Virtually pure ottrelite from the region of Ottre, Belgium

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doi: 10.1127/ejm/6/4/0547
Authors:Theye, Thomas; Fransolet, André-Mathieu
Author Affiliations:Primary:
Technische Universitat Braunschweig, Institut fur Geowissenschaften, Brunswick, Federal Republic of Germany
Universite de Liege, Belgium
Volume Title:European Journal of Mineralogy
Source:European Journal of Mineralogy, 6(4), p.547-555. Publisher: Schweizerbart'sche Verlagsbuchhandlung (Nägele u. Obermiller), Stuttgart, Federal Republic of Germany. ISSN: 0935-1221
Publication Date:1994
Note:In English. 33 refs.; illus., incl. 1 table
Summary:Very Mn-rich chloritoid with a composition near the ottrelite end-member, Mn 2+ 2 Al4O2[SiO4]2(OH)4, has been found for the first time, near the type locality Ottre in the Venn-Stavelot massif. The ottrelite occurs as minute inclusions in andalusite in metapelites that were metamorphosed at ∼ 400°C. The compositional spread of this ottrelite ranges from Mn 2+ 96 Fe 2+ 3 Mg1 to Mn 2+ 65 Fe 2+ 4 Mg31 (mol.% of divalent cations). Associated minerals are hematite, quartz, spessartine, manganian andalusite (1-12 mol.% kanonaite), muscovite, paragonite, gahnite and ardennite. Microstructural relationships are interpreted to indicate that the present mineral paragenesis with andalusite and spessartine was preceded by a paragenesis of ottrelite without garnet and andalusite, implying the disappearance of ottrelite according to the reaction ottrelite + quartz = andalusite + spessartine + H2O; thermodynamic calculations suggest that this reaction occurs at ∼ 350°C (at 2 kbar). With increasing T, chloritoid in the Mn-Mg system rapidly becomes enriched in Mg within a small T interval (XMg ∼ 0 to 0.4 within 20°C). Because of slow reaction rates at low T, chemical equilibrium was not attained, accounting for the compositional spread of this ottrelite. [R.A.H.]
Subjects:Andalusite; Genesis; Nesosilicates; Orthosilicates; Silicates; Ardennes; Belgium; Europe; Stavelot-Venn Massif; Western Europe; Ottre Belgium; Ottrelite
Abstract Numbers:95M/782
Record ID:1995001812
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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