Summary: The transformation of groutite (α-MnOOH) by heating has been studied at 300° C in air, by single-crystal and powder X-ray methods. At this temperature groutite transforms topotactically into pyrolusite (MnO₂), the a, b, and c axes of groutite becoming respectively the a, b, and c axes of pyrolusite (in pyrolusite b = a). At various stages of the transformation other weak and diffuse spots were observed on X-ray oscillation photographs, which could not be ascribed to pyrolusite. Some of these extra spots fit well to an α-Mn₂O₃ structure (isostructural with hematite), with c 14·3 and a 4·9 Å; the other few spots could not be identified.

The transformation of α-MnOOH into MnO₂ is explained by a homogeneous mechanism, with migration of protons and electrons to the crystal surface. A detailed interpretation of this mechanism is presented on the basis of the close-packing characteristics of these two structures.