High-pressure synthetic (Na_0.97Mg_0.03)(Mg_0.43Fe_0.17³⁺Si_0.40)Si₂O₆, with six-coordinated silicon, isostructural with P2/n omphacite

The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, (Na_0.97Mg_0.03)(Mg_0.43Fe_0.17³⁺Si_0.40)Si₂O₆, is isotypic with ordered P2/n omphacite. Its structure is characterized by single chains of corner-sharing SiO₄ tetra­hedra, extending along the c axis, which are crosslinked by bands of edge-sharing octa­hedra (site symmetry 2), statistically occupied by (Mg²⁺ + Fe³⁺ + Si⁴⁺). Between the bands built up of the octahedra are two non-equivalent highly distorted six-coordinated sites (site symmetry 2), statistically occupied by (Na + Mg). In contrast to omphacites, the great differences in size and charge between Mg²⁺ and Si⁴⁺ result in complete, rather than partial, ordering of Mg and Si into two distinct octa­hedral sites, whereas Fe³⁺ is disordered between the two sites. The octa­hedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe)—O and (^VISi + Fe)—O bond lengths are 2.075 and 1.850 Å, respectively.

Related literature

For structures of high-pressure synthetic clinopyroxenes with six-coordinated Si, see: Angel et al. (1988); Yang & Konzett (2005); Yang et al. (2009). For structures of ordered P2/n omphacites, see: Curtis et al. (1975); Matsumoto et al. (1975); Rossi et al. (1983). For background on the stability of clino­pyroxenes at high pressures and temperatures, see: Gasparik (1989); Konzett et al. (2005). For general background on materials with six-coordinated silicon, see: Finger & Hazen (1991). For the geologic occurrence of clinopyroxene with six-coordinated Si, see: Wang & Sueno (1996). For spectroscopic measurements on P2/n clinopyroxenes, see: Boffa Ballaran et al. (1998); Yang et al. (2009). For general information on polyhedral distortion and ionic radii, see: Robinson et al. (1971) and Shannon (1976), respectively.

Experimental

Crystal data

• (Na_0.97Mg_0.03)(Mg_0.43Fe_0.17Si_0.40)Si₂O₆

• M_r = 206.39

• Monoclinic, P 2/n

• a = 9.4432 (8) Å

• b = 8.6457 (7) Å

• c = 5.2540 (5) Å

• β = 108.003 (6)°

• V = 407.95 (6) ų

• Z = 4

• Mo Kα radiation

• μ = 1.69 mm⁻¹

• T = 293 K

• 0.06 × 0.05 × 0.05 mm

Data collection

• Bruker APEXII CCD area-detector diffractometer

• Absorption correction: multi-scan (SADABS; Sheldrick 2005) T_min = 0.906, T_max = 0.920

• 6586 measured reflections

• 1481 independent reflections

• 980 reflections with I > 2σ(I)

• R_int = 0.033

Refinement

• R[F² > 2σ(F²)] = 0.036

• wR(F²) = 0.085

• S = 1.07

• 1481 reflections

• 96 parameters

• 3 restraints

• Δρ_max = 0.51 e Å⁻³

• Δρ_min = −0.66 e Å⁻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

The coordination of silicon with oxygen in crystalline materials is crucial for our understanding of the structure and composition of the Earth's interior because together they account for 63% of the atoms in the planet. In general, silicon is four-coordinated (^IVSi) in the Earth's crust and upper mantle, but six-coordinated (^VISi) in the lower mantle (e.g., Finger & Hazen, 1991). In the Earth's transition zone (between depths of 410 and 670 km), minerals are found to contain both ^IVSi and ^VISi. Phase transitions that involve a change in the Si coordination may affect many important physical and chemical properties of materials, such as density, bulk moduli, and elasticity, which, when coupled with seismic observations, can provide vital information on the complex constituents of the Earth's mantle.

