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European Journal of Mineralogy; April/May; v. 21; no. 2; p. 397-406; DOI: 10.1127/0935-1221/2009/0021-1894
© 2009 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Articles

Ti substitution mechanisms in phlogopites from the Suwalki massif-type anorthosite, NE Poland

Olivier Namur1, Frédéric Hatert2,*, Fernande Grandjean3, Gary J. Long4, Natacha Krins5, André-Mathieu Fransolet2, Jacqueline Vander Auwera1 and Bernard Charlier1

1 Département de Géologie, UR Pétrologie et Géochimie endogènes, B20, Université de Liège, 4000 Liège, Belgium
2 Département de Géologie, Laboratoire de Minéralogie, B18, Université de Liège, 4000 Liège, Belgium
3 Département de Physique, B5, Université de Liège, 4000 Liège, Belgium
4 Department of Chemistry, University of Missouri-Rolla, Missouri 65409-0010, USA
5 Département de Chimie, Laboratoire de Chimie inorganique structurale (LCIS), B6, Université de Liège, 4000 Liège, Belgium

* Corresponding author, e-mail: fhatert{at}ulg.ac.be

Intercumulus titanian phlogopite occurs in leuco- and gabbronoritic cumulates from the Suwalki anorthosite massif, NE Poland. The degree of Ti enrichment in the micas ranges from 2.59 to 9.41 wt.% TiO2. The chemical composition is highly variable for several other elements: Al2O3 (13.07–16.75 wt.%), K2O (7.90–10.16 wt.%), FeOtot (6.92–16.69 wt.%), Fe2O3 (0.82–2.95 wt.%), and MgO (9.86–19.54 wt.%), with a Mg/(Fe + Mg) ratio ranging from 0.47 to 0.83. Substitution mechanisms for Ti are proposed, which suggest the presence of exchange vectors involving octahedral and tetrahedral cations. In samples characterized by low Ti contents (0.147–0.239 Ti a.p.f.u.), the Ti incorporation mechanism is: [6]Ti4+ + [6]{square} = 2([6]Mg2+, [6]Fe2+, [6]Mn2+), where [6]{square} corresponds to a vacancy in octahedral coordination (Ti-vacancy). In the two groups with intermediate (0.164–0.326 Ti a.p.f.u.) and high Ti contents (0.477–0.532 Ti a.p.f.u.), the Ti substitution mechanism corresponds to the reaction: [6]Ti4+ + 2([4]Al3+, [4]Fe3+) = ([6]Mg2+, [6]Fe2+, [6]Mn2+) + 2[4]Si4+ (Ti-Tschermak). The Mössbauer spectral investigation shows that 0.046–0.167 a.p.f.u. Fe3+ occur on the octahedral sites of the structure. The substitution mechanism responsible for the incorporation of Fe3+ in phlogopites from Suwalki is 3([6]Mg2+, [6]Fe2+) = 2([6]Al3+, [6]Fe3+) + [6](M3+-vacancy). The use of the Ti content of phlogopite as geothermometer reveals crystallization temperatures from 729 ± 15 to 874 ± 15 °C for the phlogopites.

Key-words: phlogopite, titanium, substitution mechanisms, Mössbauer spectroscopy, geothermometry.






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