NEW CHEMICAL AND PHYSICAL DATA FROM DETRITICAL MN-CHLORITOID AND OTTRELITE GRAINS DERIVED FROM THE CAMBRIAN STAVELOT-MASSIF, BELGIUM

New microprobe and SEM analyses of some extremely rare, yellow, detrit ical grains of Mn-rich chloritoid derived from the metamorphic rocks o f the Cambrian Stavelot Massif, Belgium, confirm the existence of ottr elite as defined by BUSTAMANTE-SANTA CRUZ (1974, 1991). The grains con tain 15.1 to 18.6 wt % MnO, corresponding to an occupation of the diva lent, octahedral Fe,Mg,Mn-site with 52 to 63 at% Mn. They are composed of a submicroscopic mixture of monoclinic and triclinic polymorphs wi th crystallographic unit cell parameters that are only slightly larger than those of Fe-chloritoid. Their infra-red spectra are comparable t o those of common Fe - Mg - chloritoid. Their DTA decomposition temper atures are systematically lower than those of Fe-chloritoid, but simil ar to those of Mg - chloritoid. Comparison with other Mn - chloritoids shows that the ottrelites are the Mn-richest members of a more or les s continuous solid solution series in which the substitution of Fe by Mn seems to be coupled with Mg, in such a way that 1 Fe is replaced by approximately 0.7 Mn + 0.3 Mg. The absence of intermediate Fe-Mg-Mn c ompositions suggests the existence of a miscibility gap between the Fe -Mg chloritoids and the Mn - chloritoids/ottrelites.