Clinopyroxenes, one of the major rock-forming minerals of the Earth's upper mantle, were long assumed to contain ^IVSi only. Studies of pyroxenes synthesized at high temperatures and pressures, however, have revealed their capacity to accommodate both ^IVSi and ^VISi (Angel et al. 1988; Konzett et al. 2005; Yang & Konzett 2005; Yang et al., 2009), pointing to their possible stabilities at higher pressures. In particular, Angel et al. (1988) reported a high-pressure Na(Mg_0.5Si_0.5)Si₂O₆ clinopyroxene (designated as NaPx hereafter), which is isostructural with ordered P2/n omphacite, with ^VISi and Mg fully ordered into two distinct octahedral sites. Later studies of NaPx-CaMgSi₂O₆ (diopside) and NaPx-NaAlSi₂O₆ (jadeite) solid solutions uncovered a symmetry transition from an ordered P2/n to a disordered C2/c structure as ^VISi content decreases (Yang & Konzett, 2005; Yang et al., 2009). The natural occurrence of a clinopyroxene containing ^VISi, (Na_0.16Mg_0.84)(Mg_0.92Si_0.08)Si₂O₆, was reported by Wang & Sueno (1996) as an inclusion in a diamond from a kimberlite in China. According to the phase stability relations for the NaPx-Mg₂Si₂O₆ (enstatite) join (Gasparik, 1989), this inclusion crystallized at pressures greater than 16.5 GPa, or at a depth within the Earth's transition zone (~500 km). The foremost implications of this finding include that (1) some portions of the Earth's upper mantle may contain a greater ratio of Na/Al than previously inferred from the xenolith chemistry, and (2) clinopyroxenes may be one of potential candidates as a silica-rich phase in the Earth's mantle. To gain more insights into the systematics on the crystal chemistry and stability field of pyroxenes with ^VISi, we conducted a structure refinement of a high-pressure synthetic NaPx-NaFeSi₂O₆ (aegirine) solid solution (designated as NaPxFe hereafter) based on the single-crystal X-ray diffraction data.

NaPxFe is isotyic with P2/n omphacite (Matsumoto et al. 1975; Curtis et al. 1975; Rossi et al. 1983) and NaPx (Angel et al., 1988; Yang et al., 2009). Its structure is characterized by a distorted closest-packed array of oxygen atoms with a layer of single silicate chains, extending along c, formed by Si1O₄ and Si2O₄ tetrahedra, alternating with a layer containing two distinct, edge-sharing octahedra M1 and M1(1), occupied by (Mg²⁺ + Fe³⁺ + Si⁴⁺). Also in the octahedral layer are two nonequivalent, considerably distorted six-coordinated sites M2 and M2(1), occupied by (Na + Mg) (Fig. 1). Note that Na in clinopyroxenes has been regarded to be eight-coordinated in all previous studies. However, our electron-density and bond-valence sum calculations indicate that Na in NaPxFe is actually six-coordinated. In contrast to omphacites, the great differences in size and charge between Mg²⁺ and Si⁴⁺ result in complete, rather than partial, ordering of Mg at M1 and Si at M1(1), while Fe is disordered between the two sites. An inspection of structure data for pyroxenes containing ^VISi shows that, as the respective contents of Mg and Si in the M1 and M1(1) sites decreases from NaPx to Na(Mg_0.45Al_0.10Si_0.45)Si₂O₆ (Angel et al., 1988; Yang et al., 2009) and NaPxFe, the average M1—O bond length decreases from 2.092 to 2.084 and 2.075 Å, respectively, whereas the average M1(1)-O bond distance increases from 1.807 Å to 1.813 and 1.850 Å, respectively. This observation is evidently a direct consequence of the differences in ionic radii among Mg²⁺ (0.72 Å), Fe³⁺ (0.645 Å), Al³⁺ (0.535 Å), and ^VISi⁴⁺ (0.40 Å) (Shannon, 1976). The coupled changes in the average M1—O and M1(1)-O bond distances from NaPx to Na(Mg_0.45Al_0.10Si_0.45)Si₂O₆ and NaPxFe lead to a substantial reduction in the size mismatch between the two edge-shared octahedra, as well as the degree of distortion of the M1 octahedron in terms of the octahedral angle variance (OAV) and octahedral quadratic elongation (OQE) (Robinson et al., 1971). The OAV and OQE values are 107.2 and 1.035, respectively, for the M1 octahedron in NaPx, 101.4 and 1.033 in Na(Mg_0.45Al_0.10Si_0.45)Si₂O₆, and 88.2 and 1.029 in NaPxFe. Interestingly, as the geometric differences between the M1 and M1(1) octahedra decreases with decreasing ^VISi content from NaPx to Na(Mg_0.45Al_0.10Si_0.45)Si₂O₆ and NaPxFe, the difference between the mean M2—O and M2(1)-O bond distances is also reduced, which is 0.123, 0.113, and 0.096 Å for NaPx, Na(Mg_0.45Al_0.10Si_0.45)Si₂O₆, and NaPxFe, respectively. Conceivably, as the ^VISi content is further decreased, the P2/n structure will eventually transform to the C2/c structure, in which M1 and M1(1) become identical, so do M2 and M2(1), and Si1 and Si2.

