The processes leading to the generation of pegmatite fabrics and rare-element ore deposits are evaluated in terms of pressure, temperature, and compositions of fluids through the combination of recent field, fluid inclusion, and experimental data. The results indicate that primary consolidation of miarolitic and massive rare-element (Li-Be-Ta) pegmatites occurs in the range of 700° to 450°C at 400 to 250 MPa. The model for pegmatite genesis proposed here involves disequilibrium crystallization of quartz and feldspars in a hydrous melt-crystal system that approaches chemical equilibrium but rarely attains it. Fluxing anionic components of boron, phosphorus, and fluorine are not essential to...