The heat capacity and entropy of calcite and aragonite, and their interpretation

• Published in: The Journal of chemical thermodynamics 1969, Vol.1 (1), p.1-11
• Main Authors: Staveley, L.A.K., Linford, R.G.
• Format: Article
• Language: English
• ISSN: 0021-9614; 1096-3626
• DOI: 10.1016/0021-9614(69)90031-7

Heat capacity measurements have been made on calcite from 10 to 303 K and on aragonite from 22 to 291 K. Assuming the two polymorphs to be perfectly ordered at 0 K, the molar entropy of calcite at 25 °C exceeds that of aragonite by (0.89±0.05) cal K −1 mol −1. This entropy difference ΔS is effectively established at quite a low temperature (about 80 K). The information which can be derived from this value of ΔS about the pressure-temperature relation for the equilibrium between calcite and aragonite is in very fair agreement with the results of recent direct experimental studies of this equilibrium. An attempt has been made to calculate the heat capacity of calcite, making use of spectroscopic information available for this crystal about the lattice vibrational frequencies. A key question briefly discussed is the division of the vibrational frequency spectrum into optical and acoustic branches and the assignment of an appropriate Debye temperature to the acoustic branch. An important point, due to Professor M. Blackman, is that the value of this temperature depends on the size of the elementary unit chosen for consideration of the vibrational frequencies. At best, the agreement between the observed and calculated C p values is only moderate, and may represent more or less the limit of what can be achieved for a crystal as complicated as calcite by an approach that uses only Debye and Einstein functions.