Structural petrology

A branch of geology that studies the structure of rocks, especially metamorphic rocks, at various stages. In metamorphic rocks, textures and structures with strong preferred orientation, such as schistosity, are observed due to the inhomogeneous movements associated with the inhomogeneous forces acting inside the rock during its formation. Schistosity is a texture in which the constituent minerals are regularly arranged in a certain direction, and is particularly easy to recognize in rocks with a strong scaly habit, such as micas and chlorites, where the faces of the scales (which correspond to the basal planes in crystallography) are aligned parallel to a certain plane. However, even in rocks that do not have a pronounced crystal habit, such as quartz and feldspars, the internal structural direction of each crystal grain, such as the principal axis of the structure of quartz, shows preferred orientation. Since the principal axis of quartz coincides with its optical axis, the preferred orientation of quartz can be determined by measuring the distribution of the optical axis direction of quartz grains in a thin section. By measuring the preferred orientation of rock-forming minerals, the direction of deformation during metamorphism can be estimated. In specimens and outcrops, not only the directions of the planes of schistosity and banded structures, but also their fold axes, microfold axes on the schistosity surfaces, and the arrangement directions of columnar minerals are measured.

[Mitsuo Hashimoto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

岩石とくに変成岩の構造をさまざまの段階で研究する地質学の一分野。変成岩には、生成のときに内部で働いた不均質な力に伴う不均質な運動によって、片理(へんり)などの定向性の強い組織や構造が認められる。片理は構成鉱物が一定の方向に規則正しく配列した組織であって、とくに雲母(うんも)類や緑泥石のように鱗片(りんぺん)状の晶癖(しょうへき)の強いものは、鱗片の面（これは結晶学的には底面に相当する）が、ある面に平行に配列するので認めやすい。しかし、石英や長石類のように顕著な晶癖をもたないものでも、それぞれの結晶粒の内部構造上の方向、たとえば石英の構造上の主軸は定向配列を示す。石英の主軸はその光軸に一致しているから、薄片内の石英粒について、光軸の方向の分布を測定すれば、石英の定向配列を知ることができる。造岩鉱物のこのような定向配列を測定することによって、変成作用における変形作用の方向を推定することができる。標本や露頭では、片理や縞(しま)状構造の面の方向はもちろん、それらの褶曲(しゅうきょく)軸や片理面上の微褶曲軸、柱状鉱物の配列方向などが測定される。

［橋本光男］

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