TWINS
An important characteristic of some mineral species is their tendency to form twinned crystals, which are strictly symmetrical intergrowths of two crystals of the same species. If more than two crystals are joined together according to the same law, the resultant compound crystal is called a repeated twin; and one may talk about trillings andfourlings, depending on the number of crystals involved. If twinning has taken place in accordance with two or more laws, the resultant twin is termed a compound or complex twin.
Twins always begin to form during the initial stages of crystal growth, and the pattern of twinning is determined by the relative positions of the crystal nuclei. The ability to form twins varies greatly from mineral to mineral. Some species, such as andalusite, for example, have not been known to form twins. Feldspar, on the other hand, forms twins in accordance with several twinning laws,
the best known of which are the Carlsbad, Baveno and Manebach laws. Quartz crystals can be twinned according to a number of less well-known laws, but twins are less common than in feldspar. Many minerals are easily recognised because of the characteristic shape of their twins. Staurolite, for instance, forms characteristic cross-like penetration twins (Plates 72, 73). One valuable criterion which is useful for recognising twinned crystals is the presence of re-entrant angles, which are common in twins but unknown in single crystals (Plate 80).