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 analysesthat are considered reliable, indicate that the chloritoid groupcan be represented by the general formula H₂FeAI₂SiO₇ with upto two-fifths of the ferrous iron replaced by magnesium, upto one-sixth by manganese, and up to one-seventh of the aluminiumreplaced by ferric iron. Chloritoid crystallizes in both monoclinicand triclinic polymorphs. The triclinic unit cell is one-halfthe monoclinic unit cell. Both polymorphs are widely distributed;they may be distinguished by their X-ray diffraction powderpatterns. In most monoclinic chloritoids the optic plane isnormal to (010), whereas in most triclinic chloritoids it isnearly parallel to (010). Replacement of iron by magnesium lowersthe indices of refraction. With increasing temperatures chloritoid breaks down to ironcordierite+hercynite+vapour at low pressures and to staurolite+almandine+hercynite+vapourat high pressures. Chloritoid was synthesized only at pressuresof about 10,000 bars, but natural chloritoids were stable athigher pressures. At lower pressures chloritoid was not synthesizedbecause of the persistence of a metastable chamosite with a7 ? basal spacing. Natural chloritoids did not decompose belowabout 600? C at these lower pressures. Stress as defined byHarker is not necessary for the growth of chloritoid. Chloritoid-bearing rocks have a high content of alumina relativeto potash, soda, lime, and mafic components, and have more ferrousoxide than magnesium oxide. In pelitic and lateritic sedimentshaving these characteristics, chloritoid is one of the firstnew 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 hydrothermalveins. Various reactions in which chloritoid is produced orconsumed are presented. ¹Present address: Research Council of Alberta, Edmonton, Alberta, Canada.