LOCAL FLUCTUATIONS IN FELDSPAR FRAMEWORKS

At any set of thermodynamic conditions a mineral will have some well d efined equilibrium crystal structure. However, this structure can be l ocally disturbed by crystal defects, such as domain walls or solute at oms. This distorted structure will only affect a finite volume within the crystal, but the need to retain continuity within the crystal mean s 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 dom ain walls can be studied quantitatively, by measuring the intensity of diffuse diffraction between pairs of twin-related Bragg peaks. In alk ali feldspar (Or(30)) at room temperature, these walls are approximate ly 25 Angstrom thick. Similarly, a single solute atom in a mineral wil l only affect a small region within a crystal. As a result, chemical m ixing will only occur in a substitutional solid solution once there is significant overlap between the strain fields around individual solut e atoms. This causes the 'plateau effect', where the properties of a p hase transition are independent of composition. In alkali feldspar, th is plateau extends from albite to 2% Or, which corresponds to a strain field radius of 10 Angstrom. These phenomena can be modelled using Gi nzburg-Landau theory, which predicts that the range of these strain fi elds will increase as the temperature is raised to T-c. This has been confirmed by measuring the thickness of twin walls asa function of tem perature.