The Rarity of Chloritoid ... Animation

The Rarity of Chloritoid

It can be seen that the paragenesis of the metapelites in the greenschist facies of a MP gradient are poorly varied. These are mainly Chl + Bt + Ms + Qtz micaschists to which garnet is added at the transition to the amphibolite facies: Chl + Bt + Ms + Grt + Qtz. However, several aluminous minerals are stable at the beginning of greenschist facies conditions (chloritoid, kyanite, staurolite), but they remain rare before the amphibolite facies.

The AFM diagrams clearly show that the occurrence or absence of chloritoid in the metapelitic series depends on the chemical composition of the rocks: aluminous or weakly aluminous. Metapelites and gneisses are often derived from low-aluminous sandstone-pelitic series (red point). The same is true for the other aluminous phases, kyanite and staurolite, which are generally lacking in greenschist facies.