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

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Garnet crystals, under the present consideration, occur abundantly in biotite-hornblende andesite which forms a small dome called Amataki-yama (Fig. 1), covering directly biotite granite, near a country town Tuda in the Prefecture of Kagawa. The prevailing size of crystals varies from 2mm to 15mm, with the crystal form dodecahedral or trapezohedral, otherwise the two combined. Though the crystals are seen as euhedral macroscopically (Fig. 10), they appear corroded by magma slightly but distinctly when they are seen under the microscope (Fig. 13), The zonal structure due to the change of chemical composition is distinct (Fig. 12). The refractive indices of the outer zone and the core of a crystal are 1.7957 and 1.8008 respectively (Tab. 3). The cell-dimensions of the same part of the same crystal are given as 11.545A and 11.553A (Tab. 5). The specific gravity of the fine grains purified from different crystals is 4.048 at 4°C (Tab. 4). The chemical composition of the garnet, computed from Kôzu's formula, using the three physical constants given above, can be given as Alm=59.91%, Py=22.20%, Sp=6.47%. And=5.71% and Gr=5.71% (Tab. 7). It is worthy to note that these chemical ratios are approximately the same as those obtained by computation from the chemical analysis made by K. Yagi in our Institute, whose analysis will be published in this journal in near-future. From the microscopical observations and also from the paragenetic relations to the associated minerals, the present garnet can be taken as a xenocrystic mineral, derived from an argillaceous rock, from which the mineral had been crystallized out before it was caught up into the andesite magma.
The present study was carried out under the guidance of Prof. S. Kôzu, to whom the writer is deeply indebted.

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