Needle twins and right-angled twins in minerals; comparison between experiment and theory

摘要: Transformation twinning in minerals forms isolated twin walls, intersecting walls with corner junctions, and wedge-shaped twins as elements of hierarchical patterns. When cut perpendicular to the have characteristic shapes, right-angled needle-shaped wall traces, which can be observed by transmission electron microscopy or optical microscopy. Theoretical geometries shapes recently derived for strain-related systems should hold most displacive order-disorder type phase transitions: (1) show curved junctions; (2) contain flat segments near needle tip if elastic behavior mineral is dominated its anisotropy; (3) additional bending forces pinning effects lead junction that make appear more blunt. Experimental studies confirmed these features occur a wide range materials. Bent were analyzed Gd 2 (MoO 4 ) 3 . Linear tips found WO , [N(CH ] .ZnBr CrAl, BiVO GdBa Cu O 7 PbZrO Parabolic K Ba(NO GeTe whereas exponential curvatures BaTiO KSCN, Pb (PO CaTiO alkali feldspars, YBa ?, MnAl. The size shape microstructure relates formation during transition subsequent annealing history. mobility after depends not only on thermal activation but also structure wall, may pinned impurities favorable structural site. Depinning energies are often large compared diffusion. This leads kinetic time scales coarsening comparable geological scales. Therefore, transformation exhibit domains indicate underwent information about