Heat capacities of synthetic hedenbergite, ferrobustamite and CaFeSi2O6 glass

Heat capacities have been measured for synthetic hedenbergite (9–647 K), ferrobustamite (5–746 K) and CaFeSi₂O₆ glass (6–380 K) by low-temperature adiabatic and differential scanning calorimetry. The heat capacity of each of these structural forms of CaFeSiO₆ exhibits anomalous behavior at low temperatures. The X-peak in the hedenbergite heat-capacity curve at 34.5 K is due to antiferromagnetic ordering of the Fe²⁺ ions. Ferrobustamite has a bump in its heat-capacity curve at temperatures less than 20 K, which could be due to weak cooperative magnetic ordering or to a Schottky anomaly. Surprisingly, a broad peak with a maximum at 68 K is present in the heat-capacity curve of the glass. If this maximum, which occurs at a higher temperature than in hedenbergite is caused by magnetic ordering, it could indicate that the range of distortions of the iron sites in the glass is quite small and that coupling between iron atoms is stronger in the glass than in the edge-shared octahedral chains of hedenbergite.

The standard entropy change, S^o_298.15 − S^o₀, is 174.2 ± 0.3, 180.5 ± 0.3 and 185.7 ± 0.4 J/mol·K for hedenbergite, ferrobustamite and CaFeSi₂O₆ glass, respectively. Ferrobustamite is partially disordered in Ca-Fe distribution at high temperatures, but the dependence of the configuratonal entropy on temperature cannot be evaluated due to a lack of information.

At high temperatures (298–1600 K), the heat capacity of hedenbergite may be represented by the equation C^o_p(J/mol·K) = 3l0.46 + 0.01257T-2039.93T−12− 1.84604× l06T−2">−12− 1.84604× 10⁶T⁻² and the heat capacity of ferrobustamite may be represented by Cop(J/mol·K)= 403.83−0.04444T+ 1.597× 10−5T2−3757.3T−12">C^o_p(J/mol·K)= 403.83−0.04444T+ 1.597× 10⁻⁵T²−3757.3T⁻¹².