Recommended nomenclature for labuntsovite-group minerals

The present report contains recommendations on the nomenclature of the labuntsovite-group (LG) minerals based on a crystal-structural classification. The labuntsovite group includes hydrous titanium and niobium silicates, both orthorhombic and monoclinic, with a structure characterized by a framework consisting of chains of (Ti,Nb)O-octahedra, linked by four-member rings of Si,O-tetrahedra. This framework contains open cavities occupied by H2O molecules and extra-framework cations. Seven subgroups are distinguished, each corresponding to different structure types. In accordance with the nomenclature recommendations, LG minerals have different root names if they belong to different subgroups, or are characterized by different prevailing cations in the (Ti,Nb)-position. The species with doubled unit cell are distinguished by the prefix “para”. In the case of structurally ordered members (space groups C2/m, I2/m), separate species within each subgroup are recognized by different root names (to distinguish the members with Ti 8 Nb or Ti ‹ Nb) and by a modifier (-K, -Mn, -Mg etc., to distinguish the members with differently occupied “key” positions). Unlike zeolites, a complex substitution of extra-framework cations is possible in monoclinic LG minerals: 2C→D(H2O)2 where C = K, Ba; D = Fe, Mg, Mn or Zn. Only one of these coupled sites (C or D) can be more than 50 % occupied, whereas the other is more than 50 % vacant and is not considered as the “key” (species-forming) position. In the case of structurally disordered members (vuoriyarvite series) with space group Cm and a large number of extra-framework positions, separate species are recognized in which different extra-framework cations (K, Na, Ca, Sr, Ba) are the most abundant in atomic proportions. Key-words: labuntsovite group, nomenclature, labuntsovite, nenadkevichite, korobitsynite, vuoriyarvite, lemmleinite, kuzmenkoite, organovaite, paralabuntsovite, tsepinite, karupmöllerite, gutkovaite.