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...$\S$8.2.1)

IT will henceforth denote International Tables for Crystallography , Vol. A, 1983 (first edition) and 1987 (second edition).

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...orbits

A crystallographic point configuration (or crystallographic orbit) (IT $\S\S$ 8.3.2 and 14.1) is an infinite set of points which is generated from one point by all symmetry operations of a given space group.

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...states.

The term isostructural is synonymous with isotypic. Isomorphy in its original definition meant close similarity of external crystal shape and analogy of chemical composition with the implication of close similarity of internal structure. Nowadays this term is used for a variety of different phenomena [series of crystal-chemical isotypes; solid-solution series (mixed crystals); atomic/ionic substitutions in crystal structures without regard to the existence or absence of isotypy]. Its use is not recommended.

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...blocks,

Blocks, rods and slabs are understood here as structure portions which are finite, or infinite in one or two dimensions, respectively.

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...brackets.

The present proposal strongly advocates putting information on coordination numbers and coordination polyhedra in the form of trailing superscripts and not as subscripts as suggested by IUPAC (1990). Although all symbols proposed here could be used as subscripts, their juxtaposition with atomic proportions (especially fractional ones) can lead to confusion even in simple cases. For example:

K_[6]Al_o,2 $^2_{\infty}[$Al_tSi_t,3O₁₀](OH)₂ (for muscovite) instead of K^[6]Al$^o_2{\,}^2_{\infty}$[Al^tSi^t₃O₁₀](OH)₂.

The IUPAC system with its longer subscripts (tet for tetrahedral instead of t etc .) can be extended to more complicated cases only with difficulty (and ambiguity) and is inappropriate for irregular coordination that can only be expressed numerically, e.g. K^[12]. The IUPAC proposal to indicate diferent types of coordination by parentheses has similar problems, since different types of brackets are used with meanings that differ from those in normal chemical formulae.

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...atoms

Henceforth the term `atom' will be applied regardless of ionization state.

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...symbols

These pictorial symbols have already been used in Acta Cryst . (Parthé, 1980).

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Copyright © 1990, 1998 International Union of Crystallography