Summary: Rhodonite, rhodochrosite, spandite, psilomelane, beldongrite, braunite, sitaparite, and vredenburgite from a collection hy Fermor have been studied by the X-ray powder diffraction method. The cell dimensions of all forms of eryptomelane—massive, horny, botryoidal, reniform, mamillated, and stalactitic—are a = 9.82 Å., c= 2.86 Å.. whereas the cell dimensions of shiny pitch-like beldongrite are a= 9.82 Å., c= 2·87 Å. The amorphous admixture associated with cryptomelane is revealed by a broad halo, 4·60 Å. to 3·90 Å., in the powder pattern. Aminoff's crystal data for braunite are discussed with a different orientation, and a new space group, I 4/mmm(D4h17), is assigned after indexing the powder pattern. Fermor' sitaparite (bixbyite) is assigned a new space group I m3 (Th5), different from that proposed by Pauling et al., on the basis of a fresh indexing of the powder pattern. Manganese-garnet from the gondite series has a cell-size of the order of spessartine, whereas the cell-size of manganese-garnet from the kodurite series varies from 11·72 to 11·95 Å. Fermor's spandite from the kodurite series is a mixture of spessartine, grossular, and andradite garnet-molecules with almandine and pyrope as minor components. Ramsdellite and γ-MnO₂ or β-MnO₂ are found in a number of samples of manganese ores.