Twin walls and hierarchical mesoscopic structures

Networks of interacting twin walls form hierarchical, mesoscopic structures in minerals. The typical thickness of transformation twins at T much less than T-C is similar to 3 nm and increases at T --> T-C \T-T-C\(-1/2). The i nternal structure of twin wails is derived to be chiral in systems with cou pled order parameters. The effect of wall bending is discussed.