Redetermination of junitoite, CaZn₂Si₂O₇·H₂O

The crystal structure of the mineral junitoite, ideally CaZn₂Si₂O₇·H₂O (calcium dizinc disilicate monohydrate), was first determined by Hamilton & Finney [Mineral. Mag. (1985), 49, 91–95] based on the space group Ama2, yielding a reliability factor R of 0.10, with isotropic displacement parameters for all non-H atoms. The present study reports a structure redetermination of junitoite using single-crystal X-ray diffraction data from a natural sample, demonstrating that the space group of this mineral is actually Aea2, which can be attained simply by shifting the origin. Topologically, the structure models in the space groups Aea2 and Ama2 are analogous, consisting of chains of corner-sharing ZnO₄ tetra­hedra parallel to the b axis, cross-linked by Si₂O₇ tetra­hedral dimers (the site symmetry of the bridging O atom is ..2) along a and c, forming a three-dimensional framework. The Ca²⁺ cations (site symmetry ..2) are situated in cavities of the framework and are bonded to five O atoms and one H₂O mol­ecule (site symmetry ..2) in a distorted octa­hedral coordination environment. However, some bond lengths, especially for the SiO₄ tetra­hedron, are noticeably different between the two structure models. Hydrogen bonding in junitoite is found between the water mol­ecule and a framework O atom.

Related literature

For junitoite, see: Williams (1976); Hamilton & Finney (1985). For junitoite-related minerals and compounds, see: Lin et al. (1999); Fleet & Liu (2001); Kolitsch et al. (2009); Yang et al. (2012). Parameters for bond-valence calculations were taken from Brese & O'Keeffe (1991).

Experimental

Crystal data

• CaZn₂Si₂O₇·H₂O

• M_r = 357.02

• Orthorhombic, A e a 2

• a = 12.530 (4) Å

• b = 6.3056 (18) Å

• c = 8.562 (3) Å

• V = 676.5 (3) ų

• Z = 4

• Mo Kα radiation

• μ = 8.21 mm⁻¹

• T = 293 K

• 0.06 × 0.06 × 0.05 mm

Data collection

• Bruker APEXII CCD area-detector diffractometer

• Absorption correction: multi-scan (SADABS; Sheldrick, 2005) T_min = 0.639, T_max = 0.684

• 2719 measured reflections

• 1257 independent reflections

• 1189 reflections with I > 2σ(I)

• R_int = 0.021

Refinement

• R[F² > 2σ(F²)] = 0.017

• wR(F²) = 0.044

• S = 1.07

• 1257 reflections

• 65 parameters

• 1 restraint

• H atoms treated by a mixture of independent and constrained refinement

• Δρ_max = 0.52 e Å⁻³

• Δρ_min = −0.65 e Å⁻³

• Absolute structure: Flack (1983), 580 Friedel pairs

• Flack parameter: 0.023 (12)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
OW5—H⋯O1	0.70 (3)	2.18 (2)	2.875 (2)	170 (3)

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Junitoite, CaZn₂Si₂O₇.H₂O, from the Christmas Mine, Gila County, Arizona was first described by Williams (1976) with orthorhombic symmetry in space group Bbm2 (non-standard setting of space group No. 40) and unit-cell parameters a = 6.309, b = 12.503, c = 8.549 Å. By adopting the standard unit-cell setting of this space group in Ama2 (a = 12.510, b = 6.318, c = 8.561 Å) for this mineral, Hamilton & Finney (1985) noted that while the Weissenberg photographic data pointed to Ama2, the X-ray diffractometer data were also compatible with the space group Aea2. Although the two space groups yielded similar reliability factors R1 ~ 0.10 with isotropic displacement parameters for all atoms (H atoms were not located), Hamilton & Finney (1985) chose Ama2 for their final structure report because it "produces less distortion of the coordination polyhedra and provides a structure in which the site symmetry of the cations is more similar to other zinc silicates". Their attempts at refinement with anisotropic displacement parameters resulted in non-positive definite displacement parameters for a number of atoms. In our efforts to understand the hydrogen bonding environments in minerals and their relationships to Raman spectra, we concluded that the structural model for junitoite needed improvement. This study reports a structure redetermination of junitoite from the type locality by means of single-crystal X-ray diffraction data, demonstrating that the space group of this mineral is actually Aea2, rather than Ama2.

