"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."