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Precipitates of alpha -cristobalite and silicate glass in UHP clinopyroxene from a Bohemian Massif eclogite

Tina R. Hill, Hiromi Konishi, Franklin Hobbs, Seungyeol Lee and Huifang Xu
Precipitates of alpha -cristobalite and silicate glass in UHP clinopyroxene from a Bohemian Massif eclogite
American Mineralogist (October 2019) 104 (10): 1402-1415

Abstract

Low-pressure metastable nanoscale crystals of alpha -cristobalite have been observed epitaxially ex-solved in cores of UHP clinopyroxene from the Bohemian Massif, Czech Republic. SAED patterns and HRTEM images detail the close structural relationship between host clinopyroxene and alpha -cristobalite precipitate: [001] (sub Di) [010]alpha , (010) (sub Di) approximately (101)alpha . TEM results indicate that alpha -cristobalite exsolved from host clinopyroxene. Non-crystalline Al-bearing silicate phases, also exsolved from UHP clinopyroxene, possesses Al/Si ratios close to eutectic compositions in the system NaAlSi (sub 3) O (sub 8) -SiO (sub 2) -H (sub 2) O system. The presence of glass exsolution suggests a high-temperature formation environment and presence of water. The alpha -cristobalite formed in a localized low-pressure, micro-environment formed through exsolution of vacancies and excess silica from the host pyroxene lattice. This micro-environment may be a result of negative density changes due to excess lower density silica exsolving from higher density pyroxene during an exsolution process that involved no localized volume change. Interface-controlled exsolution via lattice matching at the diopside/cristobalite interface, and stability changes and melting point depression due to nanoscale size effects contributed to the formation and persistence of this metastable phase. Amphibole in association with alpha -cristobalite and some non-crystalline silicate phases may be a clue to localized water quantities; silica exsolution with amphibole may have formed below the eutectic temperature and at a later stage than non-crystalline silicate phases without amphibole. Silica rods in Nove Dvory clinopyroxenes were previously thought to be quartz; however, our investigation reveals various low-pressure, high-temperature, and/or metastable phases greatly affected by the presence of vacancy and OH in clinopyroxenes. The results will help us better understand OH in the UHP pyroxene and even water release in the mantle.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 104
Serial Issue: 10
Title: Precipitates of alpha -cristobalite and silicate glass in UHP clinopyroxene from a Bohemian Massif eclogite
Affiliation: University of Wisconsin-Madison, Department of Geoscience, Madison, WI, United States
Pages: 1402-1415
Published: 201910
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 85
Accession Number: 2019-097890
Categories: General geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
N48°34'60" - N51°02'60", E12°22'60" - E18°53'60"
Secondary Affiliation: Bruker Nano, Madison, Wi, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2019
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