Plotted in Figure 2 are Raman spectra of NaPxFe and two clinopyroxenes in the NaPx-jadeite system for comparison, one with P2/n symmetry and the other C2/c (Yang et al., 2009). The detailed assignment of Raman bands for clinopyroxenes containing ^VISi has been discussed by Yang et al. (2009). Generally, the Raman spectra of pyroxenes can be classified into four regions. Region 1 includes the bands between 800 and 1200 cm^-1, which are assigned as the Si—O stretching vibrations in the SiO₄ tetrahedra. Region 2 is between 630–800 cm^-1, which includes the bands attributable to the Si—O—Si vibrations within the silicate chains. Region 3, from 400 to 630 cm^-1, includes the bands that are mainly associated with the O—Si—O bending modes of SiO₄ tetrahedra. The bands in Region 4, which spans from 50 to 400 cm^-1, are of a complex nature, chiefly due to lattice vibration modes, polyhedral librations and M—O interactions, as well as possible O—Si—O bending. As noted by Boffa Ballaran et al. (1998) and Yang et al. (2009), the C2/c-to-P2/n transformation is characterized by the splitting of many observable Raman bands in the C2/c structure into doublets in the P2/n structure, consistent with the doubled number of independent atomic sites in the P2/n structure relative to that in the C2/c structure. Compared to the Raman spectrum of P2/n Na(Mg_0.45Al_0.10Si_0.45)Si₂O₆ (Sample J2, Yang et al., 2009), most Raman bands for NaPxFe are noticeably broader, indicating the increased positional disorder of atoms in our sample, which further suggests that the chemistry of our sample is closer to the P2/n-to-C2/c transition than that of Na(Mg_0.45Al_0.10Si_0.45)Si₂O₆ (Yang et al., 2009), in agreement with the conclusion we derived above from the structure refinement.

Related literature

For structures of high-pressure synthetic clinopyroxenes with six-coordinated Si, see: Angel et al. (1988); Yang & Konzett (2005); Yang et al. (2009). For structures of ordered P2/n omphacites, see: Curtis et al. (1975); Matsumoto et al. (1975); Rossi et al. (1983). For background on the stability of clinopyroxenes at high pressures and temperatures, see: Gasparik (1989); Konzett et al. (2005). For general background on materials with six-coordinated silicon, see: Finger & Hazen (1991). For the geologic occurrence of clinopyroxene with six-coordinated Si, see: Wang & Sueno (1996). For spectroscopic measurements on P2/n clinopyroxenes, see: Boffa Ballaran et al. (1998); Yang et al. (2009). For general information on polyhedral distortion and ionic radii, see: Robinson et al. (1971) and Shannon (1976), respectively.

