Abstract
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 (Si 3Al)(Mg 5Al)O 10(OH) 8 and chamosite (Si 3Al)(Fe 5Al)O 10(OH) 8. Only few TiO 2 impurities were detected in the natural chlorite sample CCa-2. Its structural formula, obtained after subtracting the remaining TiO 2 impurities, is (Si 2.633Al 1.367)(Al 1.116 Fe 0.215 3 + Mg 2.952 Fe 1.712 2 + Mn 0.012Ca 0.011)O 10(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.
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