The solubility of natural searlesite, NaBSi2O5(OH)2, has been determined at temperatures between 25 and 100 °C at 1 atm using distilled water, 1-M NaCl, and various buffer solutions. The paths taken by different starting solutions to a steady state suggest that equilibrium was attained. When necessary, reciprocal time (t−0.5) extrapolations of the activity products (Q) were used to estimate the equilibrium constant (K) of the dissolution reaction, thus permitting the calculation of the free energy of formation of searlesite. Log K values between 25 and 100 °C fit a linear equation, log K = 8.9502 − 2660.1/T (K); of searlesite at 25 °C is – 2897.1 ± 0.8 kJ/mol.
Activity diagrams (log aH3BO3 vs. log aSiO2) for the system Na2O-B2O3-SiO2-H2O at 25 °C, constructed using thermodynamic data, show that the searlesite stability field lies between the saturation limits of quartz and amorphous silica, flanked by fields for magadiite and borax at higher and lower log aSiO2, respectively. When log aNa+/aH+ is fixed at 10.2 to approximate the compositions of many natural brines, the minimum value of log aH3BO3, necessary for searlesite crystallization is – 3. Supersaturation, however, is not a sufficient condition for the crystallization of natural searlesite. Thermodynamic calculations suggest that phillipsite may be altered by natural B-bearing brines to form searlesite and potassium feldspar at ambient temperatures. In nature the alteration reaction appears to be promoted under diagenetic conditions.