Комиссия IMA по новым минералам и названиям минералов: нормы и рекомендации по номенклатуре минералов, 1998 Полисоматические рядыPolysomatic series
Homologous structures are those that consist of structural units built on common structural principles; these structures contain the same chemical elements, although in different proportions, and differ with respect to the size of the units. A homologous series is a series of structures that can be derived from one type of basic structural unit using one type of recombination principle. Homologous series can be classified into two categories, accretional and variable-fit; combinations of the two types are known also to occur.
An accretional homologous series, also known as a polysomatic series, is one in which the types of building blocks (rods, layers, etc.) and the principles that define their mutual relationships remain preserved, but in which the sizes of these blocks vary incrementally (Veblen 1991). A member of an accretional homologous series can be regarded as a distinct species if it has the following properties: a) unique size of the fundamental building block, b) unique crystallographic unit-cell, and c) unique composition or a limited compositional range (Makovicky 1989).
Example 1: The structures of the sulfosalt minerals lillianite, eskimoite, vikingite, ourayite, gustavite and heyrovskyite can all be interpreted as consisting of alternating galena-like modules twinned on (131) of the galena motif (Makovicky & Karup-Møller 1977). The sizes of the modules, the unit-cell parameters, and the chemical compositions of these minerals are all different, which justifies their existence as separate species.
Example 2: Composite structures of members of the cylindrite group are formed of two kinds of layers, pseudohexagonal (H) and pseudotetragonal (Q). Cylindrite and franckeite have the same Q–H–Q–H sequence of stratification, but in franckeite the width of the Q layer is twice that of the Q layer of cylindrite. The two minerals are therefore regarded as separate species.
A variable-fit homologous series can also be regarded as coupled homeotypes forming a composite structure. Such a series is one in which the structure consists of two kinds of alternating, mutually non-commensurate building blocks. Each kind of building block has its own short-range periodicity, and it takes m periods of one block and n periods of the other block before they meet in the same configuration as was observed at the preselected origin. The non-commensurability of the building blocks may be one- or two-dimensional, and is usually connected with geometrical and compositional long-range modulation of both layer types (Makovicky & Hyde 1981). The period of the long-range match may vary within certain relatively broad limits because of incremental changes in the value of m or n. Because of this, the structures are infinitely adaptive, and a great number of possible variants can result. For this reason, individual members of variable-fit homologous series should not be regarded as separate species (see a later section for suggestions in the nomenclature of this group of minerals).
Example: The cylindrite structure has been interpreted as consisting of incommensurate alternating layers of pseudotetragonal and pseudohexagonal symmetry. Several different coincident lattices have been reported for this mineral (Makovicky & Hyde 1981), but these do not qualify for separate species status.
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