Alkali Feldspars with Water at 5 kb Pressure

Alkali feldspars with water at 5 kb begin to melt at an isobariceutectic: 703±2 °C, Or ₂₈.₅ (wt. per cent) recalculatedanhydrous. The liquidus of albite-H₂O at 5 kb is 758±3°C. The K feldspar-rich, water-saturated liquidus is essentiallythat of Yoder, Stewart, & Smith (1957), who furthermorefound orthoclase-H₂O to melt at 876 °C. The alkali feldspar solvus, determined by synthesis from glassand a few reversals using crystalline material, has a calculatedcritical temperature of 730 °C and a critical compositionof Or₃₁ (wt. per cent) using parametric equations (Thompson& Waldbaum, 1969). The Margules equation yields T_c = 738°C. The feldspars produced are structural equivalents oflow sanidine on the basis of their 060 and 04 powder X-ray diffractionpeaks. The critical line intersects the beginning of meltingcurve for the system Ab-Or-H₂O at 4.2 kb, 715 °C, the minimumconditions for the coexistence of two feldspars and liquid inthis system. The slope of the critical line, using Orville's (1963) 2 kbresults, is 18.3 °C/kb, leading to an intersection withthe kyanite-sillimanite curve of Richardson, Bell, & Gilbert(1968) at about 10.6 kb, 834 °C. The intersection with theandalusite-sillimanite curve of these authors (1969) falls atabout 3.5 kb, 703 °C. Such intersections may be used toestimate limits of P and T for a variety of crustal rocks. Because isobaric crystallization of feldspars in the water-deficientregion must enrich liquids in H₂O to the saturation point, haplosyeniticliquids with a finite initial water content must, at 5 kb totalpressure, eventually crystallize two feldspars.