Lanthanite-(Nd), Nd2(CO3)3·8H2O

Lanthanite-(Nd), ideally Nd2(CO3)3·8H2O [dineodymium(III) tricarbonate octahydrate], is a member of the lanthanite mineral group characterized by the general formula REE 2(CO3)3·8H2O, where REE is a 10-coordinated rare earth element. Based on single-crystal X-ray diffraction of a natural sample from Mitsukoshi, Hizen-cho, Karatsu City, Saga Prefecture, Japan, this study presents the first structure determination of lanthanite-(Nd). Its structure is very similar to that of other members of the lanthanite group. It is composed of infinite sheets made up of corner- and edge-sharing of two NdO10-polyhedra (both with site symmetry ..2) and two carbonate triangles (site symmetries ..2 and 1) parallel to the ab plane, and stacked perpendicular to c. These layers are linked to one another only through hydrogen bonding involving the water molecules.

Lanthanite-(Nd) from Bethlehem, Pennsylvania, US, was first described by Blake (1853), although it was not possible to discriminate the Nd-dominance at that time. It was not until Atencio et al. (1989) analyzed a Bethlehem sample that it was found to be Nd-rich. Lanthanite-(Nd) has since been reported from other localities, including Curitiba, Brazil (Coutinho, 1955;Ansell et al., 1976;Cesbron et al., 1979;Roberts et al., 1980;Fujimori, 1981;Svisero & Mascarenhas, 1981); Saga Prefecture, Japan (Nagashima et al., 1986); Santa Isabel, São Paulo, Brazil (Coimbra et al., 1989); and Whitianga, Coromandel Peninsula, New Zealand (Graham et al., 2007). With the exception of those found in Whitianga, all lanthanite-(Nd) samples from these localities, including the one used in this study, exhibit a predominance of Nd with sub-equal La and a notable depletion of Ce. In reference to this phenomenon, Atencio et al. (1989) stated that lanthanite minerals comprise two distinct groups: one in which the proportions of La, Ce and Nd are similar and another in which La and Nd are similar in abundance while Ce is severely depleted or entirely absent. This trend presumably stems from differences in formational conditions, but the exact mechanism(s) remain(s) unclear.
Many of the studies referenced above reported lanthanite-(Nd) unit-cell parameters, but none reported the crystal structure. This study presents the first crystal structure refinement of lanthanite-(Nd). In the course of identifying minerals for the RRUFF project (http://rruff.info), we found an un-twinned lanthanite-(Nd) sample from Mitsukoshi, Hizen-cho, Karatsu City, Saga Prefecture, Japan and performed single-crystal X-ray diffraction.
The general structure feature of lanthanite-(Nd) is that of infinite sheets of corner-and edge-sharing NdO 10 -and carbonate-polyhedra ( Fig. 1) parallel to the ab plane, and stacked perpendicular to c. The layers are linked to one another only by hydrogen bonding between water molecules (Fig. 2). This accounts for the micaceous cleavage of the lanthanite minerals ( sharing edges with the C2 carbonate group. Nd1 is bonded to two water molecules (OW1 and OW2) while Nd2 is bonded to one (OW3) that protrude from the primary sheet in the c-direction. The C1 carbonate group also projects from the sheet along c. According to bond valence calculations (Brese & O′Keeffe, 1991), without accounting for hydrogen bonding, each O-atom of the C1 carbonate group is under-bonded, with bond valence sums of 1.64 and 1.47 bond valence units for O1 and O2, respectively. The two O1 atoms are bonded only to Nd2 and C1, resulting in an underbonding that is satisfied by hydrogen bonding as an acceptor of OW1 and OW2 (Table 1). The apex O-atom, O2, is not bonded to any cation other than C1 and therefore has much larger thermal displacement parameters and a shorter bond length (1.243 (8) Å) than usually found in carbonate groups. The C1-O2 bond is close to satisfying the rigid-body criteria of equal displacement amplitudes of C1 and O2 along the C1-O2 bond direction. The bond length, corrected for rigid-body motion is 1.268 Å (Downs et al., 1992). O2 is also the acceptor of an hydrogen bond from the OW3 atom of the adjacent sheet, thus connecting the two sheets. There are not many minerals with dangling O atoms in CO 3 groups, but these features are also observed in the crystal structures of isotypic lanthanite-(La) (Shinn & Eick, 1968), lanthanite-(Ce) (Dal Negro et al., 1977) and in dawsonite, NaAlCO 3 (OH) 2 (Corazza et al., 1977). The last water molecule, OW4, is not bonded to any cation, but instead is situated between the OW1 of a given polyhedral layer and OW2 of the adjacent layer, linking the two layers together through hydrogen bonds.

Refinement
Due to similar X-ray scattering lengths, all rare earth elements were treated as Nd. The highest residual peak in the difference Fourier maps was located at (1/4, 3/4, 0.3413), 1.03 Å from Nd2, and the deepest hole at (0.2515, 0.8358, 0.2813), 0.81 Å from Nd2. H-atoms from water molecules could not be assigned reliably and were excluded from refinement.

Figure 2
The crystal structure of lanthanite-(Nd) represented with displacement ellipsoids at the 99% probability level. Blue, green, red and cyan represent Nd, C, O atoms and H 2 O molecules, respectively.

Figure 3
Photograph of the lanthanite-(Nd) specimen analyzed in this study, illustrating its platy habit. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.