The coherent solvus of disordered alkali feldspar determined with atom probe tomography

At high temperatures, alkali feldspar consists of a continuous solid-solution between the Na (albite) and K (K-feldspar) end members. Below about 600°C, a miscibility gap opens, the limits of which are determined by the so-called solvus. Alkali feldspar of intermediate composition tends to exsolve when cooled from temperatures of magmatic or metamorphic crystallization, forming an intergrowth of Na- and K-rich lamellae, a microstructure referred to as perthite. Initially, coherency is maintained across the lamellar interfaces and it may be preserved over geological times. Since the lattice parameters of alkali feldspars strongly depend on composition, the exsolution lamellae must be strained to maintain coherency at the interfaces. The elastic energy required to strain the lamellae counteracts exsolution and equilibrium compositions of coexisting coherent exsolution lamellae define a coherent solvus which lies below the solvus for strain-free phase equilibria. Segregation of Na and K and subsequent lamellar coarsening is achieved by thermally activated Na-K interdiffusion. Therefore, compositions and widths of the exsolution lamellae could be used to reconstruct cooling histories. Knowing the shape and position of the coherent solvus is key for the corresponding geo-speedometry applications.

To determine the coherent solvus, initially homogeneous disordered, gem-quality alkali feldspar was annealed at temperatures between 440°C and 560°C and at atmospheric pressure, which caused it to exsolve into coherently intergrown 10 to 20 nm wide lamellae, the compositions of which were directly determined with atom probe tomography.

The application of different annealing times showed that thermodynamic equilibrium was reached during the experiments. The obtained lamellar composition define points on the coherent solvus, which were for the first time measured directly for alkali feldspars.

Additionally, equilibrium Na-K partitioning experiments between NaCl-KCl melt and the same alkali feldspars as used for the exsolution experiments were performed at atmospheric pressure and at temperatures between 800°C and 1000°C to calibrate a thermodynamic mixing model supplemented by a model for the elastic energy required for coherent exsolution. The coherent solvus calculated from the thermodynamic model and the directly measured coherent solvus are in excellent agreement. This indicates that the developed models provide an adequate description of phase equilibria in coherent lamellar intergrowth. The directly measured coherent solvus and the models form a solid base for potential applications of geo-speedometry in coherently exsolved alkali feldspars.