The direct and indirect evidences for the oriented attachment of pre-formed crystals as a mechanism of twin formation are reviewed and discussed. Although rarer than the nucleation-stage formation, this mechanism, often overlooked, has been demonstrated in laboratory experiments, with the artificial production of twins. Moreover, the role of the oriented attachment in the formation of natural twins, although more likely to occur in environments where crystals are free of moving and interacting, is supported also by the occurrence of monoperiodic twins and of plesiotwins, whose origin cannot be explained by a nucleation-stage formation. The so far observed absence of diperiodic twins is discussed in terms of two-dimensional site-coincidence and twin obliquity.

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