Iwakiite, Mn²⁺(Fe³⁺, Mn³⁺)₂O₄, is tetragonal, space group P4₂⁄nnm, a 8.509, c 8.534Å, Z=8, and is found in rhodonite-quartz ore from the regionally metamorphosed manganese ore deposit of the Gozaisho mine, Fukushima Prefecture, Japan.
Wet chemical analysis by K. N. gave MnO 30.34, MgO 0.61, Mn₂O₃ 22.70, Fe₂O₃ 44.42, Al₂O₃ 0.32, SiO₂ 0.47, TiO₂ 0.30, SrO 0.04, Na₂O 0.02, K₂O 0.02, total 99.24%, corresponding to (Mn²⁺_0.982Mg_0.035)_Σ=1.017(Fe³⁺_1.277Mn³⁺_0.660Si_0.018 Al_0.014Ti_0.009)_Σ=1.978O₄ on the basis of O=4, or ideally Mn²⁺(Fe³⁺, Mn³⁺)₂O₄, where Fe³⁺:Mn³⁺=2:1. The strongest X-ray powder diffractions are 4.929(30) (111), 3.016(40) (202, 220), 2.570(100) (113, 311), 1.640(30) (333), 1.509(35) (404), 1.506(40) (440). They are similar to but distinct from those of jacobsite due to splitting of (404)–(440).
It is opaque. Colour is greenish black, lustre metallic. Streak is black. No cleavage. Density (g/cm³) 4.85 (meas.), 4.89 (calc.). Strongly magnetic. It is optically anisotropic. The reflection colour in air is olive grey like jacobsite without discernible reflection pleochroism. Between crossed polars the colour changes from yellowish to brownish grey.
The name is for the city name Iwaki, in which the locality is situated. Type material is deposited in National Science Museum, Shinjuku, Tokyo, Japan.

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The crystal structures of 1M-kinoshitalite and four 1M-manganese trioctahedral micas, all from Nodatamagawa mine, Iwate Prefecture, have been refined. Effect of cation substitutions in these structures is discussed based on correlations among structural and chemical compositional parameters. It was found that the ‘chemical expansion’, the expansion in the Mg octahedron by the substitution of Mn²⁺ for Mg, affects cell parameters and geometry of the tetrahedra and other polyhedra.

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Intergrowth of three phases, barium microcline, hyalophane and albite was observed under the microscope in a barium-containing alkali feldspar from the Noda-Tamagawa mine, Iwate Prefecture, Japan. It was clarified that there are two types of compositional gaps (K-rich phase-Ba-rich phase and K-rich phase-Na-rich phase) in the K-Ba-Na feldspar.

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The use of fission fragment damage tracks for dating is described. The dates are in good agreement with other radiometeric method. Annealing correction has been applied to the thermally lowered fission track ages of tektites and estimated to be 3%.

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The pyroxene-type MnSiO₃ synthesized at 10 GPa and 1200°C, is monoclinic with the cell dimensions of a=9.864(2), b=9.179(2), c=5.298(1)Å and β=108^°13′(1′). The space group is P2₁⁄c. Z=8. The structure was refined by the full-matrix least squares method to a final R=0.088 for the countercollected 1280 reflections. The structure is close to that of ferrosilite, FeSiO₃ (Burnham, 1967).

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