Kamiokite occurs as granular to thick tabular crystals with hexagonal outlines, up to 3 mm in diameter, in molybdenite-quartz veinlets in the Kamioka Ag–Pb–Zn mining area, Gifu Prefecture, central Japan. Chemical formula: (Fe
2.01Mn
0.03)
2.04Mo
2.98O
8 on the basis of O = 8, or ideally Fe
2Mo
3O
8. The mineral is hexagonal with space group
P6
3mc,
a=5.782(2),
c=10.053(3) Å, Z = 2, cell volume = 291.09 Å
3; calculated density = 6.02 gm/cm
3. Strongest X-ray powder diffraction lines and their relative intensities (subject to preferred orientation): 5.03(100), 3.55(90), 2.509(75), 2.430(55), 2.006(40). Kamiokite is opaque, iron black in color, and metallic to submetallic in luster, with black streak. Cleavage [0001] perfect, fracture even. Vickers hardness (50 g load) is about 600 kg/mm
2, Mohs hardness 4.5. Density measured 5.96 gm/cm
3. In reflected light the mineral is gray with olive tint. Strong reflection dichroism, gray to olive gray. Anisotropism is strong, from light brownish gray to dark greenish gray. Reflectances (R
max, R
min) in air: 23.6, 16.0% (480 nm); 22.9, 15.3% (546 nm); 22.2, 14.7% (589 nm); 22.1, 14.3% (657 nm). The name is for the locality.
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