Kolbeckite, ScPO₄·2H₂O, isomorphous with metavariscite

This study presents the first structural report of kolbeckite, with the ideal formula ScPO₄·2H₂O (scandium phosphate dihydrate), based on single-crystal X-ray diffraction data. Kolbeckite belongs to the metavariscite mineral group, in which each PO₄ tetra­hedron shares four vertices with four ScO₄(H₂O)₂ octa­hedra and vice versa, forming a three-dimensional network of polyhedra.

S1. Comment

A number of phosphates and arsenates belong to the orthorhombic (Pbca) variscite and monoclinic (P2₁/n) metavariscite mineral groups with a general chemical formula AXO₄·2H₂O, where A = Al³⁺, Fe³⁺, Sc³⁺, In³⁺ or Ga³⁺, and X = P⁵⁺ or As⁵⁺. The crystal chemistry of these compounds has been investigated extensively because of the biological and geochemical importance of phosphorus and arsenic, especially their roles in soils, water, and waste management (e.g. Huang & Shenker, 2004; O'Day, 2006). Recent studies have also revealed that variscite- and metavariscite-type materials possess interesting microporous and absorption properties (Tang et al., 2002, and references therein). To date, structural determinations have been conducted for the following variscite- and metavariscite-type materials: Pbca and P2₁/n Al(PO₄)·2H₂O (Kniep & Mootz, 1973; Kniep et al., 1977), Pbca and P2₁/n Fe(PO₄)·2H₂O (Moore, 1966; Song et al., 2002; Taxer & Bartl, 2004), P2₁/n In(PO₄)·2H₂O (Sugiyama et al., 1999; Tang et al., 2002), Pbca Al(AsO₄)·2H₂O (Harrison, 2000), Fe(AsO₄)·2H₂O (Hawthorne, 1976), In(AsO₄)·2H₂O (Tang et al., 2002), and Ga(PO₄)·2H₂O (Loiseau et al., 1998). The structural relationships between the two groups of compounds have been discussed by Moore (1966), Loiseau et al. (1998), and Taxer & Bartl (2004). Kolbeckite, which was also previously called eggonite or sterrettite, is a scandium phosphate mineral with the ideal chemical formula Sc(PO₄)·2H₂O and a monoclinic unit cell comparable with that of metavariscite (see the review by Hey et al., 1982). However, in spite of the long interval of time since its first description (Schrauf, 1879), the crystal structure of kolbeckite remained undetermined. This study presents the first structure refinement of kolbeckite based on single-crystal X-ray diffraction data.

The structure of kolbeckite, which is homologous with metavariscite, consists of two basic polyhedral units, the PO₄ tetrahedra and ScO₄(H₂O)₂ octahedra. Each PO₄ tetrahedron shares four vertices with four ScO₄(H₂O)₂ octahedra and vice versa, forming a three-dimensional network of polyhedra (Fig. 1). The two H₂O molecules (O5 and O6) coordinated to Sc³⁺ are in the cis position, and the Sc—O5 and Sc—O6 distances are noticeably longer than the other four Sc—O bond lengths. The average interatomic P—O (1.527 Å) and Sc—O (2.080 Å) distances in kolbeckite are in agreement with those reported in the literature. Nevertheless, the bond-valence sum calculations (Brown, 1996) indicate that P⁵⁺ is slightly under-bonded (4.926 valence units, v.u.), whereas Sc³⁺ is over-bonded (3.24 v.u.). The U₃₃ parameter of atom O5 in kolbeckite is significantly larger than its U₁₁ or U₂₂. A similar observation has also been reported for other metavariscite-type materials, such as Al(PO₄)·2H₂O (Kniep & Mootz, 1973), In(PO₄)·2H₂O (Sugiyama et al., 1999) and Fe(PO₄)·2H₂O (Song et al., 2002), thus suggesting a possible disordered distribution of this water molecule in the channels along the c axis.

As observed in other metavariscite-type compounds, the Sc—O5 and Sc—O6 bond distances of 2.116 (4) and 2.178 (4) Å, respectively, are significantly different, indicating distinct bonding environments for the two H₂O molecules. In fact, the hydrogen bonds involving atoms H1A and H1B are both shorter than those involving atoms H2A and H2B.

