Standard-state thermodynamic properties of annite, KFe3[(OH)2AlSi3O10], based on new calorimetric measurements
Dachs, Edgar; Benisek, Artur
European Journal of Mineralogy Volume 27 Number 5 (2015), p. 603 - 616
published: Oct 1, 2015
ArtNo. ESP147052705007, Price: 29.00 €
The heat capacity (C p) 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 C p 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 (C mag) to the overall C p, the C p data between 40 and 65 K were therefore excluded from the measured C p 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 (C vib) within the temperature range of the magnetic phase transition. Integrating the C p 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 C p values from published literature, are represented by the following polynomial, which can be used to compute the C p of annite at temperatures above 298 K: C p = 728.6 – 5581 · T –0.5 – 2.896 · 10–6 · T –2 + 2.957 · 108 · T –3. 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 ΔH o f = –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 S o = 422.9 ± 2.9 J/mol·K and ΔH o f,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.