Henri

In metamorphic rocks, a markedly oriented texture occurs when certain minerals are concentrated in a planar arrangement. In particular, when scale-like minerals such as mica and chlorite are arranged with their bases parallel to one another, or when columnar minerals such as amphibole are arranged with their extensions parallel to one another, the rock will have a pronounced schistosity. In the former case, the rock will not only have schistosity, but will also tend to split in the direction of the schistosity. This is called cleavage of the rock. Schistosity is most commonly seen in rocks that have been subjected to regional metamorphism under relatively low temperature conditions, i.e., crystalline schist. This is said to be because, under such conditions, the effect of deformation during metamorphic recrystallization is significant, making it easier for minerals with special crystal habits such as mica, chlorite, and amphibole to have a preferred orientation. Schistosity often occurs parallel to the bedding of the rock, but there are also many cases where the two intersect at a large angle.

[Mitsuo Hashimoto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

変成岩において、ある特定の鉱物が面状に集中配列したために生ずる、著しい定方向組織。とくに、雲母(うんも)や緑泥石のような鱗片(りんぺん)状鉱物が底面を平行にして配列する場合や、角閃(かくせん)石のような柱状鉱物が伸長方向を平行にして配列する場合には、岩石に顕著な片理が生ずる。とくに前者のような場合には、片理のみならず、岩石が片理の方向に割れやすい性質を示す。この性質を、岩石の劈開(へきかい)という。片理は比較的低温な条件の下における広域変成作用を受けた岩石、すなわち結晶片岩にもっともよく現れる。これは、そのような条件下では変成再結晶における変形作用の効果が著しいため、雲母、緑泥石、角閃石などの特殊な晶癖をもつ鉱物の定向配列が生じやすいからであるといわれている。なお、片理は、岩石の層理と平行に生じることが多いが、そうでなく、両者が大きな角度で交わる例も少なくない。

［橋本光男］

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