New data on nordite-(Ce) and the establishment of the nordite supergroup

Abstract A nomenclature scheme has been set up for the nordite supergroup of minerals, which have the general formula A2BXYZT6O17 and where A = Na; B = Na, Ca; X = Sr, Ca, Ba; Y = REE, Sr; Z = Zn, Fe, Mn, Mg and T = Si. The nordite supergroup includes nordite-(La), nordite-(Ce), ferronordite-(La), ferronordite-(Ce) and manganonordite-(Ce), as well as meieranite which is considered as an unassigned member of the nordite supergroup. In the known nordite-group minerals the Y site is occupied by rare earth elements (REE), while in meieranite the Y site is occupied by Sr. The dominant element on the tetrahedral Z site determines the prefix used in the mineral name. New rootnames must be given to species with a different dominant element on the A, B or X sites. Nordite supergroup minerals are orthorhombic, although nordite-group minerals and meieranite crystallise in the Pcca and P21nb space groups, respectively. The proposed nomenclature scheme for the nordite supergroup has been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA). In addition, new chemical and structural investigations were performed on nordite-(Ce) from Illutalik (Igdlutalik), South Greenland, leading to the first crystal structure refinement for nordite-(Ce).

During a study of nordite-group minerals from the Ilímaussaq alkaline complex, South Greenland (Gulbransen, 2020) it became apparent that the nordite-(Ce) from Illutalik (South Greenland) published by Upton et al. (1976) should be re-investigated as the chemical data results in an empirical formula with significant deviation from the ideal composition. Furthermore, it was realised that there is no published structure solution for nordite-(Ce). Consequently, new chemical and crystallographic investigations are presented on nordite-(Ce) from Illutalik, South Greenland.

Crystal chemistry of nordite-supergroup minerals
Six species are currently approved in the nordite supergroup. They can be described as silicates with the general formula A 2 BXYZT 6 O 17 , where [6][7][8] (Tables 1  and 2). In all described species, the A and B sites are dominated by Na and the X site by Sr. The Y site is either dominated by La or Ce in the nordite-group minerals, or by Sr in meieranite. In the nordite-group minerals the Z site is either dominated by Mn 2+ , Fe 2+ or Zn, whereas in meieranite Mg is the dominant cation on the Z site.
Nordite-supergroup minerals are classified as inosilicates with ring-branched 12-periodic single chains. The silicate chains (T sites) are interconnected through the tetrahedrally coordinated Z site forming layers consisting of 4-, 5-and 8-membered rings perpendicular to [010] (Fig. 1a). The T1 and T2 sites share one corner with the Z sites and two corners with other T sites. The T3 site is tricoordinated with the other T sites. The remaining apical oxygen atoms are shared with the A, A', B, X and Y sites, that form heteropolyhedral layers (Fig. 1b). These sites are connected through edge and face sharing. In the species belonging to the nordite group the A site has a 7+1 square antiprism coordination and is typically occupied by Na. In the case of meieranite, the complete ordering of Na and Sr on the A site leads to the splitting of the A site into the A and A' positions; the A position is 7-coordinated and is occupied by Na, while the A' position is 8-fold coordinated and occupied by Sr. The B site has an octahedral coordination and is occupied by Na. The X and Y sites have an 8-fold square antiprism coordination; the X site is populated by large divalent cations such as Sr and Ba, while the Y site is occupied by REE (nordite group) or by large divalent cations such as Sr in the case of meieranite ( Table 2).
The stacking of the tetrahedral and heteropolyhedral layers shows that the [8] X and [8] Y sites are located above and below the 8-membered rings, while the [8] A and [6] B sites occur above and below the 5-and 4-membered rings, respectively (Fig. 1c). In meieranite, the situation is more complex due to change from the centrosymmetric space group Pcca to the non-centrosymmetric space group P2 1 nb. In that regard, in meieranite the successive tetrahedral layers are oriented in a different way, and therefore the [7] A and [6] B sites sit above and below the 4-and 5-membered rings, the [8] A' and [8] X sites sit above and below the 5-and 8-membered rings, and the [8] Y sites are located solely above and below the 8-membered rings. The nodal net describing the tetrahedral layer of nordite-group minerals is unique among silicate minerals and is given by [(4.5.8) 8 (5 2 .8) 4 (5.8.5.8) 2 ] 2 (Hawthorne et al., 2019). However, as shown in Fig. 2, the tetrahedral layer in the nordite-group minerals and meieranite has a different symmetry, and consequently a different topology. In both nordite-group species and meieranite, the tetrahedral layer can be reconstructed from the same tetrahedral chains (Fig. 2a), in which the basic unit is one 5-membered ring pointing upwards (green), one 5-membered ring pointing downwards (red), and one 4-membered ring (blue). Simplified, in the nordite-group minerals the adjacent chains are symmetrically related through a mirror plane perpendicular to the a axis and passing through the Z site positions (Fig. 2b). In meieranite the chains are related through a 2-fold axis parallel to the b axis and going through the Z site positions (Fig. 2c).

