Gatelite-supergroup minerals: recommended nomenclature and review

Gatelite-(Ce) (Bonazzi et al., 2003), västmanlandite-(Ce) (Holtstam et al., 2005), perbøeite-(Ce) and alnaperbøeite-(Ce) (Bonazzi et al., 2014), as well as the recently approved species ferriperbøeite-(Ce) (Bindi et al., 2018) represent iso-topological epidote–törnebohmite (E–T) type polysomes. They are distinguished by the chemical composition of the E module but share the same structural topology and the common general formula A₄M₄(Si₂O₇)(SiO₄)₃(O²⁻,F⁻)(OH)₂. From this point of view, they fulfil the criteria of forming a mineral supergroup (Mills et al., 2009). Because the first member of this kind was named gatelite-(Ce), the root name of this species has priority in naming the supergroup.

The object of this paper is to present nomenclature recommendations for the supergroup, approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification, and give a condensed review of what is currently known about the minerals belonging to it.

All minerals of the gatelite supergroup have a crystal structure that can be described as a regular alternation of modules of epidote-type structure (E) and modules of törnebohmite-type structure (T). The E modules are (001) slabs, ~10.4 Å thick, with the general composition A₂M1M2M3[Si₂O₇][SiO₄]X(OH), where X = O²⁻ and/or F⁻. The T-type modules are (−102) slabs, ~7.1 Å thick, with the composition [REE₂Al(SiO₄)₂(OH)] (Fig. 1).

Specifically, the following crystallographic relations are observed: a_gat ~ 2a_epi ~ [201]_tör; b_gat ~ b_epi ~ b_tör; c_gat ~ (c_epi + a_tör). The doubling of the translation unit along the monoclinic a-axis in gatelite-(Ce) is due to a distortion away from the mirror symmetry normal to the b-axis; thus, the space group is P2₁/a instead of P2₁/m. Such symmetry deviations are indeed very slight, so that the hkl reflections with h = 2n + 1 are relatively weak (Bonazzi et al., 2003).

In the cases of perbøeite-(Ce), alnaperbøeite-(Ce) and ferriperbøeite-(Ce), no evidence of doubling of the translation unit along the a-axis was observed by means of X-ray diffraction, although the crystals were examined with long exposure times, whereas in västmanlandite-(Ce) only a weak, continuous streaking at a*/2 was recorded (Holtstam et al., 2005; Bonazzi et al., 2014; Bindi et al., 2018).

Thus, in all cases except for gatelite-(Ce), the structure was refined as an average structure in the space group P2₁/m instead of P2₁/a, disregarding the slight distortion away from the mirror plane normal to the b-axis. However, whereas the continuous streaking in västmanlandite-(Ce) is due to the offset from the (010) mirror plane of two sites (A3 and O15, occupied by REE and oxygen, respectively), in perbøeite-(Ce), alnaperbøeite-(Ce) and ferriperbøeite-(Ce), only O15 exhibits an offset from the mirror plane. Therefore, a possible streaking at a*/2 due to short-range order, if any, would be even weaker than in västmanlandite-(Ce).

All the minerals of the supergroup share the same topology, consisting of edge-sharing octahedral chains running along the monoclinic b-axis, cross-linked to each other by SiO₄ and Si₂O₇ groups. The remaining large cavities are occupied by Ca (A1) and REE ± Na (A2, A3 and A4).

In the P2₁/a model there are four independent octahedral sites: M1 octahedra form branched chains with M3 octahedra alternately attached on opposite sides, whereas M2a and M2b octahedra (which in the P2₁/m model coalesce to a unique M2 octahedron with double multiplicity) form single chains. As the homologue octahedron of the epidote structure, M3 is the largest and distorted one, and hosts always divalent cations due to the ubiquitous presence of REE³⁺ at the A2 site. On the other hand, the dominance of divalent cations (mainly Mg) in the M1 octahedron is related to the amount of F^- substituting for O²⁻ at the O4 site. As in the epidote supergroup, the amount of Mg²⁺, Fe²⁺, Al³⁺, Fe³⁺ entering M1 and M3 depends on competing ions, with a preference for the larger cations to enter M3. M2a and M2b (or M2 in the P2₁/m model), which replicate the M2 in the epidote archetype and the octahedral site in törnebohmite, are almost fully occupied by Al.

The classification of the gatelite-type minerals is based on the E structural element, as the T module is relatively constant in composition, with only minor Al³⁺-Fe³⁺ substitution.