The crystal structure of junitoite consists of chains of corner-sharing ZnO₄ tetrahedra parallel to the b axis, cross-linked by Si₂O₇ tetrahedral dimers along a and c to form a three-dimensional framework. The Ca²⁺ cations, situated in cavities of the framework, are bonded to five O atoms and one H₂O molecule in a distorted octahedral [CaO₅(H₂O)] coordination environment (Figs. 1, 2). As described below, it may be useful to consider that there is a Ca—H₂O bonded pair in the cavity. The structure of junitoite in space group Aea2 resembles that in space group Ama2 (Hamilton & Finney, 1985). In fact, as noted by Hamilton & Finney (1985), the structure model in Aea2 can be attained simply by shifting the origin of the structure model in Ama2 from (x, y, z) to (x - 1/4, y - 1/4, z). Upon this shift, the only major structural change is that the two unique Zn atoms at the 4a sites in the Ama2 structure model are transformed into a single atom at the 8b site in the Aea2 structure model. The numbers and coordination polyhedra of the distinct Ca, Si, and O sites remain unaffected. However, some bond lengths are noticeably different between the two structure models. For example, the Si—O, Zn—O, and Ca—O bond lengths range from 1.55 (5) to 1.69 (5) Å, 1.93 (4) to 1.99 (4) Å, and 2.29 (7) to 2.44 (5) Å in the Ama2 structure model, respectively, but from 1.6130 (14) to 1.6719 (12) Å, 1.9454 (13) to 1.9691 (13) Å, and 2.286 (2) to 2.439 (2) Å in the Aea2 structure model. The Si—O—Si angle within the Si₂O₇ disiilicate group is 124.8 (1)° in our study, which is slightly greater than that (122.4°) determined by Hamilton & Finney (1985).

The hydrogen bond in junitoite is found between Ow5 and O1, with Ow5 as the donor and O1 as the acceptor. This agrees with the calculated bond-valence sums of 0.42 valence units for Ow5 and 1.77 valence units for O1 by using the parameters given by Brese & O'Keeffe (1991). For numerical details of the hydrogen-bonding geometry, see: Table 1.

Remarkably, junitoite is topologically related to a group of compounds with the general formula BaM²⁺₂Si₂O₇, where M = Be (barylite and clinobarylite), Fe (andremeyerite), Cu (scottyite), and Mg, Mn, Co, and Zn in synthetic phases. These Ba-silicates are all comprised of corner-sharing MO₄ tetrahedral chains that are interlinked by Si₂O₇ tetrahedral dimers and Ba²⁺ cations, despite their diverse structural symmetries (Yang et al., 2012). Intriguingly, there is no documentation for any SrM₂Si₂O₇ compounds. It then begs the question whether the BaM₂Si₂O₇ compounds are capable of accommodating a significant amount of cations smaller than Ba²⁺. Similar to the pair (Ca²⁺ + H₂O) in junitoite, the Ba²⁺ cations in the BaM₂Si₂O₇ structures are also situated in the cavities of the framework formed by the Si₂O₇ dimers and the MO₄ tetrahedral chains. Conceivably, any substantial replacement of large Ba²⁺ by smaller divalent cations (such as Sr²⁺) would require, in addition to the other structural adjustments (such as the tilting or distortion of MO₄ and/or SiO₄ tetrahedra), a further narrowing of the Si—O—Si angle in the Si₂O₇ group in order to satisfy the bonding environment for smaller cations. This, however, would not be energetically favorable, because the Si—O—Si angles in the BaM₂Si₂O₇ compounds, ranging from 124 to 135°, are already among the smallest of disilicate materials, e.g. for high-temperature BaZn₂Si₂O₇ (Lin et al., 1999), high-pressure rare earth (RE) disilicates RE₂Si₂O₇ (Fleet & Liu, 2001) or BaKY(Si₂O₇) (Kolitsch et al., 2009). Accordingly, any sizable substitution of smaller Sr²⁺ for Ba²⁺ would worsen the bonding energetics for this site and thus destabilize the structure. For junitoite, Ca²⁺ by itself, which is even smaller than Sr²⁺, is apparently too small to occupy the cavities in the framework. Therefore, the presence of the H₂O—Ca²⁺ bonded pair is essential to stabilize its structure. By the same token, one could argue that the pair (Sr²⁺ + H₂O) together may be too large for the cavities in the structures analogous to those for the BaM₂Si₂O₇ materials, since there is no report for any SrM²⁺₂Si₂O₇.H₂O compound up to date. Based on this reasoning, we postulate that more compounds with composition CaM²⁺₂Si₂O₇.H₂O may be found in nature or synthesized in laboratories. Furthermore, it would be interesting if the Sr—H₂O pair might be found in digermanates, where this structural unit is even larger.

