Abstract: Na-saturated birnessite was synthesized by oxidizing an alkaline MnCl2 solution with gaseous O2. Transmission electron microscopy (TEM) showed no morphological change upon K-saturation of the birnessite, but selected-area diffraction (SAD) revealed structural disorder. Stepwise heating of the K-birnessite to 800°C yielded cryptomelane, as indicated by X-ray powder diffraction (XRD). Various degrees of transformation to the final cryptomelane product were observed by TEM and SAD, but not by XRD. Unaltered birnessite formed chiefly as thin, platy crystals of apparent hexagonal outline. The cryptomelane crystals that formed by heating the K-saturated birnessite were acicular. Birnessite crystals, partially transformed to cryptomelane and displaying non-integral diffraction spots, formed plates having linear striations and rods twinned at 60° to each other. These intermediate products commonly retained some of the original hexagonal appearance of the parent birnessite. Structural disorder was detected in the partially transformed crystals by SAD. TEM revealed 6.8- and 4.8-Å spacings of the (110) and (200) planes of cryptomelane, respectively. The basal planes of birnessite layers (001) appeared to correspond topotactically to the (110) plane of cryptomelane during these transformations.