Abstract: Active metamorphism of fine grained sandstone in the c.16000 year old Salton Sea geothermal system has produced a suite of chemically equilibrated coexisting authigenic alkali feldspars and re-equilibrated detrital feldspars in the 250–360°C temperature range. At c.335°C the average compositions, 2 Vs, and (t₁o+t₁m) and Z ordering parameters of coexisting authigenic feldspars are [Or_0.52Ab_97.40An_2.08, 2V_x = 91.3±4.8, (t₁o + t₁m) = 0.89±0.05, Z = 0.79±0.09], and [Or_94.42 Ab_5.10An_0.48, 2V_x = 70, (t₁o + t₁m) = 0.90, Z = 0.81]. At c.360°C authigenic albite becomes more An-rich and less ordered [Or_1.21Ab_92.83An_5.97, 2V_x = 87.5±3.4, (t₁o + t₁m) = 0.85±0.03, Z = 0.70±0.07] and K-feldspar is no longer stable. Detrital plagioclase (An up to 40%) is preserved metastably to temperatures up to c.190°C in strongly carbonate-cemented sandstone which forms part of a geothermally produced permeability cap. It undergoes rapid alkali exchange at temperatures near 200°C, and by 250°C no plagioclase with An-content over 12% is observed. At > 250°C authigenic and most detrital alkali feldspar compositions are in excellent agreement with the Bachinski and Muller (1971) microcline-low-albite solvus.