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Effects of Fe (super 2+) and Fe (super 3+) contents on cation ordering in omphacite

Fernando Camara, Fernando Nieto and Roberta Oberti
Effects of Fe (super 2+) and Fe (super 3+) contents on cation ordering in omphacite
European Journal of Mineralogy (October 1998) 10 (5): 889-906


An EPMA, TEM and single-crystal structure-refinement study of omphacites and aegirine-augites with the aegirine mole fraction (X (sub Ae) ) in the range 0-33 % (X (sub Ae + Hd) 15-47 %) is reported. The degree of order is lower than expected on the basis of both the Na contents and the T estimates from inter-crystalline exchange geothermometry; the difference is inversely proportional to X (sub Ae + Hd) (X (sub Ae) being the most important component) and a semi-quantitative relationship between X (sub Ae + Hd) and the degree of order is given. Analyses on thin sections shows that compositional zoning with decreasing X (sub Ae) corresponds with increasing degree of order. Structure refinements after annealing at various T on crystals with similar Na contents but increasing X (sub Ae + Hd) indicate that the latter affects the kinetics of the process but not the critical T of order. The Fe (super 3+) Al (sub -1) and Fe (super 2+) Mg (sub -1) substitutions affect the degree of order in omphacite because, with increasing Fe contents, the two independent octahedra in the P2/n space group becomes more and more similar due to the decreasing difference in the ionic radii involved. Thus the space group is C2/c for X (sub Ae) > 20 % and X (sub Ae + Hd) = 47 %, but is P2/n in omphacite with the same X (sub Ae) but higher X (sub Ae + Hd) . A Delta G (super *) value for anti-phase domain coarsening in Fe-rich omphacite has been derived from TEM data and T-T-t diagrams; it is approximately 20 kcal/mole higher than that proposed for Fe-free omphacite.

ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 10
Serial Issue: 5
Title: Effects of Fe (super 2+) and Fe (super 3+) contents on cation ordering in omphacite
Affiliation: CNR-Centro di Studio per la Cristallochimica e la Cristallografia, Pavia, Italy
Pages: 889-906
Published: 199810
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 31
Accession Number: 1998-070407
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
Secondary Affiliation: Universidad de Granada, ESP, Spain
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 199824
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