Coherent solvus of disordered alkali feldspar: experiment, atom probe tomography and thermodynamic model

• DOI: 10.1007/s00410-024-02150-z
• Publication Date: 2024-06-06T09:02:20Z
• Authors: D. Heuser; R. Dubosq; E. Petrishcheva; G. Bian; C. Rentenberger; C. L. Lengauer; B. Gault; G. Habler; R. Abart
• Keywords: Solvus; Alkali feldspar; Atom probe

At temperatures above about 600 °C, alkali feldspar forms a continuous solid solution between the Na and K end members. Towards lower miscibility gap opens, of intermediate composition exsolves, forming an intergrowth relatively more Na-rich K-rich lamellae. During exsolution, crystal structure usually remains coherent across lamellar interfaces, feature that may be preserved over geological times. Due to compositional dependence lattice parameters, requires lamellae are elastically strained. The associated elastic strain energy counteracts solvus delimiting misciblity for lies below free phase equilibria. To determine solvus, homogeneous gem quality was annealed at conditions falling into two-phase region diagram. Thereby approximately 10-20 nanometers wide produced. Lamellar compositions were determined with atom probe tomography defining points on solvus. In parallel, calculated using thermodynamic mixing model calibrated same as used exsolution experiments accounting intergrwoth. experimentally in excellent agreement indicating equilibria adequately described, providing sound basis interpretation relations coherently exsolved feldspar.