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Experimental and thermodynamic investigations on the stability of Mg (sub 14) Si (sub 5) O (sub 24) anhydrous phase B with relevance to Mg (sub 2) SiO (sub 4) forsterite, wadsleyite, and ringwoodite

Hiroshi Kojitani, Saki Terata, Maki Ohsawa, Daisuke Mori, Yoshiyuki Inaguma and Masaki Akaogi
Experimental and thermodynamic investigations on the stability of Mg (sub 14) Si (sub 5) O (sub 24) anhydrous phase B with relevance to Mg (sub 2) SiO (sub 4) forsterite, wadsleyite, and ringwoodite
American Mineralogist (October 2017) 102 (10): 2032-2044

Abstract

High-pressure high-temperature phase relation experiments in Mg (sub 14) Si (sub 5) O (sub 24) were performed using a 6-8 multi-anvil high-pressure apparatus in the pressure range of 12-22 GPa and temperature range of 1673-2173 K. We first found that Mg (sub 14) Si (sub 5) O (sub 24) anhydrous phase B (Anh-B) dissociates to Mg (sub 2) SiO (sub 4) wadsleysite (Wd) and MgO periclase (Per) at about 18 GPa and 1873 K. From the results of the high-pressure experiments, the phase boundaries of 5 Mg (sub 2) SiO (sub 4) forsterite (Fo) + 4 Per = Anh-B and Anh-B = 5 Wd + 4 Per were determined. In addition, the isobaric heat capacity (C (sub P) ) of Anh-B was measured by differential scanning calorimetry in the temperature range of 300-770 K and the thermal relaxation method using a Physical Property Measurement System (PPMS) in the range of 2-303 K. From the measured low-temperature C (sub P) , the standard entropy (So (sub 298.15) ) of Anh-B was determined to be 544.4(2) J/(mol.K). We also performed high-temperature X-ray diffraction measurements in the range 303-773 K to determine the thermal expansivity (alpha ) of Anh-B. The obtained C (sub P) and alpha were theoretically extrapolated to higher temperature region using a lattice vibrational model calculation partly based on Raman spectroscopic data. Thermodynamic calculations by adopting the thermochemical and thermoelastic data for Anh-B obtained in this study and the estimated formation enthalpy for Anh-B of -13 208 kJ/mol gave phase equilibrium boundaries for 5 Fo + 4 Per = Anh-B and Anh-B = 5 Wd + 4 Per that were consistent with those determined by the present high-pressure high-temperature experiments. The results clarified that, in the Mg (sub 14) Si (sub 5) O (sub 24) system, Anh-B is stable between 12 and 18 GPa at the expected temperatures of the Earth's mantle.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 102
Serial Issue: 10
Title: Experimental and thermodynamic investigations on the stability of Mg (sub 14) Si (sub 5) O (sub 24) anhydrous phase B with relevance to Mg (sub 2) SiO (sub 4) forsterite, wadsleyite, and ringwoodite
Affiliation: Gakushuin University, Department of Chemistry, Tokyo, Japan
Pages: 2032-2044
Published: 201710
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 53
Accession Number: 2017-090028
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 10 tables
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: 201747
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