This paper is concerned with three of the fairly common, manganese oxide minerals, manganite, hausmannite, and braunite. The study includes the precise determination of the minerals: (1) by X-ray diffraction and (2) spectrographic analyses. Most of the numerous specimens were collected by the writer and associates on the U. S. Geological Survey at localities in the United States, supplemented by other specimens from wellknown localities elsewhere, especially Canada and Mexico. Following determinations of the minerals, the geological environment of the specimens was reviewed in the hope of making confident interpretations of their modes of origin. In many descriptions of the minerals, even during recent years, it has seemed that their identities have not been positively determined and the criteria by which the modes of origin are interpreted leave some doubt that conclusions are correct.The present review indicates confidently that hausmannite is hypogene although recently, hausmannite from several localities has been interpreted as supergene. In those places where the geologic environment is here reviewed, the conclusion of supergene origin is open to question. On the other hand, specimens of manganite and braunite examined seem to fall readily into two groups: (1) a group in which manganite and braunite are clearly hypogene and (2) a group in which these minerals are clearly supergene. In other words, both manganite and braunite can be formed either by deposition from solutions rising from depth or by descending solutions of surficial origin. Concerning braunite, it should be noted that it is not only abundant but is the dominant oxide in deposits that have been subjected to intense metamorphism; i.e., many deposits in India.

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