Original paper
The modular structure of pyroxenes
Nespolo, Massimo; Aroyo, Mois I
European Journal of Mineralogy Volume 28 Number 1 (2016), p. 189 - 203
published: Mar 23, 2016
Abstract
The structures of pyroxenes can be described as cell-twins based on a common module (a layer) consisting of half the unit cell of the P 21/c polymorph. Atomic models of the different polymorphs are computed and the close similarity between the models and the corresponding experimental structures is shown. The cell-twin operations building the structures of pyroxenes are partial operations belonging to a special case of space groupoid which was called ''twinned space group'' in the pioneering studies by Ito started in 1935. The groupoid analysis of clino-, proto- and orthoenstatite is presented and the special place occupied by pyroxenes in the category of polytypes is discussed.
Keywords
partial symmetry • local symmetry • groups • twinned space • cell-twinning • space groupoids • pyroxenes • polytypism • enstatite