Up to now, only ET polysomes having the E module with A2 = REE have been found, probably because the high activity of lanthanides required to form törnebohmite: gatelite-(Ce), perbøeite-(Ce), and ferriperbøeite-(Ce) have E modules all belonging to the allanite group; västmanlandite-(Ce) has E = dollaseite-(Ce), and the E module of alnaperbøeite-(Ce) has no known natural analogue in the epidote supergroup (cf.Table 1). Dollaseite-(Ce), CaCeMg₂Al(Si₂O₇)(SiO₄)(OH)F, is distinct from the allanite-group members by the F-content at O4 and by having divalent ions, Mg²⁺, in both M1 and M3 (Peacor & Dunn, 1988), and forms its own group in the epidote supergroup together with khristovite-(Ce), CaCeMgAlMn²⁺(Si₂O₇)(SiO₄)(OH)F (Armbruster et al., 2006).

By analogy with the epidote supergroup (Mills et al., 2009; cf.Armbruster et al., 2006), three groups are defined to accommodate the existing accepted mineral species with the gatelite supergroup established at the higher hierarchical level. Additional groups need to be introduced if new species cannot be covered by these.

• The gatelite group includes members that can be derived from the mineral gatelite-(Ce) solely by homovalent substitutions. The key cation and anion sites for this group are: A1 = A²⁺; A2, A3, A4 = A³⁺; M1 = M³⁺; M2 = M³⁺; M3 = M²⁺; O4 = O²⁻; O10 and O11 = (OH)⁻. In other words, the dominant valence as listed above must be maintained. The three species gatelite-(Ce), perbøeite-(Ce), and ferriperbøeite-(Ce) thus constitute the presently known members of the gatelite group.

• 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^-]. Thus, the valences on the key sites are: A1 = A²⁺; A2, A3, A4 = A³⁺; M1 = M²⁺; M2 = M³⁺; M3 = M²⁺; O4 = F^-; O10 and O11 = (OH)^-. Up to now, västmanlandite-(Ce) is the unique member of this potential group.

• 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.

In line with the epidote minerals (Armbruster et al., 2006), 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 is given. If the dominant cation is different at any of these sites, a new root name is to be assigned.

In the gatelite and alnaperbøeite groups, no prefix should be added to the root name if M1 = Al; in the västmanlandite group, no prefix is added to the root name if M1 = Mg. Otherwise a proper prefix derived from the name of a chemical element is attached. In particular, when M1 = M³⁺ (gatelite and alnaperbøeite groups), the prefixes “ferri”, “mangani”, “chromo”, and “vanado” indicate dominant Fe³⁺, Mn³⁺, Cr³⁺, and V³⁺ at M1, respectively. When M1 = M²⁺ (västmanlandite group), the prefix “ferro” indicates dominant Fe²⁺ at M1. Finally, the dominant REE (at A2 + A3 + A4) is indicated with a Levinson suffix (Levinson, 1966; Bayliss & Levinson, 1988).

Some suggested names for hypothetical new members are listed in Table 1 along with the known IMA-approved species. Figure 2 shows the relations between the groups in the gatelite supergroup. Figure 3 shows the chemical relations between known and some potential new species in the gatelite supergroup.

Gatelite-supergroup minerals have been found in a variety of geological settings: granitic niobium-yttrium-fluorine family (NYF) pegmatite (Tysfjord), carbonatite and associated fenites (Biraya, Ren, Anadol), alkali-syenite (Mochalin Log placer deposit), hydrothermal-metasomatic skarns (Trimouns, Bastnäs, Norberg), with an age span of nearly two billion years (see Table 2 for references). Obviously, they exist in REE-rich assemblages, and most frequently coexist with törnebohmite-(Ce), allanite- or dollaseite-group minerals, bastnäsite-(Ce), cerite-(Ce), or fluorbritholite-(Ce). Among the more abundant associated non-REE minerals are dolomite, fluorite or quartz.

The members of the supergroup are very rare minerals, although the number of observations is increasing (Table 2). On the one hand, it is important to bear in mind that some species are easily overlooked without careful mineralogical and petrological studies; for example, ferriallanite-(Ce) and ferriperbøeite-(Ce) have similar appearance (habit, colour and lustre). Furthermore, minute intergrowths of E with ET or ET with T members have been observed (Fig. 4) so that even microchemical point analyses sometimes are inadequate for identification. On the other hand, gatelite-supergroup minerals are not ubiquitous in all REE mineral assemblages of a similar kind, as some recent detailed studies suggest (e.g., Allaz et al., 2015).

