The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists

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The analysis of the garnet-hearing andesite from Amataki, Kagawa-ken, Japan, reveals some peculiarities in the chemical composition, relatively high in Al₂O₃, K₂O and Na₂O and relatively low in FeO, Fe₂O₃, MgO and CaO, compared with the average of 113 analyses of Japanese andesite (Tab. 3). It would be reasonable to ascribe these characteristics to the contamination of mica schist in the andesitic magma. The chemical analysis of the garnet gives the following result; SiO₂=37.52, TiO₂=0.48, Al₂O₃=21.03, Fe₂O₃=2.68, FeO=25.17, MgO-=4.66, CaO=6.06, MnO=1.89, H₂O_=0.58, the total being 100.07% (Tab. 4). The chemical formula can be expressed by
(Fe'''₈ Fe''₁₆₆ Mg₅₆ Ti₃Ca₅₁ Mn₁₃)₂₉₇ (Al₁₉₂ Fe'''₈)₂₀₀ (Si₂₉₆ Al₃)₂₉₉ O₁₂₀₀ in which Ti atom is considered to replace Mg instead of Si. From the above result, the mixing ratios of the end members are given as
Alm=58.14, Py=19.44, Sp=4.48, And=8.47, Gr=9.47 mol%. It is noteworthy that these ratios agree fairly well with those obtained by T. Takéuti by computation of Kôzu's formula, using the observed physical constants (Tab. 6). The chemical composition of our garnet resembles that of the garnet characteristic in the mica schists (Tab. 7). It is interesting to note that the most of the garnets found in the andesitic rocks also fall into this category (Tab. 8). From the above statements, it is concluded that the present garnet crystals were already formed in the mica schist and then they were taken into the andcsitic magma and have remained slightly dissolved in the cooling magma as xenocrystic minerals.

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Optical orientation, refractive index and extinction angle of the anorthite crystal newly ejected in 1940 from Miyake-volcano were measured in parallel with the anorthite from the same volcano erupted in 1874, and also with the crystals of Tarumae-volcano erupted in 1909, and with the same crystal occurring as phenocryst in basaltic andesite from Mitaki, near Sendai. The optical orientations were obtained. by using Leitz's universal stage (UT4), the results being given in Tabs. 9, 12 and 13; and the refractive indices were measured by a new model of Klein's total-reflectometer in which the millor is movable for the left or right side of the glass hemi-sphere. The results are given in Tabs. 15, 16 and 17. The extinction angles measured by a usual method in this case can be seen in Tabs. 19 and 20.

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