A crystal structure refinement of uralolite, Ca2Be4(PO4)3(OH)3∙5H2O, from Weinebene, Austria

The crystal structure of uralolite, Ca2Be4(PO4)3(OH)3·5H2O, from the spodumene deposit of Weinebene, Carinthia, Austria, has been refined with X-ray single crystal data gathered on a CCD diffractometer. Uralolite is monoclinic, space group P21/n, a = 6.553(1), b = 16.005(3), c = 15.979(3) Å, β = 101.63(1)°, V = 1641.5(5) Å3. While previously only isotropic displacement parameters and no hydrogen atom positions were reported for uralolite, now anisotropic displacement parameters were used for non-hydrogen atoms and hydrogen atoms were located and refined yielding R1 = 0.038 for 3909 observed reflections. Uralolite is built up from corrugated layers [Be4(PO4)3(OH)3]4- parallel to (010), which contain Z-shaped groups of four BeO4 tetrahedra sharing corners via three OH groups and are further crosslinked by PO4 tetrahedra. Two Ca atoms in Ca(Ophosphate)5(H2O)2 coordination and an interstitial water molecule link these layers along [010]. The OH groups and the Ca-bonded H2O molecules are all involved in hydrogen bonds with O···O distances of 2.780(2) − 3.063(2) Å and O-H···O angles of 150(1) − 179(1)° excluding a bifurcated bond. The interstitial water molecule displays a distorted tetrahedral environment of O atoms and accepts and donates each two hydrogen bonds. The crystal structure exhibits a C2/c pseudosymmetry for the [Be4(PO4)3(OH)3]4- layers and the Ca atoms. However, the disposition of the water molecules and an asymmetric hydrogen bond pattern involving OH groups as well as H2O molecules are decisive for the lowering of the symmetry of the structure to the true space group P21/n.


Introduction
Uralolite is a rare hydrated calcium beryllium phosphate occurring in fibrous aggregates of minute acicular to lathlike crystals in fractures of the Weinebene spodumene pegmatites, Carinthia, Austria. A description of this deposit based on extensive pre-mining exploration was reported by Göd (1989). A rich variety of about 30 primary and more than 30 secondary minerals and their paragenetic relationships have been reported from this deposit (Niedermayr and Göd 1992;Taucher et al. 1992Taucher et al. , 1994. The presently known Be-bearing minerals of this deposit comprise primary beryl, Be 3 Al 2 Si 6 O 12 , and secondary/tertiary uralolite, Ca 2 Be 4 (PO 4 ) 3 (OH) 3 ·5H 2 O, weinebeneite, CaBe 3 (PO 4 ) 2 (OH) 2 ·4H 2 O (type locality; Walter 1992), roscherite Ca 2 Mn 2 + 5 Be 4 (PO 4 ) 6 (OH) 4 ·6H 2 O, hydoxylherderite, CaBe(PO 4 )(OH), and bavenite, Ca 4 Be 2 Al 2 Si 9 O 26 (OH) 2 (Taucher et al. 1992;Niedermayr and Göd 1992). The deposit is currently of significant economic interest for the mining of spodumene ore and production of lithium ion battery grade Li 2 CO 3 and LiOH·2H 2 O (European Lithium Ltd 2020). Only nine further occurrences of uralolite are known at present (Mindat.org 2022) and all of them with very small amounts in pegmatitic settings (Grigoriev 1964;Dunn and Gaines 1978). The properties and crystal structure of uralolite from Weinebene were previously reported by Mereiter et al. (1994). This work was based on single crystal diffraction data recorded with a Philips PW1100 four-circle diffractometer and a scintillation probe as point detector. With space group P2 1 /n, a = 6.550(1), b = 16.005(3), c = 15.969(3) 1 3 Å, β = 101.64(2)°, Z = 4 Ca 2 Be 4 (PO 4 ) 3 (OH) 3 ·5H 2 O, it yielded R 1 = 0.058 for 1651 observed out of 2878 independent reflections (θ max = 25°, MoKα radiation, 117 refined parameters). Due to a pronounced pseudosymmetry of the crystal structure with space group C2/c for a zellengleiche pseudostructure, hkl reflections with h + k = 2n + 1 were systematically weak. In order to overcome parameter correlation effects only isotropic displacement parameters were applied in this work and hydrogen atoms could not be located. With this situation in mind, it was considered worthwhile to refine the crystal structure of uralolite with CCD detector diffraction data using a larger data set with more h + k = 2n + 1 reflections in order to apply anisotropic displacement parameters for non-hydrogen atoms and to locate the hydrogen atoms.

