A new activity model for Mg-Al biotites determined through an integrated approach

Area of Science:

• Geochemistry
• Mineral Physics
• Computational Materials Science

Background:

• Accurate thermodynamic models are crucial for understanding mineral behavior and phase equilibria.
• Existing thermodynamic data for Mg-Al biotites may be based on older experimental measurements.
• Mg-Al order-disorder significantly influences the thermodynamic properties of biotites.

Purpose of the Study:

• To develop a new, physically-experimentally based activity model for Mg-Al biotites.
• To integrate experimental data (calorimetry, IR spectroscopy, phase equilibria) with DFT calculations.
• To refine thermodynamic properties (enthalpy, entropy, heat capacity) for phlogopite and eastonite end-members and their solid solutions.

Main Methods:

• Integrated approach combining experimental techniques: relaxation and differential scanning calorimetry, IR spectroscopy, hydrothermal synthesis, X-ray diffraction, and microprobe analysis.
• Density Functional Theory (DFT) calculations for deriving disordering enthalpies and thermodynamic properties.
• Analysis of existing phase-equilibrium data.

Main Results:

• A new activity model for the phlogopite-eastonite binary was formulated, incorporating Mg-Al order-disorder.
• Refined calorimetric and standard entropies for phlogopite and eastonite end-members, showing discrepancies with existing databases.
• Derived a symmetric interaction parameter for the Mg-Al biotite binary, improving phase equilibrium calculations.

Conclusions:

• The new activity model and thermodynamic data provide a more accurate representation of Mg-Al biotite behavior.
• The refined data impact calculations of mineral stability, particularly for Mg-Al biotite + quartz assemblages.
• This work offers improved thermodynamic constraints for geochemical modeling and understanding Earth processes.