Virtually pure ottrelite from the region of Ottré, Belgium
Theye, Thomas; Fransolet, André-Mathieu
European Journal of Mineralogy Volume 6 Number 4 (1994), p. 547 - 556
published: Jul 27, 1994
manuscript accepted: Mar 7, 1994
manuscript received: Nov 30, 1993
ArtNo. ESP147050604003, Price: 29.00 €
Abstract For the first time, very Mn-rich chloritoid with a composition near the ottrelite end-member, Mn2+2Al4O2[SiO4]2(OH)4, has been found near the type locality Ottre in the Venn-Stavelot Massif (Belgium). Ottrelite occurs as minute inclusions within andalusite in metapelites that were metamorphosed at about 400°C, 2 kbar. The compositional spread of this ottrelite ranges from Mn2+96Fe2+3Mg1 to Mn2+65Fe2+4Mg31 (mol.% of divalent cations). Associated minerals are hematite, quartz, spessartine, manganian andalusite (1-12 mol.% kanonaite component), muscovite, paragonite, gahnite, and ardennite. Micro structural relationships indicate that the present mineral paragenesis comprising andalusite and spessartine was preceded by a paragenesis of ottrelite without garnet and andalusite. This indicates disappearance of ottrelite according to the reaction (1) ottrelite + quartz = andalusite + spessartine + H2O. Thermodynamic calculations suggest that pure ottrelite breaks down to andalusite and garnet according to reaction (1) at about 350°C (at 2 kbar). With increasing temperature, in the Mn-Mg system, chloritoid rapidly becomes enriched in Mg within a small temperature interval (XMg~ 0 to 0.4 within 20°C). Because of slow reaction rates at low temperature, chemical equilibrium was not attained, accounting for the compositional spread of the ottrelite under present consideration.