Experimental
Using a crystal preserved from the previous study (Mereiter et al. 1994), X-ray diffraction data were collected with a Bruker Smart CCD diffractometer and graphite monochromatized MoKα radiation, λ = 0.71073 Å, from a sealed tube. A crystal measuring 0.28 × 0.08 × 0.02 mm was applied to collect six ω-scan frame sets covering a full sphere of the reciprocal space up to θ max = 30.1°. The raw data were integrated and processed with the program SAINT (Bruker 1999) and were then corrected for absorption with the multi-scan method using program SADABS (Bruker 1999). The resulting 38377 data were then merged to 4823 unique reflections (R merge = 0.050). Structure refinement was carried out with program SHELXL (Sheldrick 2015). Anisotropic temperature factors for non-hydrogen atoms were applied before a difference Fourier synthesis revealed clearly 12 of the 13 expected hydrogen atoms. The missing hydrogen atom, belonging to the interstitial water molecule of O5w, was obscured by relatively large anisotropic displacement effects of this oxygen atom, but could be reasonably inferred by geometric considerations (distorted tetrahedral environment of O5w by two H-bond donors and two adequate H-bond acceptors). All hydrogen atoms were then included in the refinement, at first unrestrained (O-H = 0.78(4) to 1.00(6) Å, mean value 0.86 Å; U iso = 0.03-0.15 Å 2 ), and then with geometric restraints: The O-H distances were fixed at 0.85 Å and H-O-H angles at 108° using hard DFIX restraints. The isotropic displacement parameters of the H atoms were divided into three groups -hydroxyl groups, water molecules of O1w to O4w, and water molecule of O5w -and each group was refined with a common U iso . These procedures were considered as useful in obtaining the most reliable and consistent information on hydrogen bonding. A check of the occupancy of the interstitial water molecule of O5w gave a population factor of 0.99(1) indicating that there is no deficiency. The final refinement converged at R 1 = 0.0380 for 3909 F o > 4σ(F o ). Crystallographic data are summarized in Table 1, atomic parameters are given in Tables 2 and  3. Selected bond lengths and bond angles are presented in Table 4, and hydrogen bond data are reported in Table 5. A crystallographic information file (CIF) with structure factors is available as electronic supplementary material. Structural graphics was generated with programs MERCURY (Macrae et al. 2006) and DIAMOND (Brandenburg 2012).

Discussion
The present study confirms the previous structure determination of Mereiter et al. (1994) but provides now very satisfactory anisotropic displacement parameters for non-hydrogen atoms, hydrogen atom positions, and an improvement in the precision of atomic positions and bond lengths. Although the e.s.d.s for bond lengths to Ca, Be, and P are reduced to about one third of their former values (Mereiter et al. 1994), the changes in geometric parameters  (Table 4). An important result is the clarification of the hydrogen bonds in uralolite, which play a distinctive role in the structure and its symmetry.

[Be 4 (PO 4 ) 3 (OH) 3 ] 4layers
The structure of uralolite is based on corrugated layers of composition [Be 4 (PO 4 ) 3 (OH) 3 ] 4which extend parallel to (010) at y = ¼ and ¾ of the monoclinic unit cell, and which are mutually linked via two Ca(H 2 O) 2 2+ fragments and an additional interstitial water molecule (Fig. 1). As shown in Fig. 2 the [Be 4 (PO 4 ) 3 (OH) 3 ] 4layer contains a unique Z-shaped group of four BeO 4 tetrahedra crosslinked by PO 4 groups. The four independent Be atoms are linked via three OH groups. Be2 and Be3 are each bonded to two OH groups and two PO 4 oxygen atoms. Be1 and Be4 in turn are each bonded to one OH group and three PO 4 oxygen atoms. Of the three independent PO 4 groups, those of P1 and P2 link three Be and have each one oxygen atom (O4, O8) in terminal position while those of P3 link four Be. Thus, in terms of the links between tetrahedra, all four BeO 4 and one PO 4 tetrahedra are four-connected and two PO 4 are threeconnected. The [Be 4 (PO 4 ) 3 (OH) 3 ] 4layer ( Fig. 1a) contains following tetrahedral rings: Three-membered BeBeP, fourmembered BeBeBeP, four-membered (BeP) 2 , six-membered (BeBeP) 2 and eight-membered (BeP) 4 rings. As pointed out previously (Mereiter et al. 1994), and remaining unchanged ever since, the four-membered rings BeBeBeP in uralolite are unique among beryllophosphates, arsenates and silicates as well as the tetrahedral architecture of the layer among silicates (Liebau 1985;Hawthorne and Huminicki 2002). For a comparison of sheet topologies in beryllium minerals including uralolite, see Huminicki and Hawthorne (2002). Bond lengths and bond angles in the BeO 4 and PO 4 tetrahedra show usual variations with Be-O ranging from 1.600(3) to 1.663(3) Å, mean value 1.632 Å, and P-O ranging from 1.5071(17) to 1.5548(16) Å, mean value 1.535 Å. In their review on the crystal chemistry of beryllium, Hawthorne and Huminicki (2002) give Be-O = 1.633 Å as grand mean tetrahedral Be-O bond distance. Bond angles O-Be-O are in the range 100.53(17)-116.62(19)°, and O-P-O angles 105.13(9)-112.50(10)°, where in both cases the lowest values represent shared edges between the Ca polyhedra and BeO 4 as well as PO 4 tetrahedra (Fig. 3). The Be-O-Be bond angles of O1h and O2h (115.60(17) and 117.20(17)°) are both involved in tetrahedral BeBeP rings. They are notably smaller than that of O3h (126.90(16)°), which bridges the two inner BeO 4 tetrahedra and is part of two BeBeBeP rings (Fig. 2).

