Introduction
Allanite, formula A1CaA2REEM3FeM1AlM2Al(Si2O7)(SiO4)O4OO10(OH), a major carrier of rare earth elements (REE), is a group member of the epidote supergroup. The crystal structure consists of two independent edge-sharing octahedral chains along the b-axis, a single M2 chain and a ‘zig-zag’ chain of M1 with M3 octahedra attached on alternate sites, and Si2O7 dimers and SiO4 islands, with cavities occupied by A1 and A2 cations (Bonazzi and Menchetti, Reference Bonazzi and Menchetti1995). Whereas the A1 site prefers Ca2+, A2 always hosts REE cations (Dollase, Reference Dollase1971; Ercit, Reference Ercit2002). The octahedral positions are distinguished on the basis of distinctive preferential occupancy of Al3+, Fe3+, Fe2+, Mg2+ and Mn2+ (Armbruster et al., Reference Armbruster, Bonazzi, Akasaka, Bermanec, Chopin, Gieré, Heuss-Assbichler, Liebscher, Menchetti, Pan and Pasero2006). The species in the group are classified by dominant cations at the A2, M1, M3 and O4 key sites. The M2 site in allanite is occupied mostly by Al3+, and iron can be incorporated as Fe3+or Fe2+on the M1 site, while the M3 site is occupied preferably by Fe2+, Mg2+, or Mn2+ (Gieré and Sorensen, Reference Gieré, Sorensen, Liebscher and Franz2004; Armbruster et al., Reference Armbruster, Bonazzi, Akasaka, Bermanec, Chopin, Gieré, Heuss-Assbichler, Liebscher, Menchetti, Pan and Pasero2006).
According to the charge-balance coupled substitution: REE3+(A2) + M2+(M3) = Ca2+(A2) + M3+(M3) (Peterson and Macfarlane, Reference Peterson and Macfarlane1993), REE substitution happens at the same time as Fe2+ incorporation. Crystal structures also show that the M3 octahedron shares edges with the REE-bearing A2 polyhedron. Because the deformation of the M3 octahedron affects the REE-rich A2 polyhedron, the presence of divalent cations in M3 sites is related to REE content, as well as iron-oxidation and dehydration processes (Reissner et al., Reference Reissner, Bismayer, Kern, Reissner, Park, Zhang, Ewing, Shelyug, Navrotsky, Paulmann, Škoda, Groat, Pöllmann and Beirau2019, Reference Reissner, Reissner, Kern, Pöllmann and Beirau2020).
During structure determination and refinement of a series of allanite-(Ce) samples, we found that the M3 site is split, a phenomenon not reported before for the epidote supergroup. Here, we present the crystal structure of allanite with a site split model at M3 and compare it with ferriallanite-(Ce) and epidote, which do not show site splitting. We summarise the differences of their refined structures and discuss the split mechanism and relationship with divalent–trivalent substitution. The crystallographic information files (cif) have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).
Sample description
The allanite-(Ce) and epidote samples (Fig. 1) were from the central zone of the Xinfeng granite, which is part of the Fogang granitic batholith in Guangdong Province, China. Allanite-(Ce) forms dark brown to black tabular and columnar crystals, up to 2 mm in size, and is associated with epidote, titanite, ilmenite, hastingsite, julgoldite-(Fe3+), biotite, feldspar and quartz. In thin sections, the crystals of allanite-(Ce) are pleochroic with yellow to dark brown colour, and some of them are subject to various degree of metamictisation. Epidote occurs as dark green tabular and columnar crystals, up to 3 mm in size. The sample of ferriallanite-(Ce) was from the supergiant carbonatite-hosted Huayangchuan ore deposit in the Qinling Orogen, Central China (Zheng et al., Reference Zheng, Chen, Wu, Jiang, Gao, Kang, Yang, Wang, Lai and Kit.2020).
Fig. 1. Single crystals of samples used for structure analysis. Allanite-(Ce) from (a) Xinfeng and (b) Gucheng, Guangdong province, China. (c) Ferriallanite-(Ce) from the Huayangchuan ore deposit in the Qinling Orogen, Central China. (d) Epidote from the Xinfeng granite, Guangdong province, China.
