VIRTUALLY PURE OTTRELITE FROM THE REGION OF OTTRE, BELGIUM

For the first time, very Mn-rich chloritoid with a composition near th e ottrelite end-member, Mn2+2Al4O2[SiO4]2(OH)4, has been found near th e type locality Ottre in the Venn-Stavelot Massif (Belgium). Ottrelite occurs as minute inclusions within andalusite in metapelites that wer e metamorphosed at about 400-degrees-C, 2 kbar. The compositional spre ad of this ottrelite ranges from Mn2+96Fe2+3Mg1 to Mn2+65Fe2+4Mg31 (Mo l.% Of divalent cations). Associated minerals are hematite, quartz, sp essartine, manganian andalusite (1-12 mol.% kanonaite component), musc ovite, paragonite, gahnite, and ardennite. Microstructural relationshi ps indicate that the present mineral paragenesis comprising andalusite and spessartine was preceded by a paragenesis of ottrelite without ga rnet and andalusite. This indicates disappearance of ottrelite accordi ng to the reaction (1) ottrelite + quartz = andalusite + spessartine H2O. Thermodynamic calculations suggest that pure ottrelite breaks do wn to andalusite and garnet according to reaction (1) at about 350-deg rees-C (at 2 kbar). With increasing temperature, in the Mn-Mg system, chloritoid rapidly becomes enriched in Mg within a small temperature i nterval (X(Mg) approximately 0 to 0.4 within 20-degrees-C). Because of slow reaction rates at low temperature, chemical equilibrium was not attained, accounting for the compositional spread of the ottrelite und er present consideration.