Abstract
Åkermanite solid solution is known as a mineral showing incommensurate to normal (I-N) phase transition at a temperature between 80 and 250°C, which depends on the chemical composition in the substitution series. To clarify the thermal behavior around the transition temperatures, crystals with various chemical compositions were synthesized by the floating zone method and the heat capacities above room temperature were measured by means of a small scale adiabatic calorimeter, using less than 2g of a sample for each measurement. The obtained I-N transition entropy values, about 1.7JK-1mol-1 in average, are almost the same, independent of composition and the transition temperature. The transition mechanisms and the value of transition entropy are discussed in terms of a statistical model related to the modulated structure.