Experimental methods
Chemical compositions were analysed with a Shimadzu EPMA-1720 electron probe microanayser at an accelerating voltage of 15 kV, a beam current of 20 nA and beam size of ~2 μm. Pure materials of SiO2, Al2O3, Fe2O3, TiO2, MgO, MnO2, CaSiO3, and a set of rare earth element phosphates, including LaP5O14, CeP5O14, PrP5O14, NdP5O14 and SmP5O14, were used for standards. The ZAF3 program provided with the instrument was used for concentration corrections.
Single-crystal diffraction data were collected on a Rigaku XtaLAB Synergy-DW diffractometer with a microfocus sealed Mo anode tube at 50 kV and 1 mA. Depending on the crystal size, the exposure time per frame was set to 2s, 3s or 5s. The experimental data were analysed with Rigaku CrysAlisPro, and all reflections were indexed on the basis of a monoclinic unit cell. The systematic absence of reflections suggests the space group P21/m. The crystal structure was solved with SHELXT and refined with SHELXL (Sheldrick et al., Reference Sheldrick, Dauter, Wilson, Hope and Sieker1993; Bourhis et al., Reference Bourhis, Dolomanov, Gildea, Howard and Puschmann2015; Sheldrick, Reference Sheldrick2015), which are included in the software Olex2 (Dolomanov et al., Reference Dolomanov, Bourhis, Gildea, Howard and Puschmann2009). The occupancies for O and Si were fixed at 1 and the occupancies for other cations were refined according to both minimum R 1 value and good agreement with empirical compositions. Anisotropic displacement parameters were refined for all atoms. Data and structure refinement for the crystals are summarised in Table 1.
Table 1. Crystal data and final structure refinement for Xinfeng allanite-(Ce), ferriallanite-(Ce) and epidote.
1 DFIX H–O = 0.96 Å
Results
Chemical compositions
The empirical formula of allanite-(Ce), from the average of 8 analyses, is calculated to be (Ca1.0REE0.9□0.1)Σ2.0(Al1.46Fe3+0.52Fe2+0.76Mg0.12Ti0.15)Σ3.01Si3O12(OH), based on 3(Si) atoms and arbitrary 12(O) and 1(OH) per formula unit (apfu). The REE in allanite-(Ce) are dominated by Ce, La, Nd and Pr, respectively with La = 0.25 apfu, Ce = 0.46 apfu, Pr = 0.06 apfu and Nd = 0.13 apfu. The calculation of charge balance reveals that Fe2+ = 0.76 apfu and Fe3+ = 0.52 apfu. The empirical formula of epidote is Ca2.0(Al2.05Fe0.94)Σ2.99Si3.0O12(OH), from the average of 10 analyses, in which Fe is classified as ferric according to charge balance (Table 2). Ferriallanite-(Ce) from the Huayangchuan deposit (Zheng et al., Reference Zheng, Chen, Wu, Jiang, Gao, Kang, Yang, Wang, Lai and Kit.2020a) has the empirical formula (Ca0.92REE0.91□0.17)Σ2.0(Al0.8Fe3+1.29Fe2+0.51Ti0.15Mg0.18Mn0.09)Σ3.02Si3.0O12[(F0.08,(OH)0.92].
Table 2. Chemical composition of allanite-(Ce), ferriallanite-(Ce) and epidote.
*Allanite-(Ce) data is for Xinfeng samples.
**Total Fe calculated as FeO and Mn as MnO; the Fe2+/Fe3+ ratio calculated based on charge balance. ‘–’ = not detected.
Crystal structures
The structures of allanite-(Ce), ferriallanite-(Ce) and epidote are all isostructural with that of the epidote supergroup (Dollase, Reference Dollase1971). Elements with <0.01 apfu were ignored in structure refinement. The fractional atomic coordinates and the occupancies of atoms at the M1, M2, M3, A1 and A2 sites of allanite-(Ce) are shown in Table 3 for comparison with ferriallanite-(Ce) and epidote. The displacement parameters are shown in Table 4. Table 3 shows that the M2 site is dominated by Al with negligible Fe3+ in all the three minerals, and M1 is dominated by Al with incorporation of minor Fe3+ in allanite-(Ce) and epidote, but Fe3+ dominates M1 in ferriallanite-(Ce). The M3 site is dominated by iron in all three minerals. It is remarkable that the M3 site is split in allanite-(Ce) but not in ferriallanite-(Ce) and epidote. To confirm this, two more crystals of allanite-(Ce) from Xinfeng and Gucheng, in the Guangdong province, China were also checked by single-crystal diffraction, and all have split M3 sites at a distance of 0.38(3) Å, which are hereafter designated as M3a and M3b. In allanite-(Ce) from Xinfeng, M3a is partially occupied by Fe0.76Mg0.12 and M3b and partially occupied by Ti0.15. In allanite-(Ce) from Gucheng, M3a is partially occupied by Fe0.74Mn0.04 and M3b is partially occupied by Ti0.11 (Table 3). The site splitting of M3 in allanite-(Ce) is required for the structure to be acceptable as the maximum peak residual is 3.60 e –3. In addition R 1 is larger (3.10%) before the site splitting (Table 6).