S2. Experimental

The specimen used in this study was synthesized in a multi-anvil apparatus at 15 GPa and 1500 °C for 23.2 h (run #JKB2006–12) and then rapidly quenched (< 5 s) to ambient conditions. The crystal chemistry was determined with a Jeol electron microprobe on the same single-crystal used for the X-ray intensity data collection. The average composition of nine analysis points yielded a chemical formula (normalized on the basis of 6 oxygen atoms and 4 cations while maintaining charge-balance) (Na_0.97Mg_0.03)(Mg_0.43Fe_0.17Si_0.40)Si₂O_6.

The Raman spectrum of (Na_0.97Mg_0.03)(Mg_0.43Fe_0.17Si_0.40)Si₂O₆ was collected from a randomly oriented crystal at 100% power on a Thermo Almega microRaman system, using a 532 nm solid-state laser, and a thermoelectrically cooled CCD detector. The laser is partially polarized with 4 cm^-1 resolution and a spot size of 1 µm.

S3. Refinement

Throughout the structure refinements, the chemical composition of the crystal was fixed to that determined from electron microprobe analysis. From crystal-chemical considerations, all Mg and ^VISi were assigned to the M1 and M1(1) sites, respectively, with the rest filled by Fe³⁺ for both sites. Because the average distance of M2—O is shorter than that of M2(1)-O, we assigned 0.03 Mg apfu to the M2 site. The highest residual peak in the difference Fourier maps was located at (0.7991, 0.0789, 0.2448), 0.54 Å from M2, and the deepest hole at (0.2602, 0.1856, 0.1853), 0.48 Å from M1(1).

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 P2/n (Na0.97Mg0.03)(Mg0.43Fe0.17Si0.40)Si2O6 clinopyroxene.
	Fig. 2. Raman spectra of clinopyroxenes. (a) P2/n (Na0.97Mg0.03)(Mg0.43Fe0.17Si0.40)Si2O6 (this study), (b) P2/n Na(Mg0.45Al0.10Si0.45)Si2O6 (Yang et al., 2009) and (c) C2/c (Na0.97Mg0.03)(Mg0.37Al0.30Si0.33)Si2O6 (Yang et al., 2009).

(sodium magnesium) [magnesium iron(III) silicon] disilicate

Crystal data

(Na0.97Mg0.03)(Mg0.43Fe0.17Si0.40)Si2O6	F(000) = 409
Mr = 206.39	Dx = 3.360 Mg m^−3
Monoclinic, P2/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yac	Cell parameters from 1071 reflections
a = 9.4432 (8) Å	θ = 3.3–35.6°
b = 8.6457 (7) Å	µ = 1.69 mm^−1
c = 5.2540 (5) Å	T = 293 K
β = 108.003 (6)°	Cube, pale gray
V = 407.95 (6) Å^3	0.06 × 0.05 × 0.05 mm
Z = 4

Data collection

Bruker APEXII CCD area-detector diffractometer	1481 independent reflections
Radiation source: fine-focus sealed tube	980 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.033
ϕ and ω scan	θmax = 32.5°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Sheldrick 2005)	h = −14→14
Tmin = 0.906, Tmax = 0.920	k = −13→10
6586 measured reflections	l = −7→7

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.036	w = 1/[σ^2(Fo^2) + (0.0264P)^2 + 0.6629P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.085	(Δ/σ)max = 0.001
S = 1.07	Δρmax = 0.51 e Å^−3
1481 reflections	Δρmin = −0.66 e Å^−3
96 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
3 restraints	Extinction coefficient: 0

