Periodic intensity distribution (PID) of mica polytypes: symbolism, structural model orientation and axial settings

Following a preliminary revisitation of the nomenclatures in use for mica p olytypes, the properties of the periodic intensity distribution (PID) funct ion, which represents the Fourier transform of the stacking sequence, are a nalysed. On the basis of the relative rotations of neighbouring layers, mic a polytypes are classified into three types; for each type, the PID exists in different subspaces of the reciprocal space. A revised procedure to comp ute the PID, in which further restrictions on the structural model orientat ion are introduced, is presented. A unifying terminology based upon the mos t common symbols used to describe mica polytypes (RTW, Z and TS) is derived ; these symbols represent the geometrical basis for the computation of the PID. Results are presented for up to four layer polytypes and are compared with the reflection conditions derived by means of Zvyagin's functions. Bot h the PID values and the reflection conditions are expressed in suitable ax ial settings and compared with previous partial reports, revealing some err ors in previous analyses. A computer program to compute PID from the stacki ng symbols is available.