Stability of CaCl2‐type and α‐PbO2‐type SiO2 at high pressure and temperature determined by in‐situ X‐ray measurements

Murakami, Motohiko; Hirose, Kei; Ono, Shigeaki; Ohishi, Yasuo

doi:10.1029/2002gl016722

Cited by 122 publications

(137 citation statements)

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“…TEM observations show that cristobalite had transformed to seifertite near the shock veins, whereas cristobalite had transformed to stishovite farther from them; that is, cristobalite transformed to seifertite in hotter portions, but to stishovite in colder portions under the same pressure. These occurrences imply that the Clapeyron slope between stishovite and seifertite is negative, even though there is some disagreement, based on deductions from synthetic experiments, about the Clapeyron slope between stishovite and seifertite (or CaCl 2 -type silica) 12,25,26 . Cristobalite becomes unstable as pressure and temperature increase in a dynamic event.…”

Section: Discussionmentioning

confidence: 99%

“…The mechanism through which cristobalite transforms to its high-pressure polymorph has not been explained. These issues are not only important for understanding the dynamic events that have occurred on the Moon but they are also clues for exploring the Earth's interior dynamics, because post-stishovite phases are expected to exist in the deep lower mantle and D 00 layer in the core-mantle boundary of the Earth 12,14 .…”

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confidence: 99%

“…A recent study, however, revealed that a lunar meteorite, Asuka 881757, contains high-pressure polymorphs of silica, coesite and stishovite formed by meteorite bombardment 3.800 Ma ago 10 . Synthetic experiments indicate that silica transforms to coesite, stishovite, CaCl 2 -type, a-PbO 2 -type (seifertite) and pyrite-type forms as pressure and temperature rise [11][12][13] . There is still no clear evidence for the existence of post-stishovite phases in lunar meteorites.…”

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Discovery of seifertite in a shocked lunar meteorite

et al. 2013

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Many craters and thick regoliths of the moon imply that it has experienced heavy meteorite bombardments. Although the existence of a high-pressure polymorph is a stark evidence for a dynamic event, few high-pressure polymorphs are found in a lunar sample. a-PbO 2 -type silica (seifertite) is an ultrahigh-pressure polymorph of silica, and is found only in a heavily shocked Martian meteorite. Here we show evidence for seifertite in a shocked lunar meteorite, Northwest Africa 4734. Cristobalite transforms to seifertite by high-pressure and -temperature condition induced by a dynamic event. Considering radio-isotopic ages determined previously, the dynamic event formed seifertite on the moon, accompanying the complete resetting of radio-isotopic ages, is B2.7 Ga ago. Our finding allows us to infer that such intense planetary collisions occurred on the moon until at least B2.7 Ga ago.

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Discovery of seifertite in a shocked lunar meteorite

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“…For comparison, results are corrected to the same pressure at 130 GPa using the theoretical bulk modulus and elastic constants (11,29). Density is shown with comparison to Fs12 silicate perovskite (ϫ symbol shown with 1% error bar) (21) and the ␣-PbO2-type (dotted line) and CaCl 2-type (dashed line) SiO2 phases (30). …”

