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II.1. Indicative symbols: modified Gard notation

Gard (1966) has described a nomenclature for fibrous calcium silicates that was modified by A. Kato and H. Schulz in the 1977 report. The modified Gard notation is recommended here because it takes into account multiple translation periods along all three axes as well as the interchange of axes relative to a standard subcell. For polytypes without interchange of axes and with multiple periodicity normal only to the layers, a simplified version of the notation may be used which is similar to that introduced by Ramsdell (1947) for SiC and is in common use for phyllosilicates. Although the modified Gard notation has been adapted here to describe polytype structures, it is evident from the examples considered below that it can also be useful in nonpolytypic cases involving permutations of axes or superstructures in one or more directions.

In this system the symbol to describe a polytypic phase consists of a mineral name and an italicized supplement, combined by a hyphen. The supplement contains four positions, with additional marks when necessary. The first position in the symbol is filled by one of the following capital letters to indicate the crystal system of the compound:

cubic = C

hexagonal = H

trigonal (with hexagonal Bravais lattice) = T

trigonal (with rhombohedral Bravais lattice) = R

tetragonal (quadratic) = Q

orthorhombic = O

monoclinic = M

triclinic (anorthic) = A.

Three lower-case letters, accompanied by numbers when necessary, follow the symmetry symbol to indicate the periodicities along the three axes (in the order a, b, c ) of the compound relative to those of the smallest subcell of the polytypic system. Thus, molybdenite-2H is written as molybdenite-Haa2c and molybdenite-3R as molybdenite-Raa3c. In cases where the pseudosymmetry is of special interest, the letter $\psi$ (abbreviation of pseudo-) is added before the symmetry symbol. For example, ${\psi}H$ means pseudo-hexagonal. Thus, pearceite is written as pearceite-${\psi}Habc$ and arseno-polybasite as pearceite-${\psi}H2a2b2c$. The permutation of axes relative to that of the parent-structure subcell is easily indicated by permuting the appropriate symbols, for example (see Appendix 1), gibbsite-${\psi}Oabc$ and gibbsite- ${\psi}M2b2ac$. A definition of these species has been given by Strunz (1966).

In order to distinguish the ordered and disordered polytypes of xonotlite, described by Gard (1966) as P121 and $P\infty$21, both of which are Ma2bc or ${\psi}Oa2bc$ in the proposed system, an additional symbol d (abbreviation for disordered) is added as a subscript to the letter involved. That is, $P\infty$21 is written as Ma_d2bc or ${\psi}Oa_d2bc$.

In order to reconcile the present notation with that of Ramsdell, the following simplification may be made. If the four-position symbol describes a tetragonal or hexagonal compound, then the first two axes have the same length, as in graphite-Haa2c. In this case, it is permissible to delete the first two axes and place the periodicity of the third axis in front of the capital letter symbolizing the crystal system. Thus graphite-Haa2c becomes graphite-2H. This rule is also applicable to polytypes with pseudo-hexagonal symmetry. In this case, the deleted letters are not always aa but may be bc, ab, ba, etc., where $b = a{\surd}$3. Thus, indialite may be given as cordierite-1H and cordierite (sensu stricto ) as cordierite-1O or cordierite- 1${\psi}H$.

In the case of the polytypic micas, 2M₁ and 2M₂ require different treatment. Thus, muscovite-2M₁ = muscovite-Mab2c or -${\psi}Hab2c$ can be shortened to muscovite-2M or -2${\psi}H$. But for muscovite-2M₂ = muscovite-Mba2c or -${\psi}Hba2c$, the long symbol should be retained if the reversal of two axes is to be described. This system, however, is not intended to replace symbols such as 2M₁ and 2M₂ that are useful and internationally accepted.

Examples of the recommended nomenclature are given in Appendix 1 for a representative number of mineral groups. The mineral names tabulated are chosen for familiarity and are intended to be illustrative, rather than exhaustive, and cover related phases by the least number of names. The Ad-Hoc Committee does not necessarily recommend that all traditional names be dropped. Polytypic non-minerals are to be referred to by their chemical formula and appropriate supplement.

Copyright © 1984, 1998 International Union of Crystallography