CNMNC guidelines for the nomenclature of polymorphs and polysomes

New guidelines for the nomenclature of polymorphs and polysomes have been approved by the CNMNC. Several cases can be distinguished: (i) Polymorphs with different crystal systems are distinguished by the prefixes cubo- (cubic), hexa- (hexagonal), tetra- (tetragonal), trigo- (trigonal), ortho- (orthorhombic), clino- (monoclinic), and anortho- (triclinic); (ii) Polymorphs with different crystal systems but showing a pseudosymmetry should show the prefix pseudo-; (iii) Polymorphs with the same crystal system but different space groups are distinguished by the prefix para-. If three or more polymorphs show the same crystal system but different space groups, the space group notation may be added as a suffix, but such a nomenclature should be avoided if possible; (iv) Polymorphs with the same space group are distinguished by the prefix para-; (v) Minerals with polymorph suffixes but with different chemical compositions cannot be considered as true polymorphs, so we recommend using the prefix meta-, which indicates a close but significantly different chemical composition; (vi) Polysomatic symbols have to be placed as a suffix, which indicates the number and types of modules which alternate in the structure as in the hogbomite supergroup, or as prefixes as in the sartorite homologous series. These recommendations have to be applied for future new mineral proposals, but the modifications of historical and well-established names have to pass through the CNMNC for approval. In order to be consistent with the new guidelines, 25 mineral names were modified: Domeykite-β becomes trigodomeykite, fergusonite-(Y)-β becomes clinofergusonite-(Y), fergusonite-(Ce)-β becomes clinofergusonite-(Ce), fergusonite-(Nd)-β becomes clinofergusonite-(Nd), ice-VII becomes cubo-ice, roselite-β becomes anorthoroselite, sulphur-β becomes clinosulphur, mertieite-II becomes mertieite, mertieite-I becomes pseudomertieite, uranophane-? becomes uranophane, uranophane-β becomes parauranophane, gersdorffite-P213 becomes gersdorffite, gersdorffite-Pa3 becomes paragersdorffite, gersdorffite-Pca21 becomes orthogersdorffite, betalomonosovite becomes paralomonosovite, lammerite-β becomes paralammerite, novacekite-I becomes hydronovacekite, novacekite-II becomes novacekite, halloysite-7A becomes halloysite, halloysite-10A becomes hydrohalloysite, metauranocircite-I becomes metauranocircite, taimyrite-I becomes taimyrite, uranocircite-II becomes uranocircite, andorite IV becomes quatrandorite, and andorite VI becomes senandorite.