The iron-titanium oxides in the granitic rocks of Southwest Japan have been examined under the microscope and analyzed with the electron probe micro-analyzer. Ilmenite containing up to 4 mole percent hematite molecule is almost sole iron-titanium oxide present as an early accessory constituent of the granitic rocks in the Outer zone, Ryoke zone, and Sanyo. Naegi zone of Southwest Japan. While, magnetite, magnetite intergrown with ilmenite, and hemo-ilmenite containing up to 20 mole percent hematite molecule are present as early accessory constituents in the Sanin-Shirakawa zone.
From the chemical composition of the granitic rocks of Southwest Japan, it is suggested that the granitic magmas in the Sanin-Shirakawa zone were more oxidized than those in other three zones.
Residual ilmenite sand deposits derived from gabbroic rocks are found only in the Ryoke zone, while residual magnetite sand deposits originated in granitic rocks are found only in the Sanin-Shirakawa zone. Thus, the distribution of residual iron sand deposits well corresponds to the generalization stated above.