Granite - Mikageishi

The name of granite and granodiorite stone. Granite quarried in the southern foot of Mt. Rokko in Hyogo Prefecture and the Mikage region of Higashinada Ward in Kobe City was originally called granite, and this became the common name for granite and other granite stones. Areas that produce this type of stone in large quantities are the Abukuma Mountains to the Tsukuba mountain range in northeastern Japan, and in southwestern Japan, from Okayama Prefecture to the southern part of Hiroshima Prefecture, and the islands around the Seto Inland Sea. They are usually named after the place of origin, such as Inada Granite or Kitagi Granite. For this reason, the stones from the Mikage region are called Hon Granite to distinguish them. This granite is a medium-grained black mica granite, which is reddish in color due to the inclusion of potassium feldspar, and is considered the most beautiful of all granites, but as it has been severely weathered and large pieces cannot be obtained, the remaining blocks are quarried and used for carvings such as stone lanterns. Diorite and gabbro are also sometimes called granite, and are called black granite because of their dark color.

The appearance of granite varies depending on the size of the crystal grains of the constituent minerals, from coarse-grained Tokuyama stone in Yamaguchi Prefecture to fine-grained Aji stone in Kagawa Prefecture. Granite also comes in a variety of colors depending on the type and amount of minerals it contains, including white granite (Inada granite in Ibaraki Prefecture) that is white with a lot of plagioclase, pink or pink granite (Hon granite in Kobe City, Mannari granite in Okayama Prefecture, Sakura granite in Fukushima Prefecture) that is pinkish with a lot of potassium feldspar, and black granite (Ukigane granite in Fukushima Prefecture, Orikabe granite in Iwate Prefecture) that is rich in colored minerals such as biotite, amphibole, and pyroxene. In quarrying, the joints and easily broken surfaces, commonly called ishime, are used. Due to the different expansion rates of quartz and feldspar, it has the disadvantages of being slightly less fire-resistant and relatively vulnerable to weathering, but because it is hard, has a beautiful appearance, and is easy to obtain in large pieces, it is the most widely used stone material for construction and civil engineering.

[Saito Yasuji]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

花崗(かこう)岩や花崗閃緑(せんりょく)岩の石材名。兵庫県六甲(ろっこう)山南麓(ろく)、神戸市東灘(ひがしなだ)区御影地方で採石される花崗岩の石材を御影石とよんだのが始まりで、それが花崗岩や花崗岩の石材の通称となった。この種の石材を多量に産する地域は、東北日本では阿武隈(あぶくま)山地から筑波(つくば)山塊にかけて、西南日本では岡山県から広島県の南部、瀬戸内海周辺の島々である。普通は産地名をつけて、稲田御影や北木(きたぎ)御影などとよばれる。そのため御影地方のものは、本(ほん)御影とよばれ区別されている。本御影は中粒の黒雲母(くろうんも)花崗岩で、肉紅色のカリ長石を含むため肉紅色を呈し、御影石のなかでもっとも美しいとされるが、風化が著しく大材が得られないため、残塊が採石され石灯籠(どうろう)などの細工や彫刻に利用される。また、閃緑岩や斑糲(はんれい)岩なども御影石とよばれることがあって、その色が黒っぽいために黒御影といわれる。

　御影石の外観は構成鉱物の結晶粒の大きさの違いで異なり、山口県の徳山石のように粗粒なものと、香川県の庵治(あじ)石のように細粒なものとがある。また含まれる鉱物の種類と量によりいろいろな色調のものがあり、斜長石が多く白色の白御影(しろみかげ)（茨城県の稲田御影）、カリ長石の多い淡紅色を帯びた桃色御影または淡紅御影（神戸市の本御影、岡山県の万成(まんなり)石、福島県の桜御影）、黒雲母・角閃石・輝石などの有色鉱物が多い黒御影（福島県の浮金(うきがね)石、岩手県の折壁(おりかべ)石）などがある。採石には節理と俗に石目(いしめ)という割れやすい面が利用される。石英と長石の膨張率が違うため耐火性にやや劣り、風化にも比較的弱いという欠点があるが、堅硬で美しい外観をもち大材が得られやすいために、建築・土木用石材としてもっとも広く利用されている。

［斎藤靖二］

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