Hornblende commonly occurs as a euhedral to subhedral prismatic crystal with a maximum length of 2 mm. It occasionally contains pyroxene and cummingtonite in rocks from the central parts of the batholith. In the core, hornblende containing pyroxene is found exclusively in rocks of gabbroic to dioritic composition. Plagioclase is occasionally found in hornblende grains.

The pleochroism of hornblende in magnetite-series rocks (yellowish green) is distinctly darker than that of hornblende in ilmenite-series rocks (greenish yellow). Hornblende in magnetite-series rocks comprises the following:

X': gray yellow to pale greenish yellow or moderate yellowish green;
Y': moderate yellowish green or moderate olive brown to light olive brown or dusky yellow;
Z': moderate yellowish green or light olive brown to light olive.

Hornblende in ilmenite-series rocks is divided into:

X': pale greenish yellow or grayish yellow;
Y': moderate greenish yellow to moderate yellowish green, dusky yellow, or light olive brown;
Z': light olive brown to light olive or pale yellowish green. (Color names are based on Rock-Color Chart Committee, 1948.)

Occasionally a weak zonal pattern is seen (Figs. VI-1-A, -B, -C, and -F and Plate 11): the greenish rim is enriched in SiO₂, FeO^* (total Fe as FeO) and MnO, and the brownish core in TiO₂, Al₂O₃, MgO, Na₂O, and K₂O (Table VI-1). However, the rim is enriched in MgO when hornblende is in contact with biotite (Fig. VI-1-A).

Plate 11

Fifty representative chemical data on amphibole from the Tokuwa batholith rocks are shown in Table VI-2. Calculation of hornblende Fe₂O₃ contents was based on the method proposed by Papike et al. (1974): Fe₂O₃ contents were calculated from the charge balance equation, Fe³⁺ = Al^IV + Na_B - (Na, K)_A - Al^VI - 2Ti⁴⁺, contents of all oxides were normalized based on 23 oxygens, and new Fe₂O₃ contents were calculated until there was no change in the derived Fe³⁺/Fe²⁺ ratio. Hornblende from the batholithic rocks is generally identified as magnesio-hornblende, ferro-hornblende, actinolitic hornblende, and ferroactinolitic hornblende with small amounts of actinolite and ferroactinolite, according to the nomenclature of Leake (1978) (Fig. VI-2). Figure VI-2 also shows that the mg values (Mg/(Mg+Fe)) of hornblende in magnetite-series rocks (0.44 to 0.73; mean 0.55) are slightly but significantly higher than those of hornblende in ilmenite-series rocks (0.42 to 0.60; mean 0.49). This may be due either to consumption of Fe to form Fe-Ti oxide minerals in magnetite-series rocks or to the effect of oxygen fugacity on Fe partitioning between the magma and hornblende. The Si content (number of Si atoms in the unit cell formula of amphibole) of hornblende in magnetite-series rocks (6.59 to 7.37) is practically identical to that of hornblende in ilmenite-series rocks (6.63 to 7.79). Hornblende in magnetiteseries rocks (0.420 to 0.896; mean 0.674) is lower in atomic Fe²⁺+Fe³⁺) ratios than hornblende in ilmenite-series rocks (0.541 to 0.926; mean 0.711). High tetrahedral Al (Al^IV) contents are generally considered to be essential to balance the associated high Fe³⁺ contents in order to maintain a charge balance between the octahedral and tetrahedral sites. The analytical data (Table VI-2) on hornblende in the Tokuwa batholith rocks were examined using a series of correlation diagrams between paired constituents. Relatively significant correlations were observed between:

(Na+K) and Al^IV
Mn and Ti (negative correlation)
Mg/(Mg+Fe) and Mn (negative correlation, Fig. VI-3)
Si and (Na+K+Al^IV) (negative correlation, Fig, VI-4-A)
(Mg+Si) and (Fe³⁺+Al^IV) (negative correlation, Fig. VI-4-B)
(Mg+2Si) and (Ti+2Al^IV) (negative correlation, Fig. VI-4-C)

Many possible substitutions control amphibole chemistry, but the most common types of amphibole solid solution can be represented by the general formula (Leake, 1978):

A_0-1B₂C^VI₅T^IV₈O₂₂(OH, F, Cl)₂

where A = Na, K, (vacancy): A site;
      B = Ca, Mn²⁺, Fe²⁺, Mg, (Na): M4 site;
      C = Al, Fe³⁺, Fe²⁺, Ti, Mg, (Cr, Mn): M1+M2+M3 sites;
and T = Si, Al, (Cr, Fe³⁺, Ti): tetrahedral site.

Robinson et al. (1971) and Czamanske et al. (1977) showed that there are at least six possible schemes of coupled substitutions which involve the charge balance within the amphibole structure, in addition to simple substitutions, such as Fe²⁺ for Mg.

1		(Na, K)A + (Al)tet. for (   )A + (Si)tet. ……… edenite
2	(a)	(Al)M1-M3 + (Al)tet. for (Mg)M1-M3 + (Si)tet. ……… tschermakite
	(b)	(Fe)M1-M3 + (Al)tet. for (Mg)M1-M3 + (Si)tet.
3	(a)	(Na)M4 + (Al)M1-M3 for (Ca)M4 + (Mg)M1-M3 ……… glaucophane
	(b)	(Na)M4 + (Fe)M1-M3 for (Ca)M4 + (Mg)M1-M3 ……… riebeckite
4		(Na)A + (Na)M4 for (   )A + (Ca)M4 ……… richterite
5		(Ti)M1-M3 + 2(Al)tet. for (Mg)M1-M3 + 2(Si)tet.
6	(a)	2(Na)M4 + (Ti)M1-M3 for 2(Ca)M4 + (Mg)M1-M3
	(b)	(Na)M4 + (Ti)M1-M3 for (Ca)M4 + (Al)M1-M3
( ): vacant

It is concluded from the observed relations that at least coupled substitution mechanisms 1, 2(a), 2(b), 3(a), 3(b), and 5 above are operative in the hornblende of the Tokuwa batholith rocks. In particular, the presence of coupled substitutions 1 and 5 is demonstrated clearly in the correlation diagrams (Figs. VI-4-A and -C).

Upon examing the relations between the bulk chemical compositions of the rocks and the corresponding hornblende compositions, the following were found:

(1) The mg values of hornblende in magnetite-series rocks of the Tokuwa batholith decrease as the SiO₂ content of the host rocks increase, whereas the mg values of hornblende in ilmenite-series rocks show a rather narrow range of variation (Fig. VI-5-A). Czamanske et at (1981) reported that mg values of hornblende in magnetite-series granitoids of the Daito and Yokota granitic bodies decrease with the increase in SiO₂ of the host rocks. This was not found in the present study.

(2) MnO contents of hornblende increase as SiO₂ contents of host rocks increase (Fig. VI-6-B).

Cummingtonite occasionally occurs as an aggregate of several subhedral grains up to 0.4 mm long at the rim of hornblende crystals. The MnO contents of cummingtonite increase with the increase in SiO₂ of host rocks (Fie. VI-6-C).