Комиссия IMA по новым минералам и названиям минералов: нормы и рекомендации по номенклатуре минералов, 1998 Политипы и политипоиды

Polytypes are substances that occur in several different structural modifications, each of which may be regarded as being built up by the stacking of layers of (nearly) identical structure and composition, and with the modifications differing only in their stacking sequence (Guinier et al. 1984). Polytypoids are substances that do not fit the strict definition of a polytype, and include minerals with the same topology and with somewhat different compositions. Polytypes and polytypoids are not regarded as separate species and, like topologically similar polymorphs, they can be distinguished by the addition of a crystallographic suffix to the mineral name, as indicated in a later section.

Example 1: Högbomite exists in a number of different hexagonal and rhombohedral polytypes owing to variations in the stacking of the basic structural layers. These polytypes are not regarded as separate mineral species, and can be distinguished by appropriate suffixes (see later).

Example 2: Pyrrhotite, Fe_1-xS, where x varies between 0 and 0.12, exists in a number of different crystallographic forms owing to variations in the degree of order of the Fe vacancies in the S lattice; because of the variable chemical composition, the different types of pyrrhotite can be regarded as polytypoids and are not regarded as separate species.


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