Twin crystals

Japanese: 双晶 - そうしょう

Two identical crystals are joined together with a plane as the plane of symmetry or a straight line as the axis of two-fold symmetry (symmetry). The plane of symmetry is called a twin plane, and the axis of symmetry is called a twin axis. When there are two or more crystals, they are called triplet twins, tetratwin twins, and agglomerate twins, respectively. In crystals with a center of symmetry, both twin planes and twin axes exist, and they are at right angles to each other. On the other hand, in crystals without a center of symmetry, only one exists, and if there is a twin plane, it is called a reflection twin, and if there is a twin axis, it is called a rotation twin. Two twinned crystals may be joined by a plane (junction surface) (junction twin), or they may penetrate each other (penetration twin, or penetration twin). It is certain that applying strain to a single crystal will result in a twin, but the cause of twinning in general is unknown.
→ Related topics Single crystal | Parallel twin crystal | Moonstone

Source : Heibonsha Encyclopedia About MyPedia Information

Japanese:

二つの同種結晶が一つの平面を対称面として，または一つの直線を２回対称軸（対称）として，結合していること。対称面を双晶面，対称軸を双晶軸という。二つ以上の結晶の場合にはそれぞれ三連晶，四連晶，集片双晶という。対称心のある結晶では双晶面，双晶軸とも存在し，両者は互いに直角。一方，対称心のない結晶では，いずれか一方しか存在せず，双晶面をもつ場合が反射双晶，双晶軸をもつ場合が回転双晶。双晶する二つの結晶は平面（接合面）で結合する場合（接合双晶）と互いに透入しあう場合（透入双晶，または貫入双晶とも）がある。一つの結晶にひずみを与えると双晶となることは確かだが，双晶の生因一般は不詳。
→関連項目単結晶｜平行連晶｜ムーンストーン

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