Biotite occurs as a euhedral to subhedral crystal of up to 5 mm in size or forms an aggregate of several grains associated intimately with hornblende grains. It frequently contains inclusions of ilmenite (magnetite), apatite, zircon, and plagioclase. It often alters slightly: biotite in magnetite-series rocks alters to an aggregate of chlorite, epidote, and ilmenite, whereas biotite in ilmenite-series rocks alters to an aggregate of chlorite, epidote, and sphene. The pleochroism of biotite in magnetite-series rocks is reddish to greenish brown, whereas that of biotite in ilmenite-series rocks is brownish red, as detailed below:

Biotite in magnetite-series rocks:

X': pale brownish yellow to pale yellowish orange;
Y', Z': dark reddish brown to dark greenish brown.

Biotite in ilmenite-series rocks:

X': pale yellow to grayish yellow;
Y', Z': dark brownish red to dark greenish brown.
(Color names are based on Rock-Color Chart Committee, 1948.)

The brownish tinge of biotite may indicate high TiO₂ contents (see Deer et al., 1962). It occasionally has kink banding.

Thirty-five representative biotite samples were newly analyzed by EPMA (Table VI-3). No compositional zoning was observed (Figs. VI-1-A and -H).

Compositional zoning may not have been present originally in biotite, or original zoning may have been obliterated by re-equilibration through cation diffusions during the postmagmatic stage.

The Mg/(Mg+Fe+Mn) ratios of biotite in magnetite-series rocks (0.35 to 0.48; mean 0.43) are similar to those of biotite in ilmenite-series rocks (0.35 to 0.45; mean 0.41), and the Si content of biotite in magnetite-series rocks (5.20 to 5.91) is slightly higher than in ilmenite-series biotite (5.16 to 5.90) (Fig, VI-7). Tainosho et al. (1979) showed the same relation between biotite in granitoids from the Ryoke, San'yo, and San'in Belts: the former two belong to ilmenite-series and the last to magnetite-series belts.

The Ti contents of biotite in magnetite-series rocks (0.29 to 0.62) are slightly higher than those of biotite in ilmenite-series rocks (0.22 to 0.56). The Mn contents of biotite from magnetite-series rocks (0.01 to 0.09) has a wider range than that in biotite from ilmeniteseries rocks (0.03 to 0.08). The Mg content of biotite in magnetite-series rocks is slightly higher than in biotite in ilmenite-series rocks.

Although a high biotite Ti content suggests complexity of composition, Figure VI-7 implies annite-siderophyllite-phlogopite-eastonite substitution.

The analytical data on biotite in the Tokuwa batholith rocks were examined using a series of correlation diagrams and the following significant correlations were observed:

Mg and Fe (negative correlation)
Mg and Ti (negative correlation)
(Ti+2Al^IV) and (Mg+2Si) (negative correlation, Fig. VI-8).

Biotite in magnetite-series rocks has a lower Al^IV value than biotite in ilmenite-series rocks. The substitution (OH, F)O may have occurred in place of the SiAl^IV substitution to compensate for the excess electric charge due to the incorporation of Fe³⁺. Wones (1980) indicated that substitution of Ti⁴⁺ in biotite for (Fe, Mg)²⁺ can be compensated for in following ways:

1     Ti + 2Al^IV for Mg + 2Si
2     Ti + (   ) for 2Mg     (   ): vacant site
3     Ti⁴⁺ + 2O^2- for (Fe, Mg)²⁺ + 2OH^-.

A fairly good negative correlation between (Ti+2Al^IV) and (Mg+2Si) for biotite in the Tokuwa batholith, as shown in Fig. VI-8, confirms that the substitution of Ti for Mg is compensated for by the substitution of Al^IV for Si (type (1)).

Morita (1970) reported on biotite compositions analyzed by H. Haramura of the University of Tokyo (Table VI-4): sample NM 65070902 collected from Tokuwa, Mitomi Village, probably belonging to magnetite-series granodiorite of the Tokuwa batholith and sample NM 65032301 collected from Hajikano, Yamato Village belonging to ilmenite-series granodiorite of the batholith. The SiO₂ contents of host rocks are practically the same (67.8%, NM 65070902; 70.1%, NM 65032301). Haramura's analyses of biotite agree with those of this study. The compositional characteristics of biotite in the two series of rocks include the following: biotite in ilmenite-series rocks is generally enriched in Al^IV and total Fe, and has a high Fe²⁺/(Fe²⁺+Fe³⁺) ratio, whereas biotite in magnetite-series rocks is enriched in Si and Mg. Manganese, Ti, and total alkali contents are nearly the same; however, biotite in ilmenite-series rocks is enriched in K and biotite in magnetite-series rocks is enriched in Na.

The following possible relations between the bulk chemical compositions of host rocks the corresponding biotite compositions were found:

(1) MnO contents of biotite increase with the increase in SiO₂ contents of host rocks (Fig. VI-6-A).

(2) The mg values of biotite slightly decrease with the increase in SiO₂ contents of host rocks, and those of biotite in both series of rocks are quite similar (Fig. VI-5). This is contradictory to withthe findings of Czamanske et al. (1981).

The chemistry of biotite from the xenoliths of biotite hornfels (serial numbers 8-10 and 8-11 in Table VI-3) is nearly the same as that of host ilmenite-series granitoids. However, the MnO contents of biotite in the xenoliths are slightly higher than those in host rocks.