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Sodium amphibole in the post-glaucophane high-pressure domain; the role of eckermannite

Harriet Howe, Alison R. Pawley and Mark D. Welch
Sodium amphibole in the post-glaucophane high-pressure domain; the role of eckermannite
American Mineralogist (June 2018) 103 (6): 989-992

Abstract

An amphibole close to eckermannite in composition, ideally Na (sub 3) Mg (sub 4) AlSi (sub 8) O (sub 22) (OH) (sub 2) , was encountered in experiments on a bulk composition close to that of glaucophane at 6.2 GPa, approximately 550-650 degrees C. The synthetic amphibole has an average composition corresponding to ANa (sub 0.96) B(Na (sub 1.80) Mg (sub 0.20) )C(Mg (sub 4) Al)T(Si (sub 7.85) Al (sub 0.15) ) O (sub 22) (OH) (sub 2) . This composition is displaced from that of end-member eckermannite by exchange vectors +0.15 (super B) Mg (super T) Al (super B) Na (sub -1) (super T) Si (sub -1) and +0.05 (super A) (super B) Mg (super A) Na (sub -1) (super B) Na (sub -1) ( = vacant site). In terms of end-members, it corresponds to 80% eckermannite + 15% Mg-katophorite, Na(NaMg)(Mg (sub 4) Al)(Si (sub 7) Al)O (sub 22) (OH) (sub 2) , + 5% Mg-winchite, (NaMg)(Mg (sub 4) Al)Si (sub 8) O (sub 22) (OH) (sub 2) , and as such is essentially binary. The absence of a glaucophane component implies that the stability of sodium amphibole at very high pressures (>4 GPa) involves eckermannitic rather than glaucophanic compositions. The stabilization of the eckermannite-pyrope tie line allows this highly Na-rich amphibole to occur even in bulk compositions that are not particularly Na-rich. In blueschist facies metabasites, it is possible that eckermannite forms by the reaction 9 jadeite + 7 talc --> 3 eckermannite + 3 pyrope + 13 coesite + 4 H (sub 2) O, above the stability limit of glaucophane that is defined by the reaction glaucophane --> 2 jadeite + talc.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 103
Serial Issue: 6
Title: Sodium amphibole in the post-glaucophane high-pressure domain; the role of eckermannite
Affiliation: University of Manchester, School of Earth and Environmental Sciences, Manchester, United Kingdom
Pages: 989-992
Published: 20180601
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 20
Accession Number: 2018-052880
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Secondary Affiliation: Natural History Museum, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2018
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