct space of about 32 Å. A similar phenomenon is observed on the dehydration of diaspore to corundum, with a spacing for the satellites of about 39 Å.
The mode of occurrence of the satellites, both in position and intensity, could correspond to a sinusoidal or square-wave structure amplitude modulation along the c-axis of the hematite and corundum crystallites, before dehydration is complete.