Magnetite in magnetite-series granitoids occurs as a euhedral (octahedral or dodecahedral) crystal of up to 5 mm in size, within or in contact with hornblende and biotite grains. The mineral occasionally occurs within plagioclase of more than An₃₅. Ilmenite commonly occurs with magnetite, and is rarely contained in the latter as lamellae In magnetite-series rocks. Very rarely a small sulfide droplet in either magnetite or ilmenite grains is found. No cloth microtexture (Ramdohr, 1953) has been observed. Iron oxide minerals replacing magnetite are occasionally seen in altered magnetite-series granitoids accompanied by sericitization of plagioclase and chloritization or epidotization of biotite. Ilmeniteseries granitoids are completely free of magnetite under ordinary microscope magnification (×400).

Sixteen representative chemical compositions of magnetite samples are reported in Table VI-7. The compositions are nearly homogeneous (Figs. VI-1-E and -I). This may be attributed to re-equilibration during cooling (Czamanske and Mihalik, 1972; Czamanske et al., 1977). Aluminum, Mn, and Mg contents of magnetite are correlated with each other (Fig. VI-12-A). When magnetite and ilmenite are in contact through a mutual boundary or grain to grain, ulvöspinel contents of magnetite are relatively high. No systematic relation of magnetite and biotite cmpositions could be confirmed.