Crystalline schist - kesshohengan (English spelling) crystalline schist

A metamorphic rock that has a structure resembling many overlapping thin plates and that tends to break into flat plates along these plates is called schist, but schist with coarse grains where mineral grains can be seen with the naked eye is called crystalline schist. In fact, metamorphic rocks that have the tendency to break into thin plates (cleavage), including fine-grained ones, are often referred to as crystalline schist. Crystalline schist has this property because the constituent minerals are arranged in a certain direction, and the preferred arrangement of rock-forming minerals in metamorphic rock is called schistosity. Schistosity is a term that refers to the textural characteristics of crystalline schist, while cleavage is a term that describes the mechanical properties of rock.

The mineral composition of crystalline schist is determined by the temperature of metamorphism and the chemical composition of the parent rock. When muddy sedimentary rocks turn into crystalline schist at low temperatures, they are composed of muscovite, chlorite, and albite, but at high temperatures they produce biotite, garnet, and kyanite. From basic igneous rocks, green schist composed of chlorite, epidote, and actinolite is produced at low temperatures, and amphibolite, composed mainly of plagioclase and hornblende, is produced at high temperatures. Crystalline schist is the product of regional metamorphism and is distributed over a vast, belt-like area called a metamorphic belt.

[Mitsuo Hashimoto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

薄い板を多数重ね合わせたような構造をもち、それに沿って平板状に割れやすい性質の変成岩を片岩というが、片岩のなかで、肉眼によって鉱物粒が認められる程度に粗粒のものを結晶片岩とよぶ。実際には細粒のものをも含めて、薄板状に割れる性質（劈開(へきかい)）をもつ変成岩を、広く結晶片岩とよぶことも少なくない。結晶片岩がこのような性質をもつのは、構成鉱物が一定の方向に配列しているためで、変成岩における造岩鉱物の定向配列を片理(へんり)という。片理は結晶片岩の組織上の特徴をいうことばであり、劈開は岩石の機械的性質についての術語である。

　結晶片岩の鉱物組成は、変成作用の温度と原岩の化学組成によって規定される。泥質堆積(たいせき)岩が低温で結晶片岩になったものは、白雲母(しろうんも)、緑泥石、曹長(そうちょう)石などからなるが、高温では黒雲母、ざくろ石、藍晶(らんしょう)石などを生ずる。塩基性火成岩からは、低温では緑泥石、緑簾(りょくれん)石、アクチノ閃(せん)石などからなる緑色片岩ができ、高温では斜長石や普通角閃石を主成分とする角閃岩が生成する。結晶片岩は広域変成作用の産物で、変成帯とよばれる広大で帯状の地域に分布する。

［橋本光男］

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