Table 3. Fractional atomic coordinates of Xinfeng allanite-(Ce) and atom occupancies compared to ferriallanite-(Ce) and epidote.
1Allanite-(Ce) and epidote from Xinfeng; 2 allanite-(Ce) from Gucheng; 3 ferriallanite-(Ce) from Huayangchuan.
Table 4. Anisotropic and equivalent isotropic displacement parameters for Xinfeng allanite-(Ce) (in Å2).
The 7-coordinated A1 is almost fully occupied by Ca in the three minerals, whereas the 11-coordinated A2 is dominated by rare earth elements in allanite-(Ce) and ferriallanite-(Ce) but is dominated by Ca in epidote. The occupancies of Ce, La, Nd and Pr for the A2 site were fixed based on chemical compositions. The tetrahedral sites Si1, Si2 and Si3 are fully occupied by Si (Fig. 2a, 2b). In structure refinement, the bond lengths (r) connected to O10 are 1.91 Å and 2.60 Å for <O10–M2>×2 and <O10–A2>, respectively. The bond valance, s, estimated by using the bond-valance equation s = exp[(r 0–r)]/B (Brown and Altermatt, Reference Brown and Altermatt1985; Brese and O'Keeffe, Reference Brese and O'Keeffe1991), sums to –1.22 for O10. This suggests that the hydrogen position is located on it, which is consistent with most reported allanite structures.
Fig. 2. (a) Molecular structure of allanite-(Ce) showing the split sites; (b) refined polyhedral style structure of allanite-(Ce) projected down the b axis.
Bond distances, polyhedral volumes, site distortion index, and bond angle variance of allanite-(Ce), ferriallanite-(Ce) and epidote are summarised in Table 5 (calculated according to quadratic elongation (Robinson et al., Reference Robinson, Gibbs and Ribbe1971) using VESTA (Momma and Izumi, Reference Momma and Izumi2011)). These results show that M1 is linked to O1, O4 and O5 to form a nearly normal octahedron in epidote, but the substitution by Fe3+ in allanite-(Ce) and ferriallanite-(Ce) leads to slight expansion and distortion of the volume of M1. The M2 cation is linked to O3, O6 and the hydroxyl O10 with little change in the M2–O bond length, leading to the smallest and most regular octahedron. M3aFe/Mg and M3bTi cations, which are split from M3, are connected to six oxygens to form the most distorted octahedron M3b (σ2allanite-M3b > σ2epidote-M3 > σ2ferriallanite-M3 > σ2allanite-M3a). The incorporation of a higher amount of Fe2+ causes the greater volume (V ferriallanite-M3 > V allanite-M3 > V epidote-M3) and higher average bond length.
Table 5. Selected bond distance (Å), polyhedral volumes (V = Å3), distortion index (σ2) and bond angle variance* (ω = degree2) for A, M, Si sites for Xinfeng allanite-(Ce).
*Bond distances, polyhedral volumes, distortion index and bond angle variance are calculated according to Robinson et al. (Reference Robinson, Gibbs and Ribbe1971) using VESTA (Momma and Izumi, Reference Momma and Izumi2011).
The mean distance of A1–O is similar among these three samples, while further variation occurs with the incorporation of REE cations in the A2 polyhedron with face-shared geometry (O7–O3–O3). O7 and O3 move away from the A1 site as the A1–O1 bond shortens. The longer bonds of A2 cause the polyhedron volume to be twice that of A1. The A2–O2 bonds shorten to balance the lowered bond strength of A2–O3, A2–O7 and A2–O10, which accompanies the unequal substitution of REE cations by Ca2+ (Table 5).
