Shale - shale

Japanese: ケツ岩(頁岩) - けつがん（英語表記）shale

A mudstone that is thin and easily flaks off, also called mudstone. Rocks that are mainly composed of clastic material less than 1/16 mm in size are called mudstones, and among mudstones, those that look like stacked pages of a book and have exfoliation properties are called shale. When exfoliation properties develop further, they are called slate. Exfoliation often appears when rocks are weathered. The terminology is confusing, and some rocks that have exfoliation properties are called mudstones, while others are called shale if they do not have any and are strongly consolidated. The exfoliation properties of shale are caused by the arrangement of particles during deposition and the compression of clay minerals and micas into parallel arrangements as the solidification process progresses.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

薄くはげやすい性質をもった泥質岩で，泥板岩ともいう。1/16mm以下の砕屑物質を主とする岩石を泥岩と呼ぶが，泥岩のうちで，本のページを重ねたようにみえ，剝離性を有するものをケツ岩と呼ぶ。さらに剝離性が発達すると粘板岩(スレート)と呼ばれる。剝離性は岩石が風化するとよく現れる。用語は混乱しており，剝離性があっても泥岩と呼んだり，逆にそれがなくても強く固結したものはケツ岩と呼んだりする。ケツ岩の剝離性は，堆積する際の粒子の配列や固結が進む過程で粘土鉱物や雲母類が圧縮されて平行に並びかわることに原因している。

出典　株式会社平凡社世界大百科事典第２版について　情報

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