Chloritoid: Its Composition, X-ray and Optical Properties, Stability, and Occurrence

The nomenclature of the chloritoid group, which includes ottrelite, is reviewed. Five new chemical analyses, and all published analyses that are considered reliable, indicate that the chloritoid group can be represented by the general formula H 2 FeAI 2 SiO 7 with up to two-fifths of the ferrous iron replaced by magnesium, up to one-sixth by manganese, and up to one-seventh of the aluminium replaced by ferric iron. Chloritoid crystallizes in both monoclinic and triclinic polymorphs. The triclinic unit cell is one-half the monoclinic unit cell. Both polymorphs are widely distributed; they may be distinguished by their X-ray diffraction powder patterns. In most monoclinic chloritoids the optic plane is normal to (010), whereas in most triclinic chloritoids it is nearly parallel to (010). Replacement of iron by magnesium lowers the indices of refraction. With increasing temperatures chloritoid breaks down to iron cordierite+hercynite+vapour at low pressures and to staurolite+almandine+hercynite+vapour at high pressures. Chloritoid was synthesized only at pressures of about 10,000 bars, but natural chloritoids were stable at higher pressures. At lower pressures chloritoid was not synthesized because of the persistence of a metastable chamosite with a 7 Å basal spacing. Natural chloritoids did not decompose below about 600° C at these lower pressures. Stress as defined by Harker is not necessary for the growth of chloritoid. Chloritoid-bearing rocks have a high content of alumina relative to potash, soda, lime, and mafic components, and have more ferrous oxide than magnesium oxide. In pelitic and lateritic sediments having these characteristics, chloritoid is one of the first new minerals to form during regional or contact metamorphism. In higher metamorphic grades it is accompanied by cordierite, andalusite, kyanite, or staurolite. It also grows in hydrothermal veins. Various reactions in which chloritoid is produced or consumed are presented.