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Ferromagnesian jeffbenite synthesized at 15 GPa and 1200 degrees C

Joseph R. Smyth, Fei Wang, E. Ercan Alp, Aaron S. Bell, Esther S. Posner and Steven D. Jacobsen
Ferromagnesian jeffbenite synthesized at 15 GPa and 1200 degrees C
American Mineralogist (March 2022) 107 (3): 405-412


Single crystals of Al-free, ferromagnesian jeffbenite up to 200 mu m in size have been synthesized at 15 GPa and 1200 degrees C in a 1200 tonne multi-anvil press from a starting composition in the forsteritefayalite-magnetite-water system. This phase has the approximate formula Mg (sub 2.62) Fe (sub 0.87) (super 2+) Fe (sub 1.63) (super 3+) Si (sub 2.88) O (sub 12) and is observed to coexist with a Ca-free clinopyroxene plus what appears to be quenched melt. The crystal structure has been refined from single-crystal X-ray diffraction data and is similar to that determined for natural Al-bearing jeffbenite, Mg (sub 3) Al (sub 2) Si (sub 3) O (sub 12) , reported from inclusions in superdeep diamonds. The structure is a tetragonal orthosilicate in space group I42d with a=6.6449(4) Aa, c=18.4823(14) Aa, and is structurally more closely related to zircon than to garnet. The T2 site is larger than T1, shares an edge with the M2 octahedron, and incorporates significant Fe (super 3+) . Because of the tetrahedral incorporation of trivalent cations, jeffbenite appears to be compositionally distinct from garnet. Previous speculations that the phase may only occur as a retrograde decompression product from bridgmanite are not supported by its direct synthesis under transition zone conditions. The phase has a calculated density of 3.93 g/cm (super 3) , which is indistinguishable from a garnet of comparable composition, and is a possible component in the mantle transition zone under oxidizing conditions or with Al-rich compositions.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 107
Serial Issue: 3
Title: Ferromagnesian jeffbenite synthesized at 15 GPa and 1200 degrees C
Affiliation: University of Colorado, Department of Geological Sciences, Boulder, CO, United States
Pages: 405-412
Published: 20220301
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 29
Accession Number: 2022-018333
Categories: Mineralogy of silicatesGeophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Northwestern University, USA, United StatesArgonne National Laboratory, USA, United StatesUniversitaet Bayreuth, DEU, Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2022, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2022
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