To the best of our knowledge, no experimental work has been undertaken to study the P–T–x stability of these minerals. Nevertheless, some analogies with the REE-rich epidote-supergroup minerals might be inferred. Allanite-(Ce) is stable up to ca. 800 °C at 1 kbar and to ≥1050 °C at 40 kbar (Affholter, 1987; Hermann, 2002). However, the greater amount of (OH)⁻ groups in the gatelite-type structure possibly decreases the T stability in comparison with the allanite-group minerals, although there are indications that törnebohmite-(Ce) may persist to at least 800 °C (Affholter, 1987; Martin et al., 2011). Studies of relevant REE-rich members of the epidote supergroup indicate that their P–T stability is highly dependent on bulk-rock and fluid chemistry (e.g., Janots et al., 2007; Budzyń et al., 2017), in particular Ca and P contents. Members of the gatelite supergroup obviously need high bulk concentrations of LREE and Al for their formation, but appear to be relatively insensitive to SiO₂ activity and fO₂. From the data provided for the different natural occurrences, it appears that they have been formed as primary mineral at least in a range from 350 °C (Trimouns; Schärer et al., 1999) to 750 °C (Ren; Ya’acoby, 2011). In some rocks, members of the supergroup have apparently survived regional metamorphism of up to ca. 600 °C/3 kbar (Bastnäs; Skelton et al., 2018) and 700 °C/5–8 kbar (Ren; Ya’acoby, 2011).

In some cases, the minerals seem to have formed at the earliest stage of REE-mineralization, more or less directly in a magmatic or hydrothermal system. In the Bastnäs-type deposits in Sweden, ferriperbøeite-(Ce) and västmanlandite-(Ce) formed at the expense of primary cerite-(Ce) or fluorbritholite-(Ce), respectively, in metasomatic reactions involving Ca–Mg minerals (dolomite, tremolite) and hydrothermal fluids (T > 400 °C) containing Si-, Fe- and Al–chloro-fluoro anion complexes (Holtstam & Andersson, 2007; Holtstam et al., 2014). In the Anadol deposit, perbøeite-(Ce) and associated törnebohmite-(Ce) are interpreted to have been formed by decomposition of allanite-(Ce), by reaction with high fO₂, low-T aqueous fluids, also producing REE-bearing epidote, quartz and iron hydroxide (Khomenko et al., 2013).

The supergroup, as it is presently known, can be described by the multicomponent system Ce₂O₃–CaO–MgO–FeO–Fe₂O₃–Al₂O₃–SiO₂–H₂O (CeCMF²ASH), with addition of F₂ or Na₂O to cover the exotic members västmanlandite-(Ce) or alnaperbøeite-(Ce) (Fig. 3). Analytical data available from literature sources are collected in a supplementary table (Table S1). As no vacancies at the cation positions have been demonstrated for the structure type, formulae are normalized to 13 cations. All analysed samples deviate more or less from the ideal composition, with mainly ^M3[Mg$Fe-12+$], ^M3[Al$Fe-12+$], ^M1[Mg$Fe-13+$], ^M1,M2[Al$Fe-13+$], ^M1,M3[MgAl₋₁], ^A[Ca(REE)₋₁], ^A[(REE)Na₋₁] and ^O4[OF₋₁] as substitution vectors. The specimens closest to end-member composition contain 84% västmanlandite (from Norberg), 80% perbøeite (type specimen), 70% ferriperbøeite (Bastnäs), 56% alnaperbøeite (type specimen), and 51% gatelite (type specimen) component, respectively. The effect of the OF₋₁ substitution is clear, largely related to the västmanlandite component, but the (OH)F₋₁ substitution operating at the OH sites in this structure type cannot be excluded a priori. Variation at the tetrahedral sites is insignificant; the average Si content for the whole sample population is 4.97 ± 0.07 atoms per formula unit (apfu). Aluminium in M2, assuming perfect ordering at the smaller M2 octahedra, ranges from 1.67 to 2.00 apfu; structure refinements and Mössbauer spectroscopy data support the presence of minor Fe³⁺ at this site for västmanlandite-(Ce) and ferriperbøeite-(Ce) (Holtstam et al. 2005; Bindi et al., 2018).

