The crystal structure of boracite

The structures of highand low-boracite have been analysed using powder and Weissenberg photographs (Me Ka, A = 0.71 A., Cu Ka, h--1.54 A.). The unit cells and space groups are: No. of molecules a (A.) b (A.) c (A.) in the unit cell High-boracite 12.1 8 (CIMg3BTOla) (at 300 ° C.) Low-boracite 8.54 8.54 12.07 4 (C1Mg3BTO is) C~v-Pca The two structures are essentially the same, both containing the same boron-oxygen network and differing only in the positions of magnesium and chlorine atoms. The network may be described as a rigid aggregate of Be4 tetrahedra and B o s e pyramids which, sharing every oxygen corner, extend indefinitely throughout the structure. A new concept of the a-fl inversion of boracite is suggested: Space group T~-F~3c