The heat capacities of natural and synthetic samples of leucite (KAlSi2O6), orthorhombic KAlSiO4, and their iron analogues, KFeSi2O6 and KFeSiO4, have been measured between 400 and 1000 K by differential scanning calorimetry. For the various phase transitions that occur, temperatures of transition and associated changes in enthalpy and entropy have been determined. The tetragonal-cubic transition in leucite spans 122 to 176 deg, depending upon the sample studied, and is characterized by two peaks on a Cp heating curve. Heat treatment of a natural leucite for one week at 1673 K lowers the transition by 24 deg. Two distinct peaks in the Cp curve, enhanced by heating, suggest that an intermediate phase exists that is stable over 17 deg. A space group of I41/acd has been assigned to this phase because it can be related to the low-temperature tetragonal leucite phase (I41/a) by mer-ohedric twinning and the high-temperature cubic leucite phase (Ia3d) by pseudomerohedric twinning, both of which are found in leucite. In contrast, KFeSi2O6 (leucite structure) has a single, sharp Cp peak at a lower transition temperature. A natural leucite consisting of 88 wt°/o KAlSi2O6 and 12 wt% KAlSi3O8 has an X-ray pattern that indicates tetragonal symmetry, yet appears to be without twinning when viewed optically. The Cp data indicate a very small transition effect which may, however, be spread out in temperature above the limit of the DSC (1000 K). The orthorhombic polymorph of KAlSiO4 undergoes one transition at 695 and one at 817 K, whereas its iron analogue, orthorhombic KFeSiO4, has a single transition at 729.5 K. The enthalpy and entropy data indicate that the single transition in orthorhombic KFeSiO4 is approximately equivalent to the sum of the two transitions in KAlSiO4.

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