Related literature

For junitoite, see: Williams (1976); Hamilton & Finney (1985). For junitoite-related minerals and compounds, see: Lin et al. (1999); Fleet & Liu (2001); Kolitsch et al. (2009); Yang et al. (2012). Parameters for bond-valence calculations were taken from Brese & O'Keeffe (1991).

S2. Experimental

The junitoite crystal used in this study is from the type locality, the Christmas Mine, Gila County, Arizona and is in the collection of the RRUFF project (http://rruff.info/R120100). Its chemical composition measured by Williams (1976) is Ca_0.98Zn_1.96Si_1.84O_6.6.1.13H₂O.

S3. Refinement

The H atom was located near Ow5 from difference Fourier syntheses and its position refined freely with a fixed isotropic displacement (U_iso = 0.03). For simplicity, the ideal chemistry, CaZn₂Si₂O₇.H₂O, was assumed during the final refinement. The highest residual peak in the difference Fourier maps was located at (0.6270, 0.1150, 0.9598), 0.69 Å from O1, and the deepest hole at (0.3072, 0.2596, 0.0874), 0.81 Å from Zn.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. Crystal structure of junitoite. The gray, green, and small blue spheres represent Ca, Ow5, and H atoms, respectively. The yellow and red tetrahedra represent ZnO4 and SiO4 groups, respectively.
	Fig. 2. Atoms in junitoite with corresponding ellipsoids at the 99.9% probability level. The gray, yellow, purple, red, and green ellipsoids represent Ca, Zn, Si, O, and Ow5, respectively. The small blue spheres represent H atoms.

calcium dizinc disilicate monohydrate

Crystal data

CaZn2Si2O7·H2O	F(000) = 696
Mr = 357.02	Dx = 3.506 Mg m^−3
Orthorhombic, Aea2	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: A 2 -2ac	Cell parameters from 1387 reflections
a = 12.530 (4) Å	θ = 4.3–33.2°
b = 6.3056 (18) Å	µ = 8.21 mm^−1
c = 8.562 (3) Å	T = 293 K
V = 676.5 (3) Å^3	Cuboid, colorless
Z = 4	0.06 × 0.06 × 0.05 mm

Data collection

Bruker APEXII CCD area-detector diffractometer	1257 independent reflections
Radiation source: fine-focus sealed tube	1189 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.021
φ and ω scan	θmax = 33.3°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Sheldrick, 2005)	h = −8→19
Tmin = 0.639, Tmax = 0.684	k = −9→8
2719 measured reflections	l = −12→13

Refinement

Refinement on F^2	Hydrogen site location: difference Fourier map
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F^2 > 2σ(F^2)] = 0.017	w = 1/[σ^2(Fo^2) + (0.0094P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.044	(Δ/σ)max = 0.001
S = 1.07	Δρmax = 0.52 e Å^−3
1257 reflections	Δρmin = −0.65 e Å^−3
65 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
1 restraint	Extinction coefficient: 0.0120 (4)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983), 580 Friedel pairs
Secondary atom site location: difference Fourier map	Absolute structure parameter: 0.023 (12)

