At any set of thermodynamic conditions a mineral will have some well defined equilibrium crystal structure. However, this structure can be locally disturbed by crystal defects, such as domain walls or solute atoms. This distorted structure will only affect a finite volume within the crystal, but the need to retain continuity within the crystal means that this volume must be non-zero. This means, for example, that the boundary between two domains will include a transition zone from one domain's crystal structure to that of the other domain. Thick twin domain walls can be studied quantitatively, by measuring the intensity of diffuse diffraction between pairs of twin-related Bragg peaks. In alkali feldspar (Or 30 ) at room temperature, these walls are approximately 25 Aa thick. Similarly, a single solute atom in a mineral will only affect a small region within a crystal. As a result, chemical mixing will only occur in a substitutional solid solution once there is significant overlap between the strain fields around individual solute atoms. This causes the "plateau effect", where the properties of a phase transition are independent of composition. In alkali feldspar, this plateau extends from albite to 2% Or, which corresponds to a strain field radius of 10 Aa. These phenomena can be modelled using Ginzburg-Landau theory, which predicts that the range of these strain fields will increase as the temperature is raised to T c . This has been confirmed by measuring the thickness of twin walls as a function of temperature.