NEEDLE TWINS AND RIGHT-ANGLED TWINS IN MINERALS - COMPARISON BETWEEN EXPERIMENT AND THEORY

Transformation twinning in minerals forms isolated twin walls, interse cting twin walls with corner junctions, and wedge-shaped twins as elem ents of hierarchical patterns. When cut perpendicular to the twin wall s, the twins have characteristic shapes, right-angled and needle-shape d wall traces, which can be observed by transmission electron microsco py or by optical microscopy. Theoretical geometries of wall shapes rec ently derived for strain-related systems should hold for most displaci ve and order-disorder type phase transitions: (1) right-angled twins s how curved junctions; (2) needle-shaped twins contain flat wall segmen ts near the needle tip if the elastic behavior of the mineral is domin ated by its anisotropy; (3) additional bending forces and pinning effe cts lead to curved walls near the junction that make the needle tip ap pear more blunt. Experimental studies confirmed that these features oc cur in a wide range of materials. Bent right-angled twins were analyze d in Gd-2(MoO4)(3) Linear needle tips were found in WO3, [N(CH3)(4)](2 ). ZnBr, CrAl, BiVO4, GdBa2Cu3O7, and PbZrO3. Parabolic tips occur in K2Ba(NO2)(4), and GeTe whereas exponential curvatures appear in BaTiO3 , KSCN, Pb-3(PO4)(2), CaTiO3, alkali feldspars, YBa2Cu3O7, and MnAl. T he size and shape of the twin microstructure relates to its formation during the phase transition and the subsequent annealing history. The mobility of the twin walls after formation depends not only on the the rmal activation but also on the structure of the wall, which may be pi nned to impurities on a favorable structural site. Depinning energies are often large compared with thermal energies for diffusion. This lea ds to kinetic time scales for twin coarsening that are comparable to g eological time scales. Therefore, transformation twins that exhibit ne edle domains not only indicate that the mineral underwent a structural phase transition but also contain information about its subsequent ge ological history.