Discovery of seifertite in a shocked lunar meteorite

Miyahara, Masaaki; Kaneko, Shohei; Ohtani, Eiji; Sakai, Takeshi; Nagase, Toshiro; Kayama, Masahiro; Nishido, Hirotsugu; Hirao, Naohisa

doi:10.1038/ncomms2733

Cited by 53 publications

(83 citation statements)

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“…Lamellae-like textures are observed in some silica grains adjacent to or near the shock-melt veins ( Fig. 1 C and D), similar to a transition texture from quartz (or cristobalite) to stishovite (13,14). TEM images indicate that the silica grains with lamellae-like texture include lamellar stishovite (Fig.…”

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confidence: 79%

“…High-pressure polymorphs of silica have been found in lunar meteorites, Martian meteorites, and carbonaceous chondrite (14)(15)(16)(17), and in terrestrial impacted rocks (13,18). Coesite is thermodynamically stable above ∼2.5 GPa (19).…”

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confidence: 99%

“…The Béréba sample used in this study has many shock-induced melt (hereafter referred to as shock melt) veins (Fig. 1A) [7][8][9][10][11][12][13][14] Or 0-1 ), silica, minor kamacite, ilmenite, chromite, and Ca-phosphate. Most of the low-Ca pyroxene has exsolution lamellae of augite.…”

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Discovery of coesite and stishovite in eucrite

Miyahara

Ohtani

Yamaguchi

et al. 2014

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

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Howardite-eucrite-diogenite meteorites (HEDs) probably originated from the asteroid 4 Vesta. We investigated one eucrite, Béréba, to clarify a dynamic event that occurred on 4 Vesta using a shock-induced high-pressure polymorph. We discovered highpressure polymorphs of silica, coesite, and stishovite originating from quartz and/or cristobalite in and around the shock-melt veins of Béréba. Lamellar stishovite formed in silica grains through a solid-state phase transition. A network-like rupture was formed and melting took place along the rupture in the silica grains. Nanosized granular coesite grains crystallized from the silica melt. Based on shock-induced high-pressure polymorphs, the estimated shock-pressure condition ranged from ∼8 to ∼13 GPa. Considering radiometric ages and shock features, the dynamic event that led to the formation of coesite and stishovite occurred ca. 4.1 Ga ago, which corresponds to the late heavy bombardment period (ca. 3.8-4.1 Ga), deduced from the lunar cataclysm. There are two giant impact basins around the south pole of 4 Vesta. Although the origin of HEDs is thought to be related to dynamic events that formed the basins ca. 1.0 Ga ago, our findings are at variance with that idea. shock metamorphism | meteoroid impact H owardite-eucrite-diogenite meteorites (HEDs) are the largest group among the achondrites. Although the origin of HEDs is still under debate (1, 2), the similarities between the reflectance spectra of HEDs and the spectra of one of the largest asteroids in the asteroid belt 4 Vesta and dynamic considerations indicate that HEDs originated from 4 Vesta (3-5). The Dawn mission supports this prediction. It has been revealed that many craters exist on 4 Vesta (6, 7), which suggests heavy meteoroid bombardment. The existence of a high-pressure polymorph in a shocked meteorite provides clear evidence for a dynamic event on its parent body (8). Some recent studies propose that 4 Vesta, similar to the Moon, might have suffered from late heavy bombardment (9-11). However, to date, no high-pressure polymorph has been found in HEDs. We now report clear evidence of highpressure polymorphs of silica, coesite, and stishovite from eucrite.We envisaged that some eucrites might contain high-pressure polymorphs because it is expected that the surface of 4 Vesta consists mainly of eucrite. We obtained one of the eucrites, Béréba, to clarify a dynamic event occurring on 4 Vesta, using the high-pressure mineral inventory. The Béréba sample used in this study has many shock-induced melt (hereafter referred to as shock melt) veins (Fig. 1A), implying that it was heavily shocked. Major constituent minerals in the host rock of Béréba are low-Ca pyroxene (Fs 59-63 En 34-37 Wo 2-3 ), augite (Fs 25-32 En 29-31 Wo 38-44 ), plagioclase (An 86-92 Ab 7-14 Or 0-1 ), silica, minor kamacite, ilmenite, chromite, and Ca-phosphate. Most of the low-Ca pyroxene has exsolution lamellae of augite. Plagioclase now transformed into maskelynite partly and/or completely. Flow-like textures appear in some maskelyn...

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Discovery of coesite and stishovite in eucrite

Miyahara

Ohtani

Yamaguchi

et al. 2014

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

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“…On the one hand, α-cristobalite is found in meteorites that lack any high-pressure silica polymorphs albeit the rest of the minerals indicate peak shock pressures in excess of 10 GPa and high temperatures, at which coesite or stishovite are expected45. On the other hand, it is documented alongside all the natural occurrences of seifertite, a post-stishovite high-pressure polymorph of SiO 2 only found in heavily shocked (25 GPa or higher) meteorites5678. According to these observations, α-cristobalite seems to be stable at variable pressure conditions (from ambient to more than 25 GPa), thereby not recording the peak transient pressure as the other associated phases.…”

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Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation

et al. 2017

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In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms—a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.

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“…Its discovery and its transition at higher pressures to stishovite 2 with silicon in octahedral coordination (SiO 6 ) demonstrated the importance of the pressure variable for understanding the deep Earth, and in effect, ushered in the new era of high-pressure mineral physics. Subsequent experiments at higher pressures and high temperatures produced the equilibrium conditions [3][4][5][6][7][8][9][10] , and revealed a number of stable post-stishovite phases, including octahedrally coordinated silica with the CaCl 2 and a-PbO 2 structures and eventually to the pyrite structure [11][12][13][14][15][16][17] . The tetrahedron-to-octahedron transition in pure silica also exemplifies the universal phenomena in all silicate minerals throughout the mantle [18][19][20] .…”

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Polymorphic phase transition mechanism of compressed coesite

Shu

Cadien

et al. 2015

Nat Commun

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Silicon dioxide is one of the most abundant natural compounds. Polymorphs of SiO 2 and their phase transitions have long been a focus of great interest and intense theoretical and experimental pursuits. Here, compressing single-crystal coesite SiO 2 under hydrostatic pressures of 26-53 GPa at room temperature, we discover a new polymorphic phase transition mechanism of coesite to post-stishovite, by means of single-crystal synchrotron X-ray diffraction experiment and first-principles computational modelling. The transition features the formation of multiple previously unknown triclinic phases of SiO 2 on the transition pathway as structural intermediates. Coexistence of the low-symmetry phases results in extensive splitting of the original coesite X-ray diffraction peaks that appear as dramatic peak broadening and weakening, resembling an amorphous material. This work sheds light on the long-debated pressure-induced amorphization phenomenon of SiO 2 , but also provides new insights into the densification mechanism of tetrahedrally bonded structures common in nature.

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Discovery of seifertite in a shocked lunar meteorite

Cited by 53 publications

References 53 publications

Discovery of coesite and stishovite in eucrite

Discovery of coesite and stishovite in eucrite

Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation

Polymorphic phase transition mechanism of compressed coesite

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