Crystal structure of heat-collapsed phases of barrerite

Barrerite is orthorhombic, with space group Amma and unit cell dimensionsof a=13.643, b=18.200, c= 17.842 A. On heating at 200-380°C it transforms on a few hours to barrerite B, with s.g. Amma and unit cell dimensions of a=13.62, b=17.12, c=16.03A. After a few days barreriteB rehydrates to barrerite C, with s.g. Amma and unit cell parameters of a=13.61, b= 17.61, c= 16.24. On heating at 400-450°C barrerite transforms in a few hours to a D phase which is also orthorhombic and has parameters a=12.90, b=16.87, c=16.18A. The heat collapsed phases B and C are characterized by major changes in the framework. A statistical beaking of of an oxygen bridge gives rise to two half occupied face-sharing tetrahedra.