High pressure transitions in the system MgAl2O4–CaAl2O4: a new hexagonal aluminous phase with implication for the lower mantle

Akaogi, Masaki; Hamada, Y; Suzuki, Toshihiro; Kobayashi, Makoto; Okada, Masakazu

doi:10.1016/s0031-9201(99)00076-x

Cited by 118 publications

(45 citation statements)

References 15 publications

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“…Previous experiments showed that MgAl 2 O 4 might also take the CF or CT structures at specific P-T conditions (5,7,9,12). This finding has implications for the widespread cation substitution of Al for Cr and between Mg and Fe in this structure type.…”

Section: Discussionmentioning

confidence: 84%

“…1). Although ferromagnesian silicate spinels were later found to break down to simple oxides (3) or stishovite plus perovskite (4), several postspinel oxides convert to a single phase with the CF or CT structure (1,2,(5)(6)(7)(8)(9)(10)(11)(12). Neither dense postspinel polymorphs nor silicate perovskites, however, have been discovered in nature and confirmed as new minerals.…”

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confidence: 99%

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Natural occurrence and synthesis of two new postspinel polymorphs of chromite

Chen

Shu

Mao

et al. 2003

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

112

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A high-pressure polymorph of chromite, the first natural sample with the calcium ferrite structure, has been discovered in the shock veins of the Suizhou meteorite. Synchrotron x-ray diffraction analyses reveal an orthorhombic CaFe 2O4-type (CF) structure. The unit-cell parameters are a ‫؍‬ 8.954(7) Å, b ‫؍‬ 2.986(2) Å, c ‫؍‬ 9.891(7) Å, V ‫؍‬ 264.5(4) Å 3 (Z ‫؍‬ 4) with space group Pnma. The new phase has a density of 5.62 g͞cm 3 , which is 9.4% denser than chromitespinel. We performed laser-heated diamond anvil cell experiments to establish that chromite-spinel transforms to CF at 12.5 GPa and then to the recently discovered CaTi 2O4-type (CT) structure above 20 GPa. With the ubiquitous presence of chromite, the CF and CT phases may be among the important index minerals for natural transition sequence and pressure and temperature conditions in mantle rocks, shock-metamorphosed terrestrial rocks, and meteorites. F orty years ago, in search of denser polymorphs of the then newly discovered silicate spinel (ringwoodite) and modified spinel (wadsleyite) that are stable at the pressure and temperature (P-T) conditions of the Earth's transition zone, Ringwood (1, 2) proposed orthorhombic CaFe 2 O 4 -type (CF) and CaTi 2 O 4 -type (CT) structures as the top candidates for ''postspinel'' transitions in the Earth's mantle (Fig. 1). Although ferromagnesian silicate spinels were later found to break down to simple oxides (3) or stishovite plus perovskite (4), several postspinel oxides convert to a single phase with the CF or CT structure (1, 2, 5-12). Neither dense postspinel polymorphs nor silicate perovskites, however, have been discovered in nature and confirmed as new minerals. Many natural high-pressure minerals presumably predominating in the Earth's mantle have been found mainly in the shock-metamorphosed meteorites, which include ringwoodite, wadsleyite, silicate ilmenite, magnesiowüs-tite, (Na,K)AlSi 3 O 8 -hollandite, and poststishovite polymorphs (13-19). Special P-T conditions developed in the impacted meteorites, for instance, a long duration of high-pressure and -temperature up to several seconds and quenching under pressure, play an important role in the formation and preservation of high-pressure phases (16). It makes the naturally shockmetamorphosed meteorites a window for discovery of new high-pressure minerals and for investigation of the constituents in the deep Earth. Preliminary examination of the Suizhou meteorite has recently revealed a shock-metamorphosed CT polymorph of chromite composition (20). In this article, we report the identification of a previously uncharacterized CF polymorph of chromite in the same meteorite. High P-T experiments demonstrate that both CF and CT are indeed quenchable polymorphs of chromite formed above 12.5 and 20 GPa, respectively.The Suizhou meteorite is a moderately shock-metamorphosed L6-chondrite, and a shock stage S5 was estimated for the meteorite according to the classification of shock metamorphism (21). The meteorite contains shock-produced melt veins ranging...

