Most of the chromite are deposited along distinctive fissure in an ultra-basic rock. The country rocks, such as serpentine and pyroxenite, which were attacked by alkaline ground water through the fissures were partly dissolved and altered into saponite filling the fissures of the serpentine and chromite bodies.
The color of the mineral from No. 7 deposit of the Wakamatsu mine is greyish white and the luster is greasy; the specific gravity is 2.27 and the hardness is 2.5. In thin section, the mineral is colorless and isotropic, but it shows weak birefringence in another part of the same specimen. The mean refractive index Nm is 1.525. The differential thermal curve (Fig. 1) of the mineral shows that the mineral is a magnesium rich montmorillonite. The writer believes that the mineral is saponite, judging from the observation of the electron micrograph (Fig. 2). The mineral reacts with the saturated benzidine, and then assumes pale blue color.
The mineral shows low content in (FeO+Fe
2O
3) and high contents in MgO, the montmorillonite of this kind, has not been reported in Japan. The formula can be expressed as (OH)
4.02 (Si
7.16 Al
0.93)
8.09 (Mg
5.84 Fe''
0.01 Fe'''
0.09 Al
0.06)
6.00((Ca/2)
0.28 Na
0.09 K
0.04)
0.41 O
19.98mH
2O, which corresponds almost to the ideal formula of saponite suggested by Ross and Hendricks.
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