Petrology of Chloritoid and Staurolite Rocks North of Stonehaven, Kincardineshire

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Abstract
In the pelitic rocks of this classical section it appears that, chemically, a Fe2O3: Al2O3 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.