Enthalpies of formation of tremolite and talc by high-temperature solutioncalorimetry - a consistent picture

Molar enthalpies of formation of natural, near end-member tremolite and tal c have been calculated from high temperature lead borate drop-solution calo rimetry. Using three mutually consistent reaction cycles for each phase, th e critical volatile component H2O was treated in fundamentally different wa ys, namely as the simple "oxide" [i.e., as the enthalpy difference (H-TCal- H-298.15) only], or as the (OH)-component in both low and high water-conten t phases, respectively. The molar enthalpies of drop solution of all solid phases participating in the various reaction cycles (brucite, diopside, ens tatite, calcite, magnesite, quartz) were independently measured. We corrobo rate that the dynamic gas flow techniques introduced by Navrotsky et al. (1 994) are critically important for very hydrous phases such as brucite, but that this dynamic technique yields the same results as traditional static d rop-solution techniques for phases with relatively lower water contents suc h as tremolite and talc. When these critical differences in H2O behavior ar e taken into account, it can be shown that drop-solution measurements on tr emolite and talc over the last twenty years actually lead to similar and co nsistent results on the enthalpy of formation of these phases; reported dif ferences are due to incorrect assumptions on the final state of H2O. Our re fined values for the enthalpy of formation from the elements for ideal end- member tremolite and talc range from -12299.2 to -12308.9 kJ/mol and -5892. 1 to -5900.2 kJ/mol, respectively, depending on the internally consistent d ata set used for the reaction cycle components. These values are identical within error to the results refined from various phase-equilibrium experime nts and show that these two fundamentally different techniques can lead to a consistent picture on them thermodynamic properties of hydrous minerals, a conclusion that has been strongly questioned in the past. Heat capacity data have been obtained for the same samples of tremolite and talc by differential scanning calorimetry in step-scanning mode in the ran ge 50-500 degreesC and 50-400 degreesC, respectively. The following best-fi t equations apply to the ideal end-members [T in Kelvin, C-p in in J/(mol.K )] in the range 298.15 to 800 K and 298.15 to 650 K, respectively: C-p (tremolite) = 1335.77586 - 0.02378 T- 1.00400 10(7) T-2 - 9678.23152 T- 0.5 C-p (talc) = 1300.66304 - 0.03007 T - 1.31510 10(7) T-2 - 8778.5468 T-0.5.