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.

The DSC data, combined with Cp values from published literature, are represented by the following polynomial, which can be used to compute the Cp of annite at temperatures above 298 K:

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.

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