Methodology
A sample from the collection of NHM in Oslo (KNR 44277) was used for the study of nordite-(Ce). The sample is from the same locality studied by Upton et al. (1976), which is a trachyte dyke on the island    of Illutalik (formerly spelled as Igdlutalik), South Greenland. Electron probe microanalyses (EPMA) were undertaken with a CAMECA SX100 housed in the Department of Geosciences, University of Oslo. The instrument is equipped with five spectrometers and was operated in wavelength dispersive mode. The instrument conditions were an acceleration voltage of 15 kV, beam current of 15 nA and a beam size of 5 μm. The following natural and synthetic standards were used: wollastonite (Si and Ca), albite (Na), pyrophanite (Mn), MgO (Mg), BaSO 4 (Ba), Sr-glass (Sr) and pure metal (Fe). The intensity data were corrected for inter-element overlaps and for matrix effects using the PAP procedure (Pouchou and Pichoir, 1984). Because of elemental overlaps that could not be successfully resolved, an Aurora Elite M90 ICPMS equipped with a Cetax LSX-213 G2+ laser (LA-ICPMS) housed at Department of Geosciences, University of Oslo was used for the following elements: Si, Ca, Zn, Rb, Y, Zr, Nb, Cs, REE 3+ , Hf, Ta, Th and U. Instrument drift was monitored using NIST610 and BCR2G while Si from EPMA was used as internal standard. Raw data were reduced using the Glitter program (Griffin et al., 2008), using a linear fit to standards within a session. The formula calculated based on 17 anions is presented in Table 3. For the single-crystal investigation, intensity data were collected at room temperature with monochromated MoKα radiation (50 kV and 1 mA) on a Rigaku Synergy-S diffractometer equipped with a HyPix-6000He detector housed at NHM in Oslo. The instrument has Kappa geometry and both data collection and subsequent data reduction, and face based absorption corrections were carried out using the Rigaku CrysAlis Pro software. The structure was solved by direct methods and refined by SHELXL (Sheldrick, 2008) using neutral atom scattering factors and the WinGX interface (Farrugia, 2012). Table 4 contains details about data collection and refinement of nordite-(Ce). The atomic coordinates, anisotropic atomic displacement parameters and the bond distances are given in Tables 5-6. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Results
Nordite-(Ce) from Illutalik occurs as single euhedral, colourlessto-white crystals up to 150 μm in size or as aggregates up to a 300 μm in size. The mineral has been found in the part of the locality that also contains narsarsukite, but not in direct contact with emeleusite. The matrix is a fine-grained aegirine and albite dyke of trachytic composition (Upton et al., 1976(Upton et al., , 1978 and accessory minerals include pectolite, a britholite-group mineral, calcite and zircon. The empirical formula for nordite-(Ce) is Na 3.06 (Sr 0.75 Ca 0.20 Ba 0.05 ) Σ1.00 (Ce 0.50 La 0.41 Nd 0.06 Pr 0.03 ) Σ1.00 (Zn 0.94 Fe 0.02 Mg 0.02 Mn 0.01 ) Σ0.99 Si 5.97 O 17 , which is similar to that reported by Upton et al. (1976). However, the new data does provide a better stoichiometry than that previously reported despite Na being slightly high and Si slightly low (Table 3). The new chemical data confirms that the 8.99 wt.% BaO reported by Upton et al. (1976) was a typo and that the correct value was 0.99 wt.% BaO. The values for Zn, La, Ce, Nd and Pr are taken from LA-ICPMS analyses. The other elements analysed by LA-ICPMS are only present in trace amounts.
The refinement of the site scattering factors shows that the A and B sites are fully occupied by Na, and the Y site by REE. The X site has a low scattering factor (35.8 e -) in comparison to a full occupancy by Sr, which is explained by the incorporation of 0.2 Ca atoms per formula unit (apfu). The incorporation of significant amount of Ca on the X site is also reported in nordite-(La) (Bakakin et al., 1970). The Z site shows only a minor deviation (28.9 e -) from the ideal value of 30, which is attributed to the incorporation of small amounts of Fe, Mg and Mn. The detailed cationic distribution on the different crystallographic sites is provided in Table 7 and is in good agreement with the chemical data and the different structural parameters.  Full-matrix least squares on F 2 R, wR (I > 2σ(I )) 1.29, 1.71 R 2 , wR 2 (all reflection) 3.44, 3.54 R int (%) 2.67 No. of refinement parameters (N par ) 138 Weighting scheme 1/(σ 2 (F o ) 2 +0.0184(P) 2 + 0.6112(P)) Δρ max , Δρ min (e -Å -3 ) -0.49/0.46 GoF (obs/all) 1.07/1.07 The bond-valence sums (BVS) calculated according to the cationic distribution are presented in Table 8.

