Description: Mineral: Tetrahedrite Description: Penetration twin on [111] Location: Cavnic (Kapnik), Romania Author: C. Palache, H. Berman, C. Frondel, The system of mineralogy. Vol I, 7th edition, John Wiley & Sons, New York, 1944. System: Cubic, point group: 43m Cell: a: 1, b: 1, c: 1, alpha: 90, beta: 90, gamma: 90 image & data © Smorf The twin operation is a 60º rotation around [111] plus a symetry in (111) plane, or, simpler, a 90º rotation around any of the crystallographic axes. But this 90º rotation will move the structure to an equivalent position. Both individuals have the crystal lattice in the same position, sharing a common volume. Then, where is the twin? Is not the same that occurs in two parallel grow crystals not considered as a twin? Very thanks for your help!

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