An Investigation of the Stability Relations of the Al-Fe Members of the Epidote Group

An attempt was made to define the stability limits and conditions of formation of the Al-Fe members of the epidote group. The various types of occurrences are discussed. Decomposition of zoisite was achieved at 475° C. in a hydrous environment in the presence of excess alumina and silica. Zoisite synthesis was not accomplished. It was found that above about 400° C. in a hydrous environment anorthite and hydro-grossularite were formed from a starting material of zoisite composition, whereas in the approximate range of 400°-200° C. a hexagonal modification of CaAl₂SiO₈ was usually also formed. Below 300° C. anorthite and hydrogrossularite were not produced. A Na-free thomsonite was occasionally encountered below 250° C. It is thought that zoisite synthesis might be achieved by inhibiting crystallization of CaAl₂Si₂O₈..