雨瀧山産柘榴石及び其の母岩の化學成分

抄録

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.