Modular features of sapphirine-type structures

Abstract

Abstract 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 octahedral ribbons) which are connected as the corresponding modules in biopyriboles. 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 order-disorder (OD) character of sapphirine is discussed in terms of one kind of OD layers, with presentation of the OD groupoid family and indication of the structures with maximum degree of order (MDO structures), as well as in terms of two kinds of layers, namely as a regular alternation of slices 〈P〉 and 〈S〉 cut from the structures of pyroxene and spinel, respectively. Finally, the diffraction features of sapphirine are discussed in relation to the different modular approaches.