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Section: Discussionmentioning

confidence: 84%

mentioning

confidence: 99%

Natural occurrence and synthesis of two new postspinel polymorphs of chromite

Chen

Shu

Mao

et al. 2003

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

112

View full text Add to dashboard Cite

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“…Experimental work has also been conducted on the stability of NAL and CF phases in the binary systems NaAlSiO 4 -MgAl 2 O 4 (Ono et al 2009) (Akaogi et al 1999). Whilst both phases are stable as part of a lower mantle eclogite assemblage, it has been shown that NAL phase (also called hexagonal phase) is stable at pressures of 13.5 GPa (~400 km), whilst CF phase is stable at 17 GPa (~510 km) depending on composition.…”

Section: Depth Of Individual Inclusion Formationmentioning

confidence: 99%

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

Thomson

Kohn²,

Bulanova

et al. 2014

Contrib Mineral Petrol

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interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (−5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have reequilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth's deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are intimately related to subducted material and record a polybaric growth history across a depth interval stretching from the lower mantle to the base of the lithosphere. It is suggested that the interaction of slabderived melts and mantle material combined with subsequent upward transport in channelised networks or a buoyant diapir explains the formation of Juina-5 diamonds. We conclude that these samples, despite originating at great mantle depths, do not provide direct information about the ambient mantle, instead, providing a snapshot of the Earth's deep carbon cycle. Keywords Diamonds · Sub-lithospheric mantle · Carbon cycle · SubductionAbstract Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ 13 C ~ −25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are Communicated by O. Müntener. Electronic supplementary materialThe online version of this article

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“…At about [26][27] GPa, this mixture has been observed to undergo a transition into an orthorhombic calcium ferrite-like structure (Decker and Kasper, 1957;Kruger et al, 1997;Funamori et al, 1998;Akaogi et al, 1999). This phase is stable over a limited pressure range as it was proposed that MgAl 2 O 4 undergoes another phase transition above 40 GPa to an orthorhombic calcium titanate-type (space group Cmcm) phase (Bertraut and Blum, 1956; Funamori et al, 1998;Catti 2001).…”

Section: A N U S C R I P Tmentioning

confidence: 99%

“…Notably, various studies have found the equations of state and/or crystal structure up to about 70 GPa (Kruger et At ambient pressure and temperature, MgAl 2 O 4 spinel is cubic with space group Fd3m (Kruger et al, 1997, Fang and. As pressure is applied, this structure becomes unstable and disassociates into a periclase (MgO) and corundum (Al 2 O 3 ) mixture around 15 GPa (Akaogi et al, 1999).…”

Section: Introductionmentioning

confidence: 99%

Elasticity and Raman and infrared spectra of MgAl2O4 spinel from density functional perturbation theory

Caracas

Banigan

2009

Physics of the Earth and Planetary Interiors

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This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractWe determine the elastic constants and the Raman spectra of spinel under pressure using density functional perturbation theory. Like other spinels, MgAl 2 O 4 shows an increasing bulk modulus and a weakly varying shear modulus under pressure. The Raman spectrum is dominated by two mains peaks whose intensities decrease under pressure. The theoretical Raman spectra of disordered spinel shows a better agreement with measurements on natural samples than the ordered one. We also investigate the high-pressure stability of different structures and find the calcium titanate type to be the stable phase beyond 45 GPa.

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High pressure transitions in the system MgAl2O4–CaAl2O4: a new hexagonal aluminous phase with implication for the lower mantle

Cited by 118 publications

References 15 publications

Natural occurrence and synthesis of two new postspinel polymorphs of chromite

Natural occurrence and synthesis of two new postspinel polymorphs of chromite

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

Elasticity and Raman and infrared spectra of MgAl2O4 spinel from density functional perturbation theory

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