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

Polymorphic minerals are those that have essentially the same chemical composition, but different crystal structures. The polymorphic forms of a mineral are regarded as different species if their structures are topologically different. However, if the crystal structures of the polymorphs have essentially the same topology, differing only in terms of a structural distortion or in the order – disorder relationship of some of the atoms comprising the structure, such polymorphs are not regarded as separate species. The names of such topologically similar polymorphs can be distinguished by the addition of crystallographic suffixes to the mineral name, as discussed in a later section.

Although the formal definition of polymorphism is restricted to substances with identical chemical compositions, this strict limitation is broadened somewhat to include relatively minor chemical variations when the topology of the structure is retained.

Example 1: Graphite and diamond are polymorphs of crystalline carbon; both have the same composition, but their structures are topologically different, and therefore minerals such as these are regarded as separate species.

Example 2: Analcime has a number of topologically similar polymorphs, e.g., cubic, tetragonal, orthorhombic, monoclinic, triclinic and possibly even trigonal, caused by relatively minor variations in symmetry due to different degrees of order of Si and Al with related different occupancies of the nearest Na structural site. Such polymorphs are not to be regarded as separate species.

Example 3: Orthoclase and microcline have essentially the same composition and topologically similar structures. According to current practice, these minerals would not be regarded as separate species, but their names are retained in the mineral lexicon for historical reasons.


источник