"In the pelitic rocks of this classical section it appears that, chemically, a Fe 2 O 3 :Al 2 O 3 ratio of about 0.40 is critical for the formation of chloritoid or staurolite. With this ratio both minerals are possible, but if the value exceeds a figure somewhere between 0.47 and 0.55 staurolite cannot form, a reddish-brown biotite appearing instead. Physically, a certain minimum temperature is necessary before staurolite can appear and below this chloritoid is the equilibrium phase."

GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States