Single-crystal metastable high-temperature C2/c clinoenstatite quenched rapidly from high temperature and high pressure

The high-temperature clinoenstatite (HT-CEn) is one of the most important MgSiO₃ pyroxene polymorphs, but the details of its structure and stability have been uncertain. The single crystal of the C2/c HT-CEn end-member is firstly synthesized by rapid pressure-temperature quenching from 15-16 GPa and 1173-2173 K. The single-crystal X-ray diffraction analysis shows unusual bonding distances and static disorder of the atoms frozen in this metastable structure. The degree of kinking of the silicate tetrahedral chains is 175° for HT-CEn. The chain angle for HP-CEn is substantially smaller (135°), but the angle for L-CEn is in the opposite direction at -160° (= 200°). The degree of kinking increases by being curved by more than 180° for the transition from HT-CEn to L-CEn. As for the reverse change from the expansion to the stretch, a potential barrier exists at the point of the continuity. It is suggested that the reason why a structure can be quenched under ambient conditions is as follows: the present HT-CEn single crystal has been formed by the isosymmetric phase transition from the high-pressure C2/c clinoenstatite (HP-CEn). The presence of HT-CEn from HP-CEn in natural rocks is an indicator of quenching history, which leads to the possibility that it exists in shocked meteorites and impact materials.

S1. Experimental

S1.1. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1.

S2. Results and discussion

Computing details

Data collection: WinAFC Version 1.03 (Rigaku Corporation, 1999); cell refinement: WinAFC Version 1.03 (Rigaku Corporation, 1999); data reduction: RADY (Sasaki, 1987); program(s) used to refine structure: RADY (Sasaki, 1987); molecular graphics: VESTA (Momma & Izumi, 2006).

Figures

(I)

Crystal data

Mg2O6Si2	F(000) = 400
Mr = 200.78	Dx = 3.240 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -C 2yc	Cell parameters from 51 reflections
a = 9.5387 (8) Å	θ = 20.1–25.3°
b = 8.6601 (7) Å	µ = 1.11 mm^−1
c = 5.2620 (4) Å	T = 296 K
β = 108.701 (6)°	Block, colourless
V = 411.73 (6) Å^3	0.12 × 0.10 × 0.10 mm
Z = 4

Data collection

Rigaku AFC7R diffractometer	θmax = 50.0°
ω–2θ scans	h = −20→20
4535 measured reflections	k = −18→18
2171 independent reflections	l = 0→11
766 reflections with F > 3.0 σ(F)	3 standard reflections every 200 reflections
Rint = 0.015	intensity decay: none

Refinement

Refinement on F	Weighting scheme based on measured s.u.'s w = 1/σ^2(F)
R[F^2 > 2σ(F^2)] = 0.029	(Δ/σ)max = 0.00021
wR(F^2) = 0.020	Δρmax = 0.73 e Å^−3
S = 1.51	Δρmin = −1.05 e Å^−3
766 reflections	Extinction correction: isotropic Type I (Becker & Coppens, 1974a and 1974b)
48 parameters	Extinction coefficient: 0.47 (8)E3

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

	x	y	z	Uiso*/Ueq
Mg1	0.0000	0.9042 (1)	0.2500	0.0052 (2)
Mg2	0.0000	0.2856 (1)	0.2500	0.0255 (4)
Si	0.29223 (6)	0.09198 (7)	0.2457 (1)	0.0102 (1)
O1	0.1130 (1)	0.0828 (2)	0.1380 (3)	0.0232 (4)
O2	0.3636 (2)	0.2577 (2)	0.3179 (3)	0.0225 (5)
O3	0.3543 (1)	0.0065 (2)	0.0258 (3)	0.0197 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mg1	0.0053 (3)	0.0042 (4)	0.0051 (4)	0.0000	0.0004 (3)	0.0000
Mg2	0.0244 (6)	0.0284 (7)	0.0192 (7)	0.0000	0.0007 (5)	0.0000
Si	0.0077 (2)	0.0121 (3)	0.0104 (3)	−0.0002 (3)	0.0026 (2)	−0.0016 (3)
O1	0.0080 (6)	0.0333 (9)	0.0263 (9)	0.0068 (7)	0.0027 (6)	−0.0095 (8)
O2	0.0303 (9)	0.0134 (7)	0.0247 (9)	−0.0022 (7)	0.0102 (8)	−0.0042 (7)
O3	0.0131 (5)	0.0280 (8)	0.0178 (8)	0.0027 (7)	0.0045 (6)	−0.0107 (7)

Geometric parameters (Å, º)

Mg1—O1^i	2.076 (2)	Mg2—O2^viii	2.260 (2)
Mg1—O1^ii	2.076 (2)	Mg2—O2^ix	2.260 (2)
Mg1—O1^iii	1.983 (1)	Mg2—O3^v	2.435 (2)
Mg1—O1^iv	1.983 (1)	Mg2—O3^vi	2.435 (2)
Mg1—O2^v	1.931 (2)	Si—O1	1.621 (1)
Mg1—O2^vi	1.931 (2)	Si—O2	1.582 (2)
Mg2—O1	2.236 (2)	Si—O3	1.636 (2)
Mg2—O1^vii	2.236 (2)	Si—O3^x	1.640 (2)
O1^i—Mg1—O1^ii	83.66 (10)	O1—Mg2—O3^v	137.53 (6)
O1^i—Mg1—O1^iii	79.97 (6)	O1—Mg2—O3^vi	119.73 (5)
O1^i—Mg1—O1^iv	95.15 (6)	O1^vii—Mg2—O2^viii	77.01 (6)
O1^i—Mg1—O2^v	169.81 (6)	O1^vii—Mg2—O2^ix	87.96 (6)
O1^i—Mg1—O2^vi	89.73 (7)	O1^vii—Mg2—O3^v	119.73 (5)
O1^ii—Mg1—O1^iii	95.15 (6)	O1^vii—Mg2—O3^vi	137.53 (6)
O1^ii—Mg1—O1^iv	79.97 (6)	O2^viii—Mg2—O2^ix	160.92 (10)
O1^ii—Mg1—O2^v	89.73 (7)	O2^viii—Mg2—O3^v	132.27 (7)
O1^ii—Mg1—O2^vi	169.81 (6)	O2^viii—Mg2—O3^vi	65.65 (5)
O1^iii—Mg1—O1^iv	173.50 (12)	O2^ix—Mg2—O3^v	65.65 (5)
O1^iii—Mg1—O2^v	92.96 (7)	O2^ix—Mg2—O3^vi	132.27 (7)
O1^iii—Mg1—O2^vi	91.31 (7)	O3^v—Mg2—O3^vi	76.47 (8)
O1^iv—Mg1—O2^v	91.31 (7)	O1—Si—O2	116.93 (9)
O1^iv—Mg1—O2^vi	92.96 (7)	O1—Si—O3	108.11 (8)
O2^v—Mg1—O2^vi	97.82 (11)	O1—Si—O3^x	108.96 (9)
O1—Mg2—O1^vii	76.51 (9)	O2—Si—O3	110.79 (10)
O1—Mg2—O2^viii	87.96 (6)	O2—Si—O3^x	104.61 (8)
O1—Mg2—O2^ix	77.01 (6)	O3—Si—O3^x	106.98 (6)

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