Chloritoid

A nesosilicate mineral. It exhibits pseudo-hexagonal plate-like crystals similar to mica or chlorite, as well as leaflet-like and block-like crystals. It is also called anhydrite. Compared to mica and chlorite, it is not very cleaved and has a high hardness. It occurs in low-grade regional metamorphic rocks along with quartz and muscovite, as a hydrothermal alteration product of certain rocks, and as a vein mineral. In Japan, it is famous for occurring in metamorphic rocks in the Kitakami Mountains, Iwate Prefecture, and the Hitoichi area of ​​Ibaraki Prefecture. The species that are extremely rich in manganese are called otterite, and are extremely rare.

[Satoshi Matsubara]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

ネソ珪(けい)酸塩鉱物の一つ。雲母(うんも)ないし緑泥石に似た擬六角板状の結晶を示すほか、葉片状、塊状の結晶をなす。硬緑泥石ともいう。雲母や緑泥石に比べると劈開(へきかい)は著しくなく、硬度は高い。低変成の広域変成岩中に、石英、白雲母などとともに産するほか、ある種の岩石の熱水変質物として、また脈状鉱物としても産する。日本では岩手県北上山地と茨城県日立地方の変成岩中のものが有名である。マンガンに著しく富む種はオットレ石といわれ、産出はきわめてまれである。

［松原　聰］

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