Section: Methodsmentioning

confidence: 99%

Iron-rich silicates in the Earth's D″ layer

Mao

Meng

Shen

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

106

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High-pressure experiments and theoretical calculations demonstrate that an iron-rich ferromagnesian silicate phase can be synthesized at the pressure-temperature conditions near the coremantle boundary. The iron-rich phase is up to 20% denser than any known silicate at the core-mantle boundary. The high mean atomic number of the silicate greatly reduces the seismic velocity and provides an explanation to the low-velocity and ultra-low-velocity zones. Formation of this previously undescribed phase from reaction between the silicate mantle and the iron core may be responsible for the unusual geophysical and geochemical signatures observed at the base of the lower mantle.core-mantle boundary ͉ high pressure ͉ mineral physics ͉ post-perovskite M odern deep-Earth mineralogical research began with highpressure experiments on iron silicate, a major component in the solid Earth. The discovery of the fayalite (Fe 2 SiO 4 ) olivine-spinel transition in 1959 (1) marked the first known transition beyond the upper mantle. The disproportionation of fayalite spinel into mixed oxides using the newly invented laser-heated and resistive-heated diamond-anvil cell in the early 1970s (2, 3) marked the first phase transition under lower mantle conditions. In the Earth's crust, upper mantle, and transition zone, iron silicates form extensive solid solutions with the magnesium endmembers in major rock-forming minerals, e.g., fayalite in ␣-(Fe,Mg) 2 SiO 4 (olivine), ferrosilite in (Fe,Mg)SiO 3 (pyroxene), almandine in (Fe,Mg) 3 Al 2 Si 3 O 12 (garnet), and fayalite spinel in ␥-(Fe,Mg) 2 SiO 4 (ringwoodite). No iron-rich silicate, however, was known to exist under the high pressuretemperature (P-T) conditions beyond the 670-km discontinuity that accounts for approximately three-quarters of the Earth's total silicates and oxides. Following Birch's 1952 postulation (4), iron-rich silicates break down to mixed oxides in the lower mantle.In the lower-mantle silicate, (Fe x Mg 1Ϫx )SiO 3 perovskite, iron can only participate as a minor component with x Ͻ 0.15 (5), even at the core-mantle boundary with an unlimited supply of iron from the core. Without a stable iron-rich silicate phase, previous explanations of the complex geochemical and geophysical signatures of the DЉ layer have been limited to heterogeneous, solid͞melt mixtures of iron-poor silicates and iron-rich metals and oxides (6, 7). Recently, MgSiO 3 has been found to transform from perovskite to CaIrO 3 structure under the P-T conditions of the DЉ layer (8)(9)(10)(11)(12). This postperovskite (ppv) phase also was observed to coexist with silicate perovskite and magnesiowüstite in experiments with orthopyroxene and olivine starting materials with x up to 0.4, but the iron content in this phase is undefined because of the unknown Fe͞Mg distributions among multiple coexisting ferromagnesian phases (13). Here, we report experimental and theoretical investigations across the ferrosilite (FeSiO 3 )-enstatite (MgSiO 3 ) join. We found that iron-rich (Fe x Mg 1Ϫx )SiO 3 with x as high as 0...

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“…Coexistence of CaCl 2 -type SiO 2 and -PbO 2 -type SiO 2 around this pressure is often reported (Murakami et al, 2003). (c) MnSiO 3 at 40 GPa, 1700 K (Ǻ).…”

Section: Figure Captionsmentioning

confidence: 99%

Stability of the perovskite structure and possibility of the transition to the post-perovskite structure in CaSiO3, FeSiO3, MnSiO3 and CoSiO3

Fujino

Nishio–Hamane

Suzuki

et al. 2009

Physics of the Earth and Planetary Interiors

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High pressure and high temperature experiments on CaSiO 3 , FeSiO 3 , MnSiO 3 and CoSiO 3 using a laser-heated diamond anvil cell combined with synchrotron X-ray diffraction were conducted to explore the perovskite structure of these compounds and the transition to the post-perovskite structure. The experimental results revealed that MnSiO 3 has a perovskite structure from relatively low pressure (ca. 20 GPa) similarly to CaSiO 3 , while the stable forms of FeSiO 3 and CoSiO 3 are mixtures of mono-oxide (NaCl structure) + high pressure polymorph of SiO 2 even at very high pressure and temperature (149 GPa and 1800 K for FeSiO 3 and 79 GPa and 2000 K for CoSiO 3 ). This strongly suggests that the crystal field stabilization energy (CFSE) of Fe 2+ with six 3d electrons and Co 2+ with seven 3d electrons at the octahedral site of mono-oxides favors a mixture of mono-oxide + SiO 2 over perovskite where Fe 2+ and Co 2+ would occupy the distorted dodecahedral sites having a smaller CFSE (Mn 2+ has five 3d electrons and has no CFSE).The structural characteristics that the orthorhombic distortion of MnSiO 3 perovskite decreases with pressure and the tolerance factor of CaSiO 3 perovskite (0.99) is far from the orthorhombic range suggest that both MnSiO 3 and CaSiO 3 perovskites will not transform to the CaIrO 3 -type post-perovskite structure even at the Earth's core-mantle boundary conditions, although CaSiO 3 perovskite has a potentiality to transform to the CaIrO 3 -type post-perovskite structure at still higher pressure as long as another type of transformation does not occur.

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Stability of CaCl₂‐type and α‐PbO₂‐type SiO₂ at high pressure and temperature determined by in‐situ X‐ray measurements

Cited by 122 publications

References 16 publications

Discovery of seifertite in a shocked lunar meteorite

Discovery of seifertite in a shocked lunar meteorite

Iron-rich silicates in the Earth's D″ layer

Stability of the perovskite structure and possibility of the transition to the post-perovskite structure in CaSiO3, FeSiO3, MnSiO3 and CoSiO3

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