Kaksoiskiteitä

Kaksoiskiteitä, often translated as "twin crystals" or "twinned crystals" in English, refers to a phenomenon in crystallography where two or more individual crystals of the same mineral species grow together in a symmetrical and orderly manner. This intergrowth is not random; it follows specific crystallographic laws. The two crystals are oriented in such a way that they appear as a single, composite crystal, often exhibiting unusual shapes or symmetry not found in a single, untwinned crystal.

The formation of twin crystals is a result of specific conditions during crystal growth or sometimes due to later deformation. The most common mechanisms involve a slight shift or rotation in the atomic arrangement during growth, leading to the formation of a mirror image or a rotated version of the original crystal lattice. This results in a boundary, known as the twin plane or twin line, where the two crystal individuals join.

Twin crystals can be identified by several features. Often, the overall symmetry of the twinned aggregate appears higher than that of the individual components. In some cases, re-entrant angles, which are internal angles that would normally be outside the crystal's faces, become visible at the junction of the twin components. These re-entrant angles are a strong indicator of twinning. Many common minerals exhibit characteristic twin crystal forms, such as the interpenetration twins of fluorite, the contact twins of gypsum (forming swallowtail shapes), and the complex lamellar twinning in feldspars. The study of twin crystals is important for mineral identification and for understanding the geological processes that formed them.