The major chemical substitutions occur at the octahedral M1 and M3 sites. The greatest variation concerns Mg²⁺, which ranges from 0 to 1.75 apfu. The Mg content is inversely correlated with Fe²⁺, indicating an extensive ferriperbøeite-(Ce)–västmanlandite-(Ce) solid solution (Fig. 5). The Rödbergsgruvan sample (Table S1) is a case with near-midpoint composition, with ~57% ferriperbøeite and 43% västmanlandite. A single analysis from the Norberg mines is also special as it shows Fe²⁺ > Mg and still contains an appreciable amount of F (0.31 apfu), corresponding to a substantial fraction of unnamed [CaCe₃][MgAl₂Fe²⁺]Si₂O₇(SiO₄)₃F(OH)₂ (root name “E”). Variation in Fe³⁺ at the M1–M3 sites is extensive (0–1.03 Fe apfu). It is noteworthy that there are no compositional points where 2 < Al apfu < 3 (Fig. 5), which means that intermediate members of the ferriperbøeite-(Ce)–perbøeite-(Ce) solid solution have not yet been found, although the corresponding epidote-supergroup minerals (i.e., members of the ferriallanite–allanite solid solution) are widely reported (Gieré & Sorensen, 2004; Vlach, 2012). There is also a limited Al–Mg substitution, similar to what occurs in the intermediate dollaseite–dissakisite compositions found in the Norberg deposits (Holtstam & Andersson, 2007). Replacement of Fe²⁺ for Al is limited to the perbøeite-(Ce)–alnaperbøeite-(Ce) join, where there is also a significant coupled Ce–Na substitution (Bonazzi et al., 2014). As shown in Fig. 5, along this join Al approaches the theoretical value of 4 apfu. However, the corresponding Na content does not reach 0.5 apfu, since the substitution of trivalent REEs by divalent Ca also contributes to charge balance.

All samples representing the ferriperbøeite-(Ce)–västmanlandite-(Ce) join show a deficit of Ca (0.80–0.97 apfu), whereas the Al-rich members seem to have Ca ≥ 1.00 apfu, which has been ascribed to an artefact related to difficulties with analysing all REE (Bonazzi et al., 2014). From structure refinements, it is shown that when extra REE atoms occur, they are accommodated at the A1 site in the E module (Holtstam et al., 2005; Bindi et al., 2018). Cerium is here consistently the dominant REE (atomic Ce/La = 2.71–1.05); the material from Ren is the richest in La₂O₃ (>20 wt%). The true La-analogue of ferriperbøeite-(Ce) has, however, recently been discovered at Mochalin Log (R. Škoda, pers. comm. 2017). Overall, there is also a significant variation in Nd₂O₃ and Sm₂O₃ contents (2.5–11.7 and 0.1–2.0 wt%, respectively). At Trimouns and Tysfjord we find Ce > Nd > La. The Tysfjord samples also have, along with a västmanlandite-(Ce) sample from Norberg (Malmkärra), the highest Y₂O₃ concentrations (0.6–1.3 wt%). The HREE (Tb–Lu) contents are, when actually measured, generally very low, with a maximum of 0.3 wt% of Er₂O₃ (Tysfjord).

It is noteworthy that no gatelite-supergroup minerals have been found to show substantial MnO contents (up to ca. 1 wt% for samples from Anadol, ca. 0.3 wt% from Trimouns, and ca. 0.2 wt% from Tysfjord), although Mn in both trivalent and divalent states is a major constituent for many members of the allanite group (see Table 1).

Impurities in these minerals are generally low, with 0.3 wt% P₂O₅ (Ren), 0.2 wt% TiO₂ (Ren). Actinides are mostly close to or below the electron microprobe detection limits, with the Tysfjord pegmatite samples as an exception (contains 0.1–0.4 wt% ThO₂). The presence of minor amounts of large cations such as Ba²⁺ and Pb²⁺, are also indicated in the EDS analyses from Anadol. The cations Th⁴⁺, Pb²⁺ and Ba²⁺ are preferably assigned to the A2–4 sites, as they occur in the epidote supergroup (Armbruster et al., 2006).

PB and LB thank the University of Florence, “Progetto di Ateneo 2015”. DH acknowledges a previous grant to study the Bastnäs-type deposits from the Swedish Research Council (contract #2003-3572). Ulf B. Andersson kindly commented on an early draft of the manuscript. The paper benefitted of improvements by two anonymous reviewers.