All minerals having a crystal structure and a chemical composition that can be described as a regular combination of modules of epidote-type structure (E) and modules of törnebohmite-type structure (T) fulfil the criteria of forming a mineral supergroup, named the gatelite supergroup. All members share the same general formula and topology, consisting of edge-sharing octahedral chains, cross-linked to each other by SiO4 and Si2O7 groups, with the remaining large cavities occupied by Ca and REE ± Na. By analogy with the epidote supergroup, three groups are defined to accommodate the existing accepted mineral species: (i) the gatelite group – includes members that can be derived from the mineral gatelite-(Ce) solely by homovalent substitutions, i.e., gatelite-(Ce), perbøeite-(Ce), and ferriperbøeite-(Ce); (ii) the västmanlandite group – includes members typified by the mineral västmanlandite-(Ce). This group is derived from gatelite-(Ce) by homovalent substitutions and one coupled heterovalent substitution of the type M1[M3+] + O4[O2−] → M1[M2+] + O4[F−]. Västmanlandite-(Ce) is the unique member of this potential group; and (iii) the alnaperbøeite group – includes members typified by the mineral alnaperbøeite-(Ce). This group is derived from gatelite-(Ce) by homovalent substitutions and one coupled heterovalent substitution of the type A2+A3+A4[A3+3] + M3[M2+] → A2+A3+A4[A3+2.5A+0.5] + M3[M3+]. Alnaperbøeite-(Ce) is the unique known member of this potential group. We recommend that the nomenclature of the gatelite supergroup be based on the criterion of occupancy of key cation sites. In particular, A1 and M3 (and, in principle, M2) determine the root name. If the dominant cations at A1, M3 (and M2) exactly match those of an approved species, the same fixed root name – modified by an appropriate prefix – is given. If the dominant cation is different at any of these sites, a new root name is assigned.
The members of the gatelite supergroup are rare minerals, although the number of occurrences is increasing. They have been found in REE-rich assemblages in a variety of geological settings, including granitic NYF pegmatites, carbonatite and associated fenites, alkali-syenites, and hydrothermal-metasomatic skarns. Most frequently, they coexist with törnebohmite-(Ce), allanite- or dollaseite-group minerals, bastnäsite-(Ce), cerite-(Ce), or fluorbritholite-(Ce). Analytical data available from the literature show extensive solid solution along the ferriperbøeite-(Ce)–västmanlandite-(Ce) and the perbøeite-(Ce)–alnaperbøeite-(Ce) joins, whereas there are few or no samples testifying to västmanlandite-(Ce)–gatelite-(Ce) or perbøeite-(Ce)–ferriperbøeite-(Ce) solid solutions. The present nomenclature recommendations were approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification.