Heat capacities and entropies from 8 to 1000 K of langbeinite (K₂Mg₂(SO₄)₃), anhydrite (CaSO₄) and of gypsum (CaSO₄·2H₂O)

Heat capacities of K₂Mg₂(SO₄)₃ (langbeinite) and CaSO₄ (anhydrite) were measured from approximately 8 to 1000 K by combined adiabatic shield calorimetry (8-365 K) and differential scanning calorimetry (350-1000 K). Heat capacities were also measured on natural crystals of gypsum (CaSO₄ · 2H₂O) between 8.1 and 323.5 K. The molar entropies at 298.15 K, S_m^o(298.15 K), are 378.8 ± 0.6, 107.4 ± 0.2 and 193.8 ± 0.3 J K⁻¹ mol⁻¹ for langbeinite, anhydrite and gypsum, respectively. The heat capacity in J K⁻¹ mol⁻¹ of langbeinite can be represented by the equation C_p,m^o(K₂Mg₂(SO₄)₃T) = 535.9 + 0.11011T-1.0200 × 10⁶/T²-4.909 × 10⁻⁵T² -4040.2/T^0.5 between 300 and 1000 K with an average deviation of ± 0.4%. For anhydrite the heat capacity between 300 and 1000 K is given by C_p,m^o(CaSO₄,T) = 372.8 - 0.1574 T +1.695 × 10⁶/T² + 7.993 × 10⁻⁵T² - 4330.8/T^0.5 with an average deviation of ±0.4%.

Combining our heat-capacity and entropy data with the solution calorimetric results of Kelley et al. (U.S. Bur. Mines Tech. Paper, 625, 1941) yields an equilibrium temperature for the reaction gypsum → anhydrite + 2 water of 314.7 K (41.5 ° C).

Our observations are in agreement with the conclusions of Speer and Salje (Phys. Chem. Miner., 13 (1986) 17); we see no evidence in our heat capacity measurements for the transformation of cubic langbeinite (P2₁3) to a low temperature orthorhombic (P2₁2₁2₁) form as is seen in the isostructural Co, Zn, Ca, Mn and Cd langbeinites.

Although Bond (Bell Sys. Tech. J., 22 (1943) 145) reported that langbeinite was piezoelectric at room temperature, we found no evidence in our C_p^o measurements for a Curie temperature above which langbeinite would no longer be piezoelectric.