[M-ZZ52] Geology and culture

[MZZ52-06] Petrography of 'Oshima Ishi' stone, Imabari, Ehime prefecture

Takahito Onishi², *Satoshi SAITO¹ (1.Graduate School of Science and Engineering, Ehime University, 2.Faculty of Science, Ehime University)

Keywords:granite, Oshima Ishi, microcrack

Oshima Ishi' quarried at northern Oshima island in Imabari city, Ehime prefecture, is one of famous stone in Japan used for tombstone, monuments etc. This stone is gray colored fine-grained granitoid. The 'Oshima Ishi' develops three mutually perpendicular cleavage planes including rift plane, grain plain and hardway plane called ‘Me’, ‘Niban’ and ‘Shiwa’ in Oshima, respectively. Among three planes, subtle differences in color tone are observed after polishing stone surface.
In this study, we have performed detained petrography describing mineralogy, modal composition and microcrack distribution for 'Oshima Ishi' samples collected from 14 quarries.
Based on modal compositions, 'Oshima Ishi' samples are classified as hornblende-biotite granodiorite, biotite granite, and hornblende-biotite monzogranite. They are quite homogeneous and significant differences in modal compositions are not observed between three planes. Microcracks of 0.1-5 mm in length are developed within quartz grains. The abundance of microcracks along three planes increase from hardway plane through grain plane to rift plane. Distribution of microcracks can probably explain the subtle differences in color tone between three planes of 'Oshima Ishi'.