A calorimetric study of biotite solid solutions

Biotite is an important rock-forming mineral. Most biotites in nature are solid-solutions, dominated by the Fe-Mg end-member components annite and phlogopite, and the aluminious Fe-Mg end-members siderophyllite and eastonite. The standard state thermodynamic properties of these end- members and the Fe-Mg, Fe-Al, and Mg-Al mixing properties of biotite are thus required for petrologic applications such as, for example, geothermobarometry and phase diagram calculations. Despite of their importance in the geosciences, a calorimetric-based thermodynamic description of biotite is lacking. In order to rectify this deficiency, it is proposed to investigate the thermodynamic mixing behavior of four biotite solid-solutions, namely the binaries annite-phlogopite, annite-siderophyllite, phlogopite-eastonite and annite-eastonite. Solid-solution compositions for these binaries will be synthesized at elevated pressures and temperatures and then characterized using standard analytical techniques. The synthetic biotites will then be studied by: 1) Relaxation calorimetry, i.e. low-temperature heat capacity measurements between 2 K and 300 K using a Physical Properties Measurements System, 2) Differential scanning calorimetry, i.e. heat capacity measurements between 280 K and 760 K, 3) Infrared spectroscopy, and 4) Ab-initio calculations. The planned research will allow to establish a new, calorimetric-based activity model for biotite solid solutions for general use in the geosciences.

P28724: Final report - public relation summary English Biotites are important rock forming minerals in nature and occur in a variety of metamorphic and magmatic rocks. The main goal of this project was to quantify the thermodynamic mixing behaviour of the important biotite binaries annite (Ann)-phlogopite (Phl), annite-siderophyllite (Sid), phlogopite-eastonite (Eas) and annite-eastonite. The general aim was to formulate a new activity model for Fe-Mg-Al biotites through an integrated approach combining experimental results from calorimetry (low-temperature relaxation calorimetry using a Physical Properties Measurements System (PPMS), differential scanning calorimetry (DSC) above room temperature) and from line-broadening in infrared (IR) spectra with the analysis of existing phase-equilibrium data and with density functional theory (DFT) calculations using the software CASTEP. Main project results are: Excess entropies and volumes of mixing are zero for all important biotite binaries, so that biotite has excess Free energies that are independent of pressure and temperature. Standard entropies of the biotite endmembers Ann, Phl, Eas and Sid were determined calorimetrically and/or by DFT calculations. Published phase-equilibrium data, combined with line-broadening in IR spectra and DFT calculations, were used to derive enthalpy of formation values of biotite endmembers and to constrain the enthalpic mixing behaviour of biotite binaries in a single-step least squares procedure. Negative deviation from thermodynamic ideality is predicted along the Ann-Sid join, positive deviation for the Phl-Eas binary, and Fe-Mg mixing in biotite along the Ann-Phl join is predicted to deviate negatively from ideality for Fe-rich, switching to positive deviation at intermediate compositions. Mg-Al and Fe-Mg ordering in biotite as function of temperature was quantified from DFT calculations. Activities based on the new model are closer to ideality than would be computed from existing biotite activity models. The impact of the new biotite activity model on phase relations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) was studied in various examples relevant in geosciences. With the results from this research project, the construction of more reliable phase diagrams involving biotite is possible, a prerequisite to decipher the geodynamic evolution of mountain ranges, like the Alps. As a general outcome, the usefulness of first principles calculations as a new method in geosciences in order to derive thermodynamic endmember and mixing properties of petrologically relevant solid solutions was demonstrated.