The literature on feldspar unit-cell parameters and phase transitions is reviewed, and the data are used to examine systematics of alkali feldspar expansion as a function of composition, temperature and pressure. Several different trends in expansion behavior are identified, and these trends may be linked by phase transitions. Compositional expansion can be divided into two main trends (I and II), possibly separated by a phase transition like that occurring in microcline at low temperature. Trend I is characterized by a coupled b–c expansion, whereas in Trend II, b and c are uncoupled. Trend I is divided into two parts (IA and IB) at the displacive triclinic–monoclinic transition in disordered feldspars or at the analogous changes in ordered AlSi3 feldspars. Thermal expansion may or may not follow Trend I, depending on the size of the cavity cation (M). The trends and phase transitions are analyzed in terms of the available structure refinements. It is thought that the expansion is dominated by the framework and not by expansion of the short M–O bonds.
The thermodynamic nature of the transitions is discussed. The monoclinic–triclinic symmetry change is an almost pure second-order transition, whereas the microcline transition involves hysteresis and may be a smeared (partially martensitic ?) transition.