Crystal chemistry of aegirine as an indicator of P-T conditions

One metamorphic andfour magmatic aegirines, andtwo end-member aegirines synthesizedat atmospheric pressure and different temperatures, were investigated by single-crystal X-ray diffraction. The limited compositional differences allow the polyhedral volumes to be almost constant in all the aegirines investigated( VM1 & 11.0 A ¢ª 3; VM2 & 26.3 A ¢ª 3; VT & 2.21 A ¢ª 3). However, differences in polyhedral distortions are responsible for the cell-volume variations, reflected mainly in change of a and b cell parameters. Cell volume is relatedonly partly to the composition of these aegirines: with increasing temperature of formation, an increase in the unit-cell volume of about 1.2 A ¢ª 3 is observed, while a significant contraction of the cell volume takes place at high-pressure conditions of formation. As the difference in cell volume between the two synthetic aegirines is ascribed to the different conditions of synthesis temperature, the same interpretation could be adopted for the differences observedin natural aegirines.