S2. Experimental

The kolbeckite specimen used in this study is from Hot Springs County, Arkansas, USA, and is in the collection of the RRUFF project (deposition No. R060981; https://rruff.info). The average chemical composition (15 point analyses), (Sc_0.94V³⁺_0.03Fe³⁺_0.02Al_0.01)_Σ=1P_1.00O₄·2H₂O, was determined with a CAMECA SX50 electron microprobe (https://rruff.info). All crystals examined are severely twinned, with the twin axis along a. Kolbeckite crystals from another locality, Christy Pit, Magnet Cove, Arkansas, USA (RRUFF deposition No. R061040), display the same feature.

S3. Refinement

The twin refinement with the twin law (1 0 0, 0 − 1 0, 0 0 − 1) reduced the R₁ factor from 0.105 to 0.052. The chemical analysis showed the presence of small amounts of V³⁺, Fe³⁺ and Al³⁺ in the sample, but the final refinement assumed a full occupancy of the octahedral site by Sc³⁺ only, as the overall effects of these trace amounts of elements on the final structure results are negligible. Four H atoms were located in the difference Fourier syntheses and their positions refined freely. The isotropic displacement parameters of the H atoms, however, were refined with the constraint that the U_iso values of atoms H1A and H1B were 1.2 times U_eq of their parent atom O5, and those of atoms H2A and H2B were 1.2 times U_eq of their parent atom O6. The highest residual peak in the difference Fourier map was located at (0.248, 0.662, 0.740), 1.28 Å from atom O3, and the deepest hole at (0.100, 0.831, 0.252), 0.45 Å from Sc.

Computing details

Data collection: APEX2 (Bruker, 2003); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: SHELXTL (Bruker, 1997).

Figures

Fig. 1. The crystal structure of kolbeckite, viewed along the a axis. The tetrahedra and octahedra represent the PO4 and ScO4(H2O)2 groups, respectively. Small spheres indicate the H atoms.

scandium phosphate dihydrate

Crystal data

ScPO4·2H2O	F(000) = 352
Mr = 175.96	Dx = 2.370 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 2244 reflections
a = 5.4258 (4) Å	θ = 6.1–65.6°
b = 10.2027 (8) Å	µ = 1.76 mm^−1
c = 8.9074 (7) Å	T = 293 K
β = 90.502 (5)°	Block, green-yellow
V = 493.08 (7) Å^3	0.07 × 0.06 × 0.06 mm
Z = 4

Data collection

Bruker SMART CCD area-detector diffractometer	1858 independent reflections
Radiation source: fine-focus sealed tube	1600 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.042
ϕ and ω scans	θmax = 33.0°, θmin = 2.0°
Absorption correction: multi-scan (SADABS; Sheldrick, 2005)	h = −8→8
Tmin = 0.887, Tmax = 0.902	k = −15→15
8954 measured reflections	l = −13→13

