Microfabrics as energy minimisers: Rotation recrystallisation as an example

Microfabrics are discussed as features that minimise Helmholtz energy in a system undergoing deformation and metamorphism. The energy minimisation process leads to inhomogeneous deformations and hence the formation of microfabrics at a number of scales. This process together with the requirement for compatibility both with the imposed deformation and local gradients in deformation means the microfabric must refine on smaller and smaller scales in a self-similar manner leading to fractal geometries. Nine independent levels of refinement are necessary to match the nine independent components of a general imposed deformation gradient. The process of rotation recrystallisation is proposed as one example of self similar refinement so that crystallographic preferred orientations (CPO) associated with rotation recrystallisation are presented as fractals whose fractal dimensions reflect the conditions of deformation. Compatibility also has implications for the formation of other microstructures such as non-rational deformation lamellae. The evolution and orientations of microfabrics that minimise energy are related to the history of the imposed deformation gradient (as was originally proposed by Sander) and not the strain tensor as is commonly assumed. As examples, possible models for CPO development in deformed quartz aggregates by rotation recrystallisation and the development of deformation lamellae in deformed quartz grains are presented.

Research highlights► Metamorphic and deformed microfabrics result from minimisation of Helmholtz energy. ► The fabric geometry must be compatible with the imposed deformation gradients. ► The microfabric refines on smaller and smaller scales leading to fractal geometries. ► Fabrics that form by these processes are controlled in orientation and symmetry by the kinematics of the deformation and not the finite strain.