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Phase diagram and P-V-T equation of state of Al-bearing seifertite at lowermost mantle conditions

Denis Andrault, Reidar G. Tronnes, Zuzana Konopkova, Wolfgang Morgenroth, Hanns P. Liermann, Guillaume Morard and Mohamed Mezouar
Phase diagram and P-V-T equation of state of Al-bearing seifertite at lowermost mantle conditions
American Mineralogist (October 2014) 99 (10): 2035-2042

Abstract

We investigated the properties of Al-bearing SiO (sub 2) (with 4 or 6 wt% Al (sub 2) O (sub 3) ) at pressures and temperatures corresponding to the lowermost mantle, using laser-heated diamond-anvil cell coupled with synchrotron-based in situ X-ray diffraction. The phase transition from CaCl (sub 2) -structured to alpha -PbO (sub 2) -structured (seifertite) polymorphs occurs between 113 and 119 GPa at 2500 K. The range of pressure where the two phases coexist is small. There is a slight decrease of the transition pressure with increasing Al-content. We propose a tentative phase diagram reporting the minerals composition as a function of pressure in the SiO (sub 2) -Al (sub 2) O (sub 3) system. We also refine the P-V-T equation of state of Al-bearing seifertite based on volume measurements up to more than 160 GPa and 4000 K [V (sub 0) = 92.73(10) Aa (super 3) , K (sub 0) = 304.2(3.0) GPa, K' (sub 0) = 4.59 (fixed), Theta (sub D0) = 1130 K (fixed), gamma (sub 0) = 1.61(3)]. At 300 K, the volume decrease at the CaCl (sub 2) to alpha -PbO (sub 2) transition is 0.5(1)%, a value slightly lower than the 0.6% reported previously for Al-free samples. At high temperature, the Gruneisen parameter of seifertite is found to be similar to that of stishovite. Nevertheless, the Delta V/V across the CaCl (sub 2) -form to seifertite transition is found to increase slightly with increasing temperature. Across the phase transition, volume changes can be translated into density changes only when the Al substitution mechanisms in both CaCl (sub 2) -form and seifertite are defined. The analysis of all available data sets suggests different substitution mechanisms for the two SiO (sub 2) polymorphs. Al-substitution could occur via O-vacancies in the CaCl (sub 2) -form and via extra interstitial Al in seifertite. This would result in a density increase of 2.2(3)% at 300 K for SiO (sub 2) in basaltic lithologies. Alternatively, the same Al-substitution mechanism in both of the SiO (sub 2) -dominated phases would yield a density increase of 0.5(1)%.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 99
Serial Issue: 10
Title: Phase diagram and P-V-T equation of state of Al-bearing seifertite at lowermost mantle conditions
Affiliation: Universite B. Pascal, Laboratoire Magmas et Volcans, Clermont-Ferrand, France
Pages: 2035-2042
Published: 201410
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 31
Accession Number: 2014-099437
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: University of Oslo, NOR, NorwayDeutsches Elektronen-Synchrotron, DEU, GermanyGoethe University of Frankfurt, DEU, GermanyInstitut de Mineralogie, de Physique des Mineraux et de Cosmochimie, FRA, FranceEuropean Synchrotron Radiation Facility, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201450
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