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.052	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.168	w = 1/[σ^2(Fo^2) + (0.0963P)^2 + 1.2135P] where P = (Fo^2 + 2Fc^2)/3
S = 1.11	(Δ/σ)max = 0.001
1858 reflections	Δρmax = 1.13 e Å^−3
90 parameters	Δρmin = −1.21 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.007 (5)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Sc1	0.41180 (14)	0.33385 (6)	0.29798 (9)	0.01239 (19)
P1	0.91347 (18)	0.15360 (9)	0.17940 (11)	0.0121 (2)
O1	1.1656 (6)	0.1873 (3)	0.2483 (4)	0.0210 (6)
O2	0.8938 (7)	0.2064 (3)	0.0202 (4)	0.0253 (7)
O3	0.7131 (5)	0.2153 (3)	0.2753 (4)	0.0187 (6)
O4	0.8864 (5)	0.0043 (3)	0.1816 (4)	0.0172 (5)
O5	0.1035 (7)	0.4609 (4)	0.2955 (7)	0.0486 (15)
H1A	0.154 (16)	0.531 (7)	0.278 (10)	0.058*
H1B	−0.039 (11)	0.465 (8)	0.289 (11)	0.058*
O6	0.4277 (10)	0.3663 (5)	0.0562 (4)	0.0416 (10)
H2A	0.319 (14)	0.342 (8)	−0.002 (8)	0.050*
H2B	0.548 (16)	0.324 (7)	0.033 (10)	0.050*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sc1	0.0104 (3)	0.0108 (3)	0.0160 (3)	0.0001 (2)	0.0003 (2)	−0.0006 (2)
P1	0.0104 (4)	0.0107 (4)	0.0150 (4)	0.0006 (3)	0.0006 (3)	−0.0002 (3)
O1	0.0120 (12)	0.0151 (13)	0.0357 (18)	−0.0005 (10)	−0.0042 (11)	−0.0058 (12)
O2	0.0409 (19)	0.0200 (14)	0.0151 (13)	0.0039 (14)	0.0014 (14)	0.0015 (11)
O3	0.0164 (13)	0.0182 (13)	0.0216 (14)	0.0048 (10)	0.0047 (10)	0.0006 (11)
O4	0.0136 (12)	0.0116 (11)	0.0265 (15)	0.0002 (9)	−0.0008 (11)	0.0003 (10)
O5	0.0131 (14)	0.0149 (15)	0.118 (5)	0.0017 (12)	0.003 (2)	0.007 (2)
O6	0.053 (3)	0.054 (3)	0.0180 (17)	−0.017 (2)	−0.0051 (18)	0.0002 (17)

Geometric parameters (Å, º)

Sc1—O2^i	2.025 (3)	P1—O3	1.525 (3)
Sc1—O3	2.045 (3)	P1—O4	1.531 (3)
Sc1—O1^ii	2.051 (3)	P1—O1	1.533 (3)
Sc1—O4^iii	2.062 (3)	O5—H1A	0.78 (6)
Sc1—O5	2.116 (4)	O5—H1B	0.78 (6)
Sc1—O6	2.181 (4)	O6—H2A	0.82 (5)
P1—O2	1.519 (3)	O6—H2B	0.81 (4)
O2^i—Sc1—O3	91.27 (14)	O1^ii—Sc1—O6	85.92 (16)
O2^i—Sc1—O1^ii	91.51 (15)	O4^iii—Sc1—O6	86.18 (16)
O3—Sc1—O1	93.85 (13)	O5—Sc1—O6	86.2 (2)
O2^i—Sc1—O4^iii	96.60 (14)	O2—P1—O3	109.38 (19)
O3—Sc1—O4^iii	94.77 (12)	O2—P1—O4	111.03 (19)
O1^ii—Sc1—O4^iii	168.02 (13)	O3—P1—O4	109.56 (17)
O2^i—Sc1—O5	95.1 (2)	O2—P1—O1	110.4 (2)
O3—Sc1—O5	173.6 (2)	O3—P1—O1	108.74 (19)
O1^ii—Sc1—O5	86.04 (14)	O4—P1—O1	107.63 (17)
O4^iii—Sc1—O5	84.46 (13)	H1A—O5—H1B	106 (8)
O2^i—Sc1—O6	177.00 (16)	H2A—O6—H2B	105 (7)
O3—Sc1—O6	87.36 (18)

Symmetry codes: (i) x−1/2, −y+1/2, z+1/2; (ii) x−1, y, z; (iii) −x+3/2, y+1/2, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O5—H1A···O1^iii	0.78 (6)	1.89 (6)	2.658 (5)	169 (10)
O5—H1B···O4^iv	0.78 (6)	1.95 (6)	2.704 (4)	165 (10)
O6—H2A···O3^v	0.81 (5)	2.16 (6)	2.874 (5)	147 (9)
O6—H2B···O2	0.80 (6)	2.27 (6)	3.029 (7)	168 (9)

Symmetry codes: (iii) −x+3/2, y+1/2, −z+1/2; (iv) −x+1/2, y+1/2, −z+1/2; (v) x−1/2, −y+1/2, z−1/2.