Nomenclature scheme
Following the recommendations of Mills et al. (2009) on the standardisation of mineral group hierarchies we propose the establishment of the nordite supergroup, subdivided into the nordite group containing the following five mineral species: nordite-(La), nordite-(Ce), ferronordite-(La), ferronordite-(Ce) and manganonordite-(Ce) ( Table 1). Due to structural and compositional differences meieranite is considered as an unassigned member of the nordite supergroup and it will be the first mineral of the potential meieranite group if a related species is reported. The supergroup is named according to the first described species, nordite-(La) (Gerasimovsky, 1941). Members of the nordite supergroup are classified according to the following rules. The rootname is dependent on the dominant cation located on the X site (Table 2). In the nordite group, the nordite rootname is applied to species with X = Sr, and new rootnames will be applied to species with X ≠ Sr. In the potential   Note: bond-valence parameters are recalculated according to the site occupancies (see Table 5) and taken from Brown and Altermatt (1985) for the all the cations apart Si, for which the parameters from Gagné and Hawthorne (2015) have been used. VS: valence sums calculated from the site popluation.
meieranite group, the meieranite rootname is applied to species with X = Sr and new rootnames are required for species with X ≠ Sr. In the case of the species belonging to the nordite group the Levinson suffix (Bayliss and Levinson, 1988) is added to the names to indicate the dominant REE on the crystallographic Y site.
In both nordite and meieranite groups a prefix is added according to the dominant chemical composition of the tetrahedral Z site; magnesio (Mg), mangano (Mn 2+ ), ferro (Fe 2+ ) and zinco (Zn). The first described nordite-group mineral, nordite-(La), has Z = Zn, and consequently the prefix zinco is not used in approved or new species belonging to the nordite group. The prefix rule is the same in the potential meieranite group when Z = Mg. The first described species (meieranite) has Z = Mg, therefore the prefix magnesio is not added.
The published data on nordite-group minerals indicate that only limited cationic substitutions occur on the A and B sites. Although, in the case of end-members with A ≠ Na or B ≠ Na a new rootname must be used and the rules given above have to be applied. Meieranite has so far been reported from only one locality, and therefore it is challenging to predict all potential new members. The nomenclature scheme proposed herein provides to the mineralogical community a tool for the classification of nordite-supergroup minerals according to their crystal-chemical properties.