The thermodynamic properties of dumortierite Si₃B[Al_6.75 □_0.25O_17.25(OH)_0.75]

The heat capacities of dumortierite have been measured from 7 to 1000 K by quasi-adiabatic low-temperature and differential scanning calorimetry. The corrected calorimeteric entropy at 298.15 K calculated from the experimental data is 330.2 ± 5.0 J/(mol.K) based upon the chemistry of the sample, average compositions proposed by Alexander et al. (1986), and correction to the structural formula given by Moore and Araki (1978). Moore and Araki (1978) have shown the Al(1) site to be disordered, and chemical analyses suggest that there is some Al substitution for Si. A minimum configurational entropy of 4.74 J/(mol.K) should be added to the corrected calorimetric entropy for third law calculations. The enthalpy and Gibbs free energy of formation of dumortierite from the elements have been estimated from synthesis and decomposition data and are -9109 ± 20 and -8568 ± 20 kJ/mol, respectively, at 298.15 K and 1 bar. -Authors