Fig. 3: Thermodynamic modelling of the stability of Na-K-feldspars and the evolution of solids and solution composition during hydrothermal reaction.

Starting compositions of fluids and minerals are listed in Supplementary Table 2. Calculated solvus (co-existence of Na-rich and K-rich alkali feldspar) in the alkali feldspar-aqueous system (A) and Lippmann diagram for the aqueous–alkali feldspar–plagioclase system (B), as well as its upper-right fragment (C) calculated at P = 2 kbar, T = 600 °C. The solid and dashed lines in the Lippmann diagram are the stable and metastable segments, respectively, of the solidus and solutus curves. The peritectic point is where the aqueous phase co-exists with the two solid solution phases. X(Ab) is the percentage of albite end-member in each phase, and X(Na⁺,aq) the composition of the co-existing aqueous fluid (X(Na⁺,aq) = a(Na⁺)/[a(Na⁺) + a(K⁺)], where a is activity). In B the symbol ξ represents reaction progress. ξ is the fraction of sanidine replaced by albite, assuming negligible dissolution of sanidine. This value highlights the solution compositions at different stages of sanidine replacement by albite, based on mass balance calculations in our experiments. This indicates that the bulk solution is expected to become saturated with respect to K-feldspar at ξ~0.4.