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Crystal chemistry of synthetic Mn-bearing anorthite; incorporation of MnAl (sub 2) Si (sub 2) O (sub 8) end-member into feldspar

Tomoaki Matsui and Mitsuyoshi Kimata
Crystal chemistry of synthetic Mn-bearing anorthite; incorporation of MnAl (sub 2) Si (sub 2) O (sub 8) end-member into feldspar (in EMPG-VI; Experimental mineralogy, petrology and geochemistry, E. A. J. Burke (editor), C. Chopin (editor), W. V. Maresch (editor), L. Ungaretti (editor) and C. Willaime (editor))
European Journal of Mineralogy (April 1997) 9 (2): 333-344

Abstract

A series of feldspars with intermediate compositions along the join CaAl (sub 2) Si (sub 2) O (sub 4) -MnAl (sub 2) Si (sub 2) O (sub 4) have been synthesized at 1550 degrees C in an N (sub 2) atmosphere, followed by rapid cooling. Mn may play a role as a mineralizer for megacrystallization of anorthite. The Mn appears to be incorporated into the extra-framework M site of the feldspar structure, with a maximum limit of approximately 25 mol.% MnAl (sub 2) Si (sub 2) O (sub 4) . The Mn-bearing anorthite Ca (sub 0.715) Mn (sub 0.196) Na (sub 0.045<P6M><$B0><P255MV>0.044) Al (sub 1.867) Si (sub 2.133) O (sub 4) has a 8.131, b 12.847, c 7.069 Aa, alpha 94.03, beta 115.89, Gamma 90.72 degrees , V 661.8 Aa (super 3) , space group C<$E 1 back 40 up 40 ->, Z = 4; its structure has been refined to R 3.9% and shows the distribution of Ca and Mn over three split M sites together with disordered Al/Si distribution. Solid solution of the <P6M><$B0><P255D>Si (sub 4) O (sub 8) end-member (the excess silica component) contributes to the shrinkage of the M site making it more favourable for Mn to enter the anorthite structure. The rarity of natural Mn feldspars cannot therefore be attributed to an intrinsic exclusion of Mn from the structure, but to the unusual combination of T, P and chemical conditions required for its crystallization.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 9
Serial Issue: 2
Title: Crystal chemistry of synthetic Mn-bearing anorthite; incorporation of MnAl (sub 2) Si (sub 2) O (sub 8) end-member into feldspar
Title: EMPG-VI; Experimental mineralogy, petrology and geochemistry
Author(s): Matsui, TomoakiKimata, Mitsuyoshi
Author(s): Burke, E. A. J.editor
Author(s): Chopin, C.editor
Author(s): Maresch, W. V.editor
Author(s): Ungaretti, L.editor
Author(s): Willaime, C.editor
Affiliation: University of Tsukuba, Institute of Geoscience, Ibaraki, Japan
Pages: 333-344
Published: 199704
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
Meeting name: Sixth international symposium on Experimental mineralogy, petrology and geochemistry
Meeting location: Bayreuth, DEU, Federal Republic of Germany
Meeting date: 19960410April 10-13, 1996
References: 44
Accession Number: 1997-036947
Categories: Mineralogy of silicates
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: 8 tables
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 199712
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