OD interpretation of Mg-vermiculite. Symbolism and X-ray identification of its polytypes

Abstract

In order to establish identification criteria for polytypes of Mg-vermiculite, all its 44 polytypes with maximum degree of order (MDO polytypes, identical with 'standard', 'simple' or 'regular' polytypes) were systematically derived and their respective diffraction patterns calculated. According to the diffractions ± 20l (± 13l), all polytypes of Mg-vermiculite can be categorized into four groups; diffractions 02l suffice, then, to give an unambiguous determination of any MDO polytype. The geometrical analysis leading to the derivation of MDO polytypes was based on the idealized model of the vermiculite structure which can be interpreted as an OD structure (four kinds of layers, category IV), which in turn makes it possible to apply consistently the apparatus of the OD theory. A practical example is also included.