Ca coordination
The coordination of the two independent Ca atoms and how they connect the [Be 4 (PO 4 ) 3 (OH) 3 ] 4layers in detail is shown in Fig. 3. Both Ca atoms are seven coordinated and form CaO 5 (H 2 O) 2 polyhedra, where the five phosphate oxygen atoms adopt a roughly equatorial arrangement about Ca and the two water molecules are in approximately axial  Fig. 3; atoms with primed labels are used to distinguish them from their inversion related equivalents). Despite the apparent similarity of the environments of Ca1 and Ca2, their water molecules show significant differences in their positions relative to Ca, in H 2 O orientations, and in hydrogen bonding. At comparable distances between Ca1-Ca1' = 3.954(1) Å and Ca2-Ca2' = 3.802(1) Å, the separation between O1w and O2w' is 3.668(2) Å, a value much larger than between O3w and O4w', 2.831(2) Å, which represents a hydrogen bond (Fig. 3) addressed below.

Pseudosymmetry
The pseudosymmetry of the crystal structure of uralolite has been pointed out previously (Mereiter et al. 1994). In Fig. 1a it can be noted that in addition to the symmetry elements , and much more so with regard of the positions of their hydrogen atoms and hydrogen bonds. Qualitatively, the same is valid for the pair O1w and O3w (corresponding height difference 0.65 Å). These features lead to the differences in Ca coordination outlined above and in Fig. 3. A simple proof for the pseudosymmetry is that the crystal structure can be refined in space group C2/c using only reflections with h + k = 2n, and with one Ca (Ca1), two Be (Be1, Be2), one Oh (O1h), one PO 4 tetrahedron (P1, O1 through O4) and O9, O10 of P3 in general positions (Z = 8 in C2/c, Z = 4 in P2 1 /n), whereas P3 and O3h adopt special positions on twofold axes. The water molecules O1w through O4w show up in half occupied split position pairs, and O5w near a twofold axis and also in a half-occupied general position. Using anisotropic displacement parameters for all atoms except water oxygen atoms and neglecting H atoms the structure can be refined in space group C2/c to R 1 = 0.0403 for 2128 F o > 4σ(F o ) (h + k = 2n only). The outlined features show that the entire system of hydrogen bonds starting with the asymmetry of the three Be-bonded OH groups and ending with all water molecules are decisive for the true space group symmetry P2 1 /n of uralolite and its deviation from C2/c pseudosymmetry.

Structural relationships
Overviews on beryllium in earth sciences including beryllium minerals and their crystal chemistry have been given in the review volume 50 of the Mineralogical Society of America   (Lindbloom et al. 1974). Ca is 7-coordinated with mean Ca-O = 2.469 Å disregarding a further oxygen at 3.09 Å. The CaO 7 polyhedron shares edges with a BeO 4 and a PO 4 tetrahedron (not adjacent edges). The O-T-O angles (T = Be, P) corresponding to the shared edges are the smallest in this structure (102.8 and 103.9°), just like in uralolite.
Hydroxylherderite, CaBe(PO 4 )(OH), has a layer structure built up from alternating 3-connected BeO 3 (OH) and PO 4 tetrahedra forming (BeP) 2 and (BeP) 4 tetrahedral rings where the Be-bonded OH group and a phosphate oxygen atom are in terminal positions. All hydroxylherderites investigated so far have a significant OH by F substitution leading to ideal herderite CaBe(PO 4 )F (Harlow and Hawthorne 2008). Ca is 8-coordinated in the shape of a square antiprism with a mean bond length of Ca-O ≈ 2.50 Å, where four of the ligands are two Be-bonded OH-groups and two terminal PO 4 oxygen atoms. The polyhedron shares edges with two BeO 4 tetrahedra, but not with PO 4 , and the corresponding O-Be-O bond angles are small (103-106°). Each CaO 8 polyhedron shares edges with three adjacent Ca polyhedra.
Finally, fransoletite and its dimorph parafransoletite, Ca 3 Be 2 (PO 4 ) 2 (PO 3 OH) 2 ·4H 2 O (Kampf 1992), stand out by bearing OH groups not on BeO 4 but on PO 4 tetrahedra. Here 4-connected BeO 4 and 3-connected PO 4 tetrahedra (half of them bearing OH) form infinite chains of (BeP) 2 rings crosslinked by CaO 4 (H 2 O) 2 and CaO 5 (H 2 O) 2 polyhedra. Fig. 3 Comparison of two independent Ca atoms in uralolite and their coordination. Both assemblies are centrosymmetric; a few atoms were labeled with primes to distinguish them from their equivalents. Note the difference in H 2 O orientations and internal hydrogen bond patterns of the two assemblies