Shunso Ishihara studied the opaque minerals in common granitic rocks and classified them into two series: magnetite-series and ilmenite-series. The magnetite-series contains magnetite (0.1–2 vol.%), ilmenite, hematite, pyrite, sphene, epidote, and high ferric/ferrous (and high Mg/Fe) biotite, while the ilmenite-series contains ilmenite (less than 0.1 vol.%), pyrrhotite, graphite, muscovite, and low ferric/ferrous (and low Mg/Fe) biotite. The mineral assemblages suggest that the magnetite-series granitoids formed under higher oxygen fugacity than the ilmenite-series granitoids during solidification. The boundary between the two series is near the Ni–NiO buffer. The magnetite-series granitoids are thought to have formed in a deep level (upper mantle and lowest crust) without interaction with carbon-bearing materials, while the ilmenite-series granitoids formed in the middle to lower continental crust and interacted with carbon-bearing metamorphic and sedimentary rocks. The magnetite-series granitoids are associated with porphyry copper-molybdenum deposits, while the ilmenite-series granitoids are associated with greisen-type tin-wolframite deposits. The lack of porphyry copper deposits in Mesozoic orogenic belts in East Asia is related to the general scarcity of magnetite-series granitoids in this region. The study discusses the distribution and mineralogical characteristics of the two series of granitoids in Japan, including their occurrence in different tectonic units and belts. The magnetite-series granitoids are more common in the central Sanin belt, while the ilmenite-series granitoids are more common in the northern part of the Sanyo belt. The distribution of the two series is illustrated in Figure 1. The mineralogical characteristics of the two series are distinct, with the magnetite-series granitoids containing a higher volume percentage of opaque minerals (0.1–2 vol.%), mostly magnetite, while the ilmenite-series granitoids are practically free of opaque oxides. The magnetite-series granitoids have higher ferric/ferrous ratios and higher Mg/Fe ratios in biotite, indicating higher oxygen fugacity. The ilmenite-series granitoids have lower ferric/ferrous ratios and lower Mg/Fe ratios in biotite, indicating lower oxygen fugacity. The genetic implications of the two series are discussed, with the magnetite-series granitoids forming under higher oxygen fugacity conditions and the ilmenite-series granitoids forming under lower oxygen fugacity conditions. The presence of graphite in the ilmenite-series granitoids suggests a reducing environment, while the presence of magnetite in the magnetite-seriesShunso Ishihara studied the opaque minerals in common granitic rocks and classified them into two series: magnetite-series and ilmenite-series. The magnetite-series contains magnetite (0.1–2 vol.%), ilmenite, hematite, pyrite, sphene, epidote, and high ferric/ferrous (and high Mg/Fe) biotite, while the ilmenite-series contains ilmenite (less than 0.1 vol.%), pyrrhotite, graphite, muscovite, and low ferric/ferrous (and low Mg/Fe) biotite. The mineral assemblages suggest that the magnetite-series granitoids formed under higher oxygen fugacity than the ilmenite-series granitoids during solidification. The boundary between the two series is near the Ni–NiO buffer. The magnetite-series granitoids are thought to have formed in a deep level (upper mantle and lowest crust) without interaction with carbon-bearing materials, while the ilmenite-series granitoids formed in the middle to lower continental crust and interacted with carbon-bearing metamorphic and sedimentary rocks. The magnetite-series granitoids are associated with porphyry copper-molybdenum deposits, while the ilmenite-series granitoids are associated with greisen-type tin-wolframite deposits. The lack of porphyry copper deposits in Mesozoic orogenic belts in East Asia is related to the general scarcity of magnetite-series granitoids in this region. The study discusses the distribution and mineralogical characteristics of the two series of granitoids in Japan, including their occurrence in different tectonic units and belts. The magnetite-series granitoids are more common in the central Sanin belt, while the ilmenite-series granitoids are more common in the northern part of the Sanyo belt. The distribution of the two series is illustrated in Figure 1. The mineralogical characteristics of the two series are distinct, with the magnetite-series granitoids containing a higher volume percentage of opaque minerals (0.1–2 vol.%), mostly magnetite, while the ilmenite-series granitoids are practically free of opaque oxides. The magnetite-series granitoids have higher ferric/ferrous ratios and higher Mg/Fe ratios in biotite, indicating higher oxygen fugacity. The ilmenite-series granitoids have lower ferric/ferrous ratios and lower Mg/Fe ratios in biotite, indicating lower oxygen fugacity. The genetic implications of the two series are discussed, with the magnetite-series granitoids forming under higher oxygen fugacity conditions and the ilmenite-series granitoids forming under lower oxygen fugacity conditions. The presence of graphite in the ilmenite-series granitoids suggests a reducing environment, while the presence of magnetite in the magnetite-series