MODULAR FEATURES OF SAPPHIRINE-TYPE STRUCTURES

The various modular approaches to the structure type of sapphirine are presented and discussed. Like biopyriboles sapphirine is built up by TOT rod modules (where T stands for tetrahedral chains and O for octah edral ribbons) which are connected as the corresponding modules in bio pyriboles. Two distinct orientations (S,Z) of the modules are possible ; their structures and stacking ways are constrained by the condition of cubic close packing of oxygen atoms. The various possible stacking ways are presented and the homogeneous sequences are indicated. The or der-disorder (OD) character of sapphirine is discussed in terms of one kind of OD layers, with presentation of the OD groupoid family and in dication of the structures with maximum degree of order (MDO structure s), as well as in terms of two kinds of layers, namely as a regular al ternation of slices [P] and [S] cut from the structures of pyroxene an d spinel, respectively. Finally, the diffraction features of sapphirin e are discussed in relation to the different modular approaches.