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.

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