Cancrinite: Crystal structure, phase transitions, and dehydration behavior with temperature

The structural behavior of a cancrinite, Na_5.96Ca_1.52[Al₆Si₆O₂₄](CO₃)_1.57·1.75H₂O, was determined by using in situ synchrotron X-ray powder diffraction data [λ = 0.91806(5) Å] at room pressure and from 25 to 982 °C. The sample was heated at a rate of about 9.5 °C/min, and X-ray traces were collected at about 15 °C intervals. The satellite reflections in cancrinite were lost at about 504 °C, where a phase transition occurs. All the unit-cell parameters for cancrinite also show a discontinuity at 504 °C. Initially, the [Ca·CO₃] clusters and their vacancies are ordered in the channels, and this ordering is destroyed on heating to give rise to the phase transition. Cancrinite loses water continuously until about 625 °C; thereafter an anhydrous cancrinite phase exists. From 25 to 952 °C, a minimal amount of CO₂ is lost from the structure. Over this temperature range, the average <Al-O-Si> bridging angle, which is an indication of the degree of rotation of the tetrahedra, increases from 143.7(4) to 147.7(5)°. Rotations of the tetrahedra are caused by expansion of the Na1-O2 bond lengths.