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
Ca	0.0000	0.5000	0.51882 (6)	0.00899 (12)
Zn	0.252164 (9)	0.244531 (17)	0.13494 (6)	0.00808 (7)
Si	0.11827 (4)	0.00274 (4)	0.39406 (7)	0.00615 (9)
O1	0.12311 (9)	0.2992 (2)	0.01003 (15)	0.0107 (3)
O2	0.12739 (10)	0.21991 (19)	0.49272 (17)	0.0099 (3)
O3	0.20596 (13)	−0.00482 (13)	0.25347 (15)	0.0101 (3)
O4	0.0000	0.0000	0.3037 (2)	0.0082 (3)
OW5	0.0000	0.5000	0.2518 (3)	0.0209 (7)
H	0.035 (3)	0.448 (4)	0.200 (3)	0.030*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca	0.0106 (2)	0.0078 (2)	0.0085 (2)	0.00019 (13)	0.000	0.000
Zn	0.00755 (10)	0.00739 (10)	0.00931 (11)	−0.00067 (7)	0.00070 (15)	−0.00072 (18)
Si	0.00563 (18)	0.00687 (18)	0.00596 (19)	−0.00022 (11)	0.00019 (16)	0.00035 (16)
O1	0.0084 (5)	0.0112 (5)	0.0126 (8)	−0.0010 (4)	−0.0020 (4)	0.0046 (5)
O2	0.0086 (5)	0.0109 (5)	0.0102 (8)	0.0019 (4)	−0.0026 (4)	−0.0035 (5)
O3	0.0124 (7)	0.0080 (6)	0.0099 (6)	−0.0001 (3)	0.0045 (4)	0.0003 (4)
O4	0.0063 (8)	0.0112 (8)	0.0071 (9)	0.0000 (4)	0.000	0.000
OW5	0.0272 (15)	0.0278 (18)	0.0076 (11)	0.0100 (7)	0.000	0.000

Geometric parameters (Å, º)

Ca—OW5	2.286 (2)	Zn—O3^v	1.9501 (11)
Ca—O2^i	2.3910 (12)	Zn—O3	1.9589 (11)
Ca—O2	2.3910 (12)	Zn—O1	1.9691 (13)
Ca—O1^ii	2.4381 (13)	Si—O2	1.6130 (14)
Ca—O1^iii	2.4381 (13)	Si—O1^vi	1.6239 (14)
Ca—O4^ii	2.439 (2)	Si—O3	1.6305 (16)
Zn—O2^iv	1.9454 (13)	Si—O4	1.6719 (12)
OW5—Ca—O2^i	84.64 (4)	O1^iii—Ca—O4^ii	91.77 (4)
OW5—Ca—O2	84.64 (4)	O2^iv—Zn—O3^v	100.47 (6)
O2^i—Ca—O2	169.28 (8)	O2^iv—Zn—O3	119.31 (6)
OW5—Ca—O1^ii	88.23 (4)	O3^v—Zn—O3	117.42 (7)
O2^i—Ca—O1^ii	81.25 (6)	O2^iv—Zn—O1	108.13 (8)
O2—Ca—O1^ii	98.41 (6)	O3^v—Zn—O1	111.21 (6)
OW5—Ca—O1^iii	88.23 (4)	O3—Zn—O1	100.32 (6)
O2^i—Ca—O1^iii	98.41 (6)	O2—Si—O1^vi	110.37 (11)
O2—Ca—O1^iii	81.25 (6)	O2—Si—O3	111.33 (7)
O1^ii—Ca—O1^iii	176.46 (7)	O1^vi—Si—O3	113.77 (7)
OW5—Ca—O4^ii	180.0	O2—Si—O4	108.30 (6)
O2^i—Ca—O4^ii	95.36 (4)	O1^vi—Si—O4	107.94 (6)
O2—Ca—O4^ii	95.36 (4)	O3—Si—O4	104.79 (10)
O1^ii—Ca—O4^ii	91.77 (4)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
OW5—H···O1	0.70 (3)	2.18 (2)	2.875 (2)	170 (3)