Manjiroite, a new manganese dioxide mineral, occurs in the oxidation zone of rhodonite-tephroite-rhodochrosite bedded ore deposits of Kohare Mine, Iwate Prefecture, Japan, running along the boundaries between chert and schalstein of Permian age. It is associated with pyrolusite, nsutite, birnessite, cryptomelane and goethite. Manjiroite is dense compact masses up to 10×8×5cm., with marked conchoidal fracture. Colour dark brownish-gray, luster dull, streak brownish-black. No cleavage, sp. gr. 4.29, Vickers hardness 181 av. Under the microscope opaque, distinctly anisotropic with weak pleochroism.
Analysis gives MnO₂ 85.79, MnO 3.17, CuO 0.03, CoO none, ZnO 0.03, MgO 0.18, CaO 0.22, Na₂O 2.99, K₂O 1.39, BaO 0.16, Al₂O₃ 0.62, Fe₂O₃ 0.40, TiO₂ none, SiO₂ 0.12, H₂O- 0.68, H₂O+3.92, sum 99.71%. This corresponds to (Na_0.73 K_0.22 Ca_0.03 Ba_0.01) _0.99 (Mn⁴⁺_7.46 Mn₂₊^0.34 A1_0.09 Fe_0.04 Mg _0.03) _7.96 O₁₆⋅1.64H₂O or (Na, K) Mn⁴⁺₈ O₁₆⋅nH₂O (probably n<2). The DTA curve shows endothermal effects at 530°, 905°, and 980°C. X-ray study shows it to be tetragonal, a_O 9.916, c_O 2.864A, isostructural with cryptomelane. There is probably an isomorphous series between cryptomelane and majiroite. The strongest lines of the X-ray pattern are 2.406 (100) (121), 7.02 (98) (110), 3.14 (92) (130), 4.94 (77) (200), 2.160 (69) (301), 1.839 (46) (141), 1.548 (46) (251), 2.332 (38) (330), 1.431 (38) (002).
The name is given in honour of Dr. Manjiro Watanabe, mineralogist, economic geologist and Emeritus Professor of Tohoku University, Japan. The mineral, Manjiroite, has been approved by the Commission on New Mineral and Mineral-Name, I. M. A.

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Hydrothermal reactions of some clay minerals with sodiumhydroxide were carried out at water vapor pressure (110kg/cm²) and temperature 300°C, with an autoclave of Morey type. With an increasing amount of NaOH, talc was converted into a montmorillonite-like phase, achloritic phase and finally a serpentine phase and brucite. By heat treatmeat of the synthesized samples at 1000°C, enstatite was formed from a sample synthesized under presence of a small amount of NaOH, forsterite from medium quantity, and periclase from a large quantity.
Natural magnesium rich chlorite, i.e. leuchtenbergite, was also converted into a serpentine mineral and brucite under a great quantity of NaOH. In the case of magnesium poor minerals, e.g. pyrophyllite, kaolinite, montmorillonite, sericite, and analcime was synthesized from pyrophyllite and montmorillonite and nepheline hydrate I was formed from kaolinite and sericite under presence of a small amount of NaOH. Synthetic cancrinite and nosean were synthesized from pyrophyllite, kaolinite, sericite and montmorillonite under presence of a large quantity of NaOH. Nepheline was formed from the mixture of synthetic cancriuite and nosean after heating at 1000°C for an hour.

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