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.
Supporting information
CCDC reference: 966424
Crystal data, data collection and structure refinement details are summarized in Table 1.
S2. Results and discussion
top
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).
Crystal data top
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) Å3 | 0.12 × 0.10 × 0.10 mm |
Z = 4 | |
Data collection top
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 top
Refinement on F | Weighting scheme based on measured s.u.'s w = 1/σ2(F) |
R[F2 > 2σ(F2)] = 0.029 | (Δ/σ)max = 0.00021 |
wR(F2) = 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 |
Crystal data top
Mg2O6Si2 | V = 411.73 (6) Å3 |
Mr = 200.78 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 9.5387 (8) Å | µ = 1.11 mm−1 |
b = 8.6601 (7) Å | T = 296 K |
c = 5.2620 (4) Å | 0.12 × 0.10 × 0.10 mm |
β = 108.701 (6)° | |
Data collection top
Rigaku AFC7R diffractometer | Rint = 0.015 |
4535 measured reflections | 3 standard reflections every 200 reflections |
2171 independent reflections | intensity decay: none |
766 reflections with F > 3.0 σ(F) | |
Refinement top
R[F2 > 2σ(F2)] = 0.029 | 48 parameters |
wR(F2) = 0.020 | Δρmax = 0.73 e Å−3 |
S = 1.51 | Δρmin = −1.05 e Å−3 |
766 reflections | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | 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 (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
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) top
Mg1—O1i | 2.076 (2) | Mg2—O2viii | 2.260 (2) |
Mg1—O1ii | 2.076 (2) | Mg2—O2ix | 2.260 (2) |
Mg1—O1iii | 1.983 (1) | Mg2—O3v | 2.435 (2) |
Mg1—O1iv | 1.983 (1) | Mg2—O3vi | 2.435 (2) |
Mg1—O2v | 1.931 (2) | Si—O1 | 1.621 (1) |
Mg1—O2vi | 1.931 (2) | Si—O2 | 1.582 (2) |
Mg2—O1 | 2.236 (2) | Si—O3 | 1.636 (2) |
Mg2—O1vii | 2.236 (2) | Si—O3x | 1.640 (2) |
| | | |
O1i—Mg1—O1ii | 83.66 (10) | O1—Mg2—O3v | 137.53 (6) |
O1i—Mg1—O1iii | 79.97 (6) | O1—Mg2—O3vi | 119.73 (5) |
O1i—Mg1—O1iv | 95.15 (6) | O1vii—Mg2—O2viii | 77.01 (6) |
O1i—Mg1—O2v | 169.81 (6) | O1vii—Mg2—O2ix | 87.96 (6) |
O1i—Mg1—O2vi | 89.73 (7) | O1vii—Mg2—O3v | 119.73 (5) |
O1ii—Mg1—O1iii | 95.15 (6) | O1vii—Mg2—O3vi | 137.53 (6) |
O1ii—Mg1—O1iv | 79.97 (6) | O2viii—Mg2—O2ix | 160.92 (10) |
O1ii—Mg1—O2v | 89.73 (7) | O2viii—Mg2—O3v | 132.27 (7) |
O1ii—Mg1—O2vi | 169.81 (6) | O2viii—Mg2—O3vi | 65.65 (5) |
O1iii—Mg1—O1iv | 173.50 (12) | O2ix—Mg2—O3v | 65.65 (5) |
O1iii—Mg1—O2v | 92.96 (7) | O2ix—Mg2—O3vi | 132.27 (7) |
O1iii—Mg1—O2vi | 91.31 (7) | O3v—Mg2—O3vi | 76.47 (8) |
O1iv—Mg1—O2v | 91.31 (7) | O1—Si—O2 | 116.93 (9) |
O1iv—Mg1—O2vi | 92.96 (7) | O1—Si—O3 | 108.11 (8) |
O2v—Mg1—O2vi | 97.82 (11) | O1—Si—O3x | 108.96 (9) |
O1—Mg2—O1vii | 76.51 (9) | O2—Si—O3 | 110.79 (10) |
O1—Mg2—O2viii | 87.96 (6) | O2—Si—O3x | 104.61 (8) |
O1—Mg2—O2ix | 77.01 (6) | O3—Si—O3x | 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. |
Experimental details
Crystal data |
Chemical formula | Mg2O6Si2 |
Mr | 200.78 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 296 |
a, b, c (Å) | 9.5387 (8), 8.6601 (7), 5.2620 (4) |
β (°) | 108.701 (6) |
V (Å3) | 411.73 (6) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 1.11 |
Crystal size (mm) | 0.12 × 0.10 × 0.10 |
|
Data collection |
Diffractometer | Rigaku AFC7R diffractometer |
Absorption correction | – |
No. of measured, independent and observed [F > 3.0 σ(F)] reflections | 4535, 2171, 766 |
Rint | 0.015 |
(sin θ/λ)max (Å−1) | 1.078 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.029, 0.020, 1.51 |
No. of reflections | 766 |
No. of parameters | 48 |
No. of restraints | ? |
Δρmax, Δρmin (e Å−3) | 0.73, −1.05 |