Tetrahedra are the most stable unit in the allanite structure. The distance between Si and O in every tetrahedron can be considered as a constant (Table 5), and there is no substitution of Si by other cations. The Si3 island near the b axis shares O2–O2’ edges with the A2 octahedron, and thus compresses the O2–Si3–O2’ angle to 103.78(17)° and causes a larger variation of bond angles in allanite-(Ce) (ωSi 3 = 27.1 > ωSi 1 = 9.9 > ωSi 2 = 2.5). The Si1 and Si2 islands are linked to form an Si2O7 dimer in which O9 connects Si1 and Si2 with an Si1–O9–Si2 angle of 143.7(2)°. With the incorporation of REE, the Si1–O9–Si2 angle reduces from 153.39(14)° [epidote] to 143.6(2)° [ferriallanite-(Ce)) and 143.7(2)° (allanite-(Ce)].
Discussion
Refined structure data for the allanite subgroup are listed in Table 6. Much previous work has ignored the residual of the Q peaks, but here we use it to emphasise the site splitting model of Fe2+/Mg2+ or Fe2+/Mn2+ – Ti4+ in allanite when refining the structure. A better R factor and more reasonable Q peak residuals were obtained with the site split model of allanite, whereas the one atom model was feasible for ferriallanite-(Ce) and epidote. This shows that Fe2+ substituted for Fe3+ or Fe3+ substituted for Al3+ in M3 cannot cause the site split. Thus, we conclude that heterovalent substitution leads to the crystal site splitting phenomenon. Similar examples were reported in intermediate plagioclases where the Na+/Ca2+ substitution series occurs as continuous solid solutions (Yamamoto et al., Reference Yamamoto, Nakazawa, Kitamura and Morimoto1984; Steurer and Jagodzinski, Reference Steurer and Jagodzinski1988).
Table 6. Cation site occupancies and refined parameters of Xinfeng allanite-(Ce) with site split model compared with epidote, ferriallanite and other ‘allanites’.
*This work; aDollase (Reference Dollase1971); bOrlandi and Pasero (Reference Orlandi and Pasero2006); cHoshino et al. (Reference Hoshino, Kimata, Nishida, Kyono, Shimizu and Takizawa2005); dKartashov (Reference Kartashov2002); eNagashima et al. (Reference Nagashima, Imaoka and Nakashima2011).
The size of the M3 octahedron follows the order ferriallanite-(Ce) > allanite-(Ce) > epidote, suggesting that Fe2+ content dominates the expansion of M3. The volume of allanite octahedra decreases in the sequence M3bO6 = M3aO6 > M1O6 > M2O6, which implies that site splitting also contributes to the expansion of M3. Moreover, the large deformation index of M3 in these three minerals, especially in site M3b, implies that the distortion of the crystal cell is not only caused by different ionic radii but also by the possible crystal site splitting.
Allanite is a common accessory mineral in granite (Hanson et al., Reference Hanson, Falster, Simmons and Brown2012) and volcanic (Chesner and Ettlinger, Reference Chesner and Ettlinger1989; Hoshino et al., Reference Hoshino, Kimata, Chesner, Nishida, Shimizu and Akasaka2010) and metamorphic rocks (Wing et al., Reference Wing, Ferry and Harrison2003). As a major carrier of rare earth elements (REE), the coupled substitution: Ca2+(A2) + Fe3+/Al3+(M3) = REE3+(A2) + Fe2+/Mg2+/Mn2+(M3) between M3 and A2 sites maintains charge balance in allanite-(Ce). This substitution mechanism is responsible for REE incorporation in the allanites studied.
Conclusions
Based on crystal chemistry and structure refinement of allanite-(Ce), ferriallanite-(Ce) and epidote under the constraint of their compositions, we conclude that the M3 site splitting model helps to obtain a better structure that accommodates heterovalent substitution in allanite subgroup minerals. A higher distortion index implies that both different ionic radii and the site split contribute to crystal lattice distortion.
Acknowledgements
This research was funded by the National Natural Science Foundation of China (grant No.42072054) and Central South University (grant No. 2018zzts034).
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1180/mgm.2021.106
