Thermodynamic properties of chlorite CCa-2. Heat capacities, heat contents and entropies

The heat capacities of the international reference clay mineral chlorite CCa-2 from Flagstaff Hill, California, were measured by low temperature adiabatic calorimetry and differential scanning calorimetry, from 5 to 520 K (at 1 bar). The studied chlorite is a Fe-bearing trioctahedral chlorite with an intermediary composition between ideal clinochlore (Si3Al)(Mg5Al)O10(OH)8 and chamosite (Si3Al)(Fe5Al)O10(OH)8. Only few TiO2 impurities were detected in the natural chlorite sample CCa-2. Its structural formula, obtained after subtracting the remaining TiO2 impurities, is (Si2.633Al1.367)(Al1.116Fe0.2153+Mg2.952Fe1.7122+Mn0.012Ca0.011)O10(OH)8. From the heat capacity results, the entropy, standard entropy of formation and heat content of the chlorite were deduced. At 298.15 K, the heat capacity of the chlorite is 547.02 (±0.27) J mol−1 K−1 and the molar entropy is 469.4 (±2.9) J mol−1 K−1. The standard molar entropy of formation of the clay mineral from the elements is −2169.4 (±4.0) J mol−1 K−1.