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	Occ. (<1)
M2	0.7500	0.04980 (16)	0.2500	0.0129 (3)	0.9400 (1)
M2MG	0.7500	0.04980 (16)	0.2500	0.0129 (3)	0.0600 (1)
M2(1)	0.7500	0.45601 (17)	0.7500	0.0169 (3)
M1	0.7500	0.65310 (10)	0.2500	0.00495 (18)	0.8600 (1)
M1Fe	0.7500	0.65310 (10)	0.2500	0.00495 (18)	0.1400 (1)
M1(1)	0.7500	0.84745 (9)	0.7500	0.00855 (17)	0.8000 (1)
M11Fe	0.7500	0.84745 (9)	0.7500	0.00855 (17)	0.2000 (1)
Si1	0.04344 (7)	0.84714 (7)	0.22988 (13)	0.00748 (14)
Si2	0.03811 (7)	0.66398 (7)	0.73666 (13)	0.00731 (14)
O1	0.86242 (18)	0.8417 (2)	0.1106 (4)	0.0133 (4)
O2	0.85803 (18)	0.6879 (2)	0.6524 (4)	0.0129 (4)
O3	0.1201 (2)	0.0136 (2)	0.3071 (4)	0.0130 (4)
O4	0.1012 (2)	0.4949 (2)	0.7949 (4)	0.0141 (4)
O5	0.10966 (18)	0.7652 (2)	0.0132 (3)	0.0105 (3)
O6	0.09535 (18)	0.74992 (19)	0.5087 (3)	0.0112 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
M2	0.0162 (7)	0.0106 (7)	0.0093 (7)	0.000	0.0002 (5)	0.000
M2MG	0.0162 (7)	0.0106 (7)	0.0093 (7)	0.000	0.0002 (5)	0.000
M2(1)	0.0210 (7)	0.0107 (7)	0.0128 (8)	0.000	−0.0037 (6)	0.000
M1	0.0060 (4)	0.0045 (4)	0.0037 (4)	0.000	0.0006 (3)	0.000
M1Fe	0.0060 (4)	0.0045 (4)	0.0037 (4)	0.000	0.0006 (3)	0.000
M1(1)	0.0094 (3)	0.0085 (4)	0.0073 (4)	0.000	0.0020 (3)	0.000
M11Fe	0.0094 (3)	0.0085 (4)	0.0073 (4)	0.000	0.0020 (3)	0.000
Si1	0.0081 (3)	0.0074 (3)	0.0065 (3)	−0.0002 (2)	0.0016 (2)	−0.0005 (2)
Si2	0.0075 (3)	0.0076 (3)	0.0065 (3)	0.0000 (2)	0.0018 (2)	0.0004 (2)
O1	0.0103 (7)	0.0128 (9)	0.0165 (9)	0.0007 (6)	0.0039 (7)	−0.0023 (7)
O2	0.0074 (7)	0.0173 (9)	0.0132 (9)	−0.0013 (6)	0.0018 (6)	0.0035 (7)
O3	0.0200 (9)	0.0074 (8)	0.0106 (9)	−0.0014 (6)	0.0033 (7)	−0.0009 (6)
O4	0.0188 (9)	0.0104 (9)	0.0132 (9)	0.0006 (7)	0.0050 (7)	0.0002 (7)
O5	0.0098 (7)	0.0136 (9)	0.0078 (8)	0.0009 (6)	0.0024 (6)	−0.0018 (6)
O6	0.0125 (8)	0.0129 (9)	0.0086 (8)	−0.0017 (6)	0.0037 (6)	0.0017 (6)

Geometric parameters (Å, º)

