Spinodal decomposition in alkali feldspar studied by atom probe tomography

Area of Science:

• Mineral Physics
• Materials Science
• Geochemistry

Background:

• Spinodal decomposition is a critical process in feldspar mineral systems, influencing their microstructural evolution and properties.
• Understanding phase separation kinetics and equilibrium in alkali feldspars is essential for interpreting geological processes and material behavior.
• Previous studies relied on lower-resolution techniques, limiting detailed analysis of nanoscale phase transformations.

Purpose of the Study:

• To investigate the early stages of spinodal decomposition in alkali feldspar using atom probe tomography (APT) and electron microscopy.
• To quantitatively analyze the chemical separation and microstructural evolution during exsolution.
• To determine equilibrium Na-K partitioning and interfacial excess energy.

Main Methods:

• Preparation of gem-quality alkali feldspar from Madagascar orthoclase via ion-exchange with molten (NaK)Cl salt.
• Annealing experiments at 550°C to induce spinodal decomposition.
• High-resolution analysis using atom probe tomography (APT) for chemical composition and electron microscopy for microstructure.

Main Results:

• Observation of coherent lamellar intergrowth of Na-rich and K-rich phases after annealing.
• Attainment of chemical equilibrium and Na-K partitioning within four days, followed by microstructural coarsening.
• Lamellar wavelength increased from ~17 nm (4 days) to ~30 nm (16 days); APT provided nm-resolution chemical data consistent with the coherent solvus.

Conclusions:

• APT enables quantitative chemical analysis at the nanoscale, crucial for studying early-stage exsolution in alkali feldspars.
• The study provides insights into phase relations within nm-scaled coherent intergrowths.
• The findings validate the Cahn-Hilliard theory for quantifying interfacial excess energy in exsolving systems.