Topotactic relations among pyrolusite, manganite, and Mn₅O₈: A high resolution transmission electron microscopy investigation

Pyrolusite (MnO₂, tetragonal) is commonly created in the natural environment by the oxidation of manganite (MnOOH, monoclinic). Such secondary pyrolusite typically displays anomalous, nontetragonal characteristics. In this high-resolution transmission electron microscopy (hrtem) study, Mn₅O₈ (monoclinic) was found intergrown with pyrolusite that had formed from manganite. This occurrence of Mn₅O₈ is of interest for several reasons: (1) Mn₅O₈ has been identified in a natural manganite-pyrolusite mixture. Assuming that it is not an artifact of hrtem observation, this is the first report of naturally occurring Mn₅O₈. (2) Aligned monoclinic Mn₅O₈ intergrown with pyrolusite may be partly responsible for the nontetragonal character of secondary pyrolusite. (3) Mn₅O₈-pyrolusite intergrowths occur adjacent to manganite. This occurrence suggests that the oxidation of manganite may form both pyrolusite and Mn₅O₈.

Another new Mn oxide has been identified through selected-area electron diffraction. We hypothesize that this phase is a structural modification of pyrolusite. Ordering of OH groups and Mn atoms of different oxidation states is a possible explanation for this modification.