The heat capacity (Cp) of synthetic annite was measured using a Physical Properties Measurement System (PPMS) for temperatures ranging from 2 to 300 K, and by differential scanning calorimetry (DSC) for temperatures between 282 and 363 K. The sample was synthesized in a hydrothermal apparatus at 2 kbar and 800°C, with a logfO2 of −20. It was characterized using electron microprobe analysis, Mössbauer spectroscopy, XRD, SEM, and single-crystal structural refinement methods and contained the well-established minimum possible Fe3+ content of 10% (i.e., Fe3+/Fetot = 0.1), representing the closest possible synthetic composition to ‘ideal’ annite.
A distinct kink in the slope of Cp plotted against T at around 49 K indicated a heat-capacity anomaly, which is interpreted to have been caused by a magnetic ordering phase transition. In order to compute the magnetic contribution (Cmag) to the overall Cp, the Cp data between 40 and 65 K were therefore excluded from the measured Cp data set and the remaining data fitted to a combination of Debye, Einstein, and Schottky functions. These functions then served as a model for estimating the vibrational part of the heat capacity (Cvib) within the temperature range of the magnetic phase transition.
Integrating the Cp data with respect to temperature yields a calorimetric entropy of 411.4±2.9 J/mol·K at 298 K for ‘pure’ annite (excluding any configurational terms). The magnetic entropy amounts to only 1.5 J/mol·K, which is much less than predicted theoretically.
Applying the calorimetrically determined entropy of annite (and adding a configurational entropy of 11.53 J/mol·K, representing Al-avoidance) to published phase-equilibrium data on the reaction annite + quartz = sanidine + fayalite + H2O resulted in a standard enthalpy of formation for annite of ΔHof=−5132.5±2.0 kJ/mol.
The published experimental data on the reaction annite = sanidine + magnetite + H2 (hydrogen-sensor experiments and conventional oxygen-buffer brackets) were analysed using the calorimetrically derived entropy of annite. With the exception of magnetite–wüstite buffered experiments, all experimental constraints could be satisfied using the annite standard-state properties So =422.9±2.9 J/mol·K and ΔHof, values ranging between 5128 and −5135 kJ/mol. A plot is presented showing contours for the Fe3+/Fetot ratio of annite within the assemblage annite–sanidine–magnetite (+H2) as function of logfO2 and temperature.