M2—O1^i	2.319 (2)	M1—O2	2.0658 (18)
M2—O1^ii	2.319 (2)	M1—O2^xi	2.0658 (18)
M2—O3^iii	2.3356 (18)	M1—O1^xi	2.1917 (19)
M2—O3^iv	2.3356 (18)	M1—O1	2.1917 (19)
M2—O6^v	2.3651 (19)	M1(1)—O3^v	1.8071 (19)
M2—O6^vi	2.3651 (19)	M1(1)—O3^vii	1.8071 (19)
M2—O5^vii	2.7127 (19)	M1(1)—O1^xii	1.8645 (18)
M2—O5^viii	2.7127 (19)	M1(1)—O1^xi	1.8645 (18)
M2(1)—O2^ix	2.376 (2)	M1(1)—O2^ix	1.8788 (19)
M2(1)—O2	2.376 (2)	M1(1)—O2	1.8788 (19)
M2(1)—O4^x	2.4073 (19)	Si1—O3^xiii	1.6058 (19)
M2(1)—O4^vi	2.4073 (19)	Si1—O5	1.6216 (18)
M2(1)—O5^vii	2.438 (2)	Si1—O6	1.6276 (18)
M2(1)—O5^v	2.438 (2)	Si1—O1^xiv	1.6294 (18)
M2(1)—O6^vii	2.8944 (19)	Si2—O4	1.5725 (19)
M2(1)—O6^v	2.8944 (19)	Si2—O2^xiv	1.6326 (18)
M1—O4^vi	1.9678 (19)	Si2—O6	1.6366 (18)
M1—O4^v	1.9678 (19)	Si2—O5^xii	1.6510 (18)
O4^vi—M1—O4^v	98.88 (11)	O3^v—M1(1)—O2^ix	170.80 (8)
O4^vi—M1—O2	96.72 (7)	O3^vii—M1(1)—O2^ix	89.37 (8)
O4^v—M1—O2	94.15 (7)	O1^xii—M1(1)—O2^ix	83.67 (8)
O4^vi—M1—O2^xi	94.15 (7)	O1^xi—M1(1)—O2^ix	94.09 (8)
O4^v—M1—O2^xi	96.72 (7)	O3^v—M1(1)—O2	89.37 (8)
O2—M1—O2^xi	163.26 (11)	O3^vii—M1(1)—O2	170.80 (8)
O4^vi—M1—O1^xi	90.36 (7)	O1^xii—M1(1)—O2	94.09 (8)
O4^v—M1—O1^xi	164.04 (7)	O1^xi—M1(1)—O2	83.67 (8)
O2—M1—O1^xi	71.74 (7)	O2^ix—M1(1)—O2	85.52 (11)
O2^xi—M1—O1^xi	95.54 (7)	O3^xiii—Si1—O5	109.12 (10)
O4^vi—M1—O1	164.04 (7)	O3^xiii—Si1—O6	104.39 (10)
O4^v—M1—O1	90.36 (7)	O5—Si1—O6	109.41 (9)
O2—M1—O1	95.54 (7)	O3^xiii—Si1—O1^xiv	117.40 (10)
O2^xi—M1—O1	71.74 (7)	O5—Si1—O1^xiv	107.69 (9)
O1^xi—M1—O1	83.83 (10)	O6—Si1—O1^xiv	108.64 (10)
O3^v—M1(1)—O3^vii	96.67 (12)	O4—Si2—O2^xiv	118.08 (10)
O3^v—M1(1)—O1^xii	89.08 (8)	O4—Si2—O6	111.93 (10)
O3^vii—M1(1)—O1^xii	92.94 (8)	O2^xiv—Si2—O6	107.21 (9)
O3^v—M1(1)—O1^xi	92.94 (8)	O4—Si2—O5^xii	107.11 (10)
O3^vii—M1(1)—O1^xi	89.08 (8)	O2^xiv—Si2—O5^xii	106.27 (9)
O1^xii—M1(1)—O1^xi	176.97 (12)	O6—Si2—O5^xii	105.39 (9)

Symmetry codes: (i) −x+3/2, y−1, −z+1/2; (ii) x, y−1, z; (iii) x+1/2, −y, z−1/2; (iv) −x+1, −y, −z+1; (v) −x+1, −y+1, −z+1; (vi) x+1/2, −y+1, z−1/2; (vii) x+1/2, −y+1, z+1/2; (viii) −x+1, −y+1, −z; (ix) −x+3/2, y, −z+3/2; (x) −x+1, −y+1, −z+2; (xi) −x+3/2, y, −z+1/2; (xii) x, y, z+1; (xiii) x, y+1, z; (xiv) x−1, y, z.