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Magnesiostaurolite and zincostaurolite; mineral description with a petrogenetic and crystal-chemical update

Christian Chopin, Bruno Goffe, Luciano Ungaretti and Roberta Oberti
Magnesiostaurolite and zincostaurolite; mineral description with a petrogenetic and crystal-chemical update
European Journal of Mineralogy (February 2003) 15 (1): 167-176

Abstract

Magnesiostaurolite, ideally (super A) [] (sub 4) (super B) Mg (sub 4) (super C) Al (sub 16) (super D) (Al (sub 2) [] (sub 2) ) (super T) Si (sub 8) O (sub 40) (super X) [(OH) (sub 2) O (sub 6) ], occurs together with talc, clinochlore and kyanite (or alone, armouring corundum) as inclusions in pyrope megablasts from the ultra-high-pressure metamorphic terrane of the Dora-Maira massif, Italian Western Alps. It is transparent colourless in thin section; non pleochroic, biaxial with a 2V angle close to 90 degrees ; the birefringence is low (<0.010) with a mean value of n = 1.709(2) at 592 nm. A few crystals show a "tweed" texture under crossed nicols. The calculated density is 3.54 g/cm (super 3) . Zincostaurolite, ideally (super A) [] (sub 4) (super B) Zn (sub 4) (super C) Al (sub 16) (super D) (Al (sub 2) [] (sub 2) ) (super T) Si (sub 8) O (sub 40) (super X) [(OH) (sub 2) O (sub 6) ], occurs with kyanite, muscovite, margarite, + or -chloritoid, gahnite and either quartz or diaspore in a karst-filling meta-argillite of the Mesozoic Barrhorn series, Zermatt valley, Swiss Western Alps; it may have formed concurrently with kyanite from the breakdown of gahnite + pyrophyllite + diaspore, i.e. near 400 degrees C. Zincostaurolite is non pleochroic and biaxial positive, with positive elongation (gamma = c); alpha = 1.722(2) and gamma = 1.734(2) at 592 nm. The calculated density is 3.78 g/cm (super 3) . Both magnesiostaurolite and zincostaurolite samples are monoclinic, C2/m, with the beta angle equal or very close to 90 degrees , which implies a very low degree of cation order; this feature is confirmed by the nearly equal site-scattering values and the similar mean bond lengths refined at the relevant pairs of sites; it is unrelated to metamorphic grade. Electron and ion microprobe analyses were combined to new single-crystal structure refinements to give the following crystal-chemical formulae: magnesiostaurolite, (super A) (Fe (super 2+) (sub 0.16) Mg (sub 0.72) [] (sub 3.12) ) (super B) (Mg (sub 1.86) Li (sub 0.94) Zn (sub 0.02) [] (sub 1.18) ) (super C) (Al (sub 15.96) Ti (sub 0.04) ) (super D) (Al (sub 1.58) Mg (sub 0.45) [] (sub 1.97) ) (super T) (Si (sub 7.96) Al (sub 0.04) ) O (sub 40) (super X) [(OH) (sub 3.98) O (sub 4.02) ]; zincostaurolite, (super A) (Fe (super 2+) (sub 0.13) Mg (sub 0.10) [] (sub 3.77) ) (super B) (Zn (sub 2.45) Li (sub 0.51) Fe (super 2+) (sub 0.20) [] (sub 0.84) ) (super C) (Al (sub 15.98) Ti (sub 0.02) ) (super D) (Al (sub 1.95) Mg (sub 0.09) [] (sub 1.96) ) (super T) Si (sub 8) O (sub 40) (super X) [(OH) (sub 3.67) O (sub 4.33) ]. The magnesiostaurolite crystal, with several hundreds ppm BeO, probably shows the highest Li and Be contents ever measured in staurolite. The increased occupancy of the M4 octahedron at the expense of the T2 tetrahedron from Fe- or Zn-rich to Mg-rich staurolite may be the key to the Mg-staurolite paradox (a high-pressure phase with expectedly four-fold coordinated Mg) and to the complex thermodynamic behaviour of the staurolite series (e.g. reversal in Fe-Mg partitioning with garnet).


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 15
Serial Issue: 1
Title: Magnesiostaurolite and zincostaurolite; mineral description with a petrogenetic and crystal-chemical update
Affiliation: Ecole Normale Superieure, Laboratoire de Geologie, Paris, France
Pages: 167-176
Published: 200302
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 57
Accession Number: 2003-026006
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
N44°19'60" - N45°10'00", E07°04'60" - E07°25'00"
Secondary Affiliation: Consiglio Nazionalle delle Ricerche, ITA, ItalyUniversita di Pavia, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute.
Update Code: 200309
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