Oligoclase - Oligoclase

Japanese: 灰曹長石 - かいそうちょうせき（英語表記）oligoclase

One of six types of plagioclase. Currently, in mineralogy, it is not treated as an independent type and is classified as albite. Its chemical composition is defined by the ratio of albite to anorthite components. Although it is difficult to distinguish it from other plagioclase with the naked eye, most of the plagioclase that makes up phenocrysts in granite and granite porphyry, and the so-called gneiss found in Japan, is anorthite. It also occurs in volcanic rocks such as hornfels, trachyte, and trachyandesite of mudstone origin, as well as green schist, quartz schist, and mica schist with a metamorphic degree of epidote to amphibole facies. The English name comes from the Greek, meaning that it has less cleavage than albite. The Japanese name is based on its chemical composition.

[Satoshi Matsubara]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

斜長石を6種に細分したときの1種。現在、鉱物学上は独立種として扱わず、曹長石に分類される。化学組成上、曹長石と灰長石成分の割合で定義される。肉眼による他の斜長石との識別は困難であるが、花崗岩(かこうがん)、花崗斑岩(はんがん)の斑晶、日本産のいわゆる片麻岩を構成する斜長石の多くは灰曹長石である。ほかに、緑簾石(りょくれんせき)～角閃(かくせん)岩相程度の変成度の緑色片岩、石英片岩、雲母(うんも)片岩など、泥質岩起源のホルンフェルス、粗面岩、粗面安山岩などの火山岩中に産する。英名は、曹長石より劈開(へきかい)が乏しいという意味のギリシア語に由来する。和名は化学組成による。

［松原　聰］