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, detritical grains of Mn-rich chloritoid derived from the metamorphic rocks of the Cambrian Stavelot Massif, Belgium, confirm the existence of ottrelite as defined by BUSTAMANTE-SANTA CRUZ (1974, 1991). The grains contain 15.1 to 18.6 wt % MnO, corresponding to an occupation of the divalent, octahedral Fe,Mg,Mn-site with 52 to 63 at% Mn. They are composed of a submicroscopic mixture of monoclinic and triclinic polymorphs with crystallographic unit cell parameters that are only slightly larger than those of Fe-chloritoid. Their infra-red spectra are comparable to those of common Fe - Mg - chloritoid. Their DTA decomposition temperatures are systematically lower than those of Fe-chloritoid, but similar to those of Mg - chloritoid. Comparison with other Mn - chloritoids shows that the ottrelites are the Mn-richest members of a more or less 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 compositions suggests the existence of a miscibility gap between the Fe-Mg chloritoids and the Mn - chloritoids/ottrelites.