Abstract: A IIb orthochlorite (brunsvigite), from the chloritic metabasalt in the Middletown Valley of Maryland, was altered to a regularly interstratified chlorite—vermiculite after four months reaction in saturated bromine water on the steambath. The artificial weathering product resembles the regularly interstratified chlorite—vermiculite in a soil in Adams County, Pennsylvania, that has developed in greenstone similar to the chloritic metabasalt in the Middletown Valley of Maryland. The results of this and previous investigations on artificial and natural weathering of chlorites indicate that oxidation of structural Fe2+ is an important reaction in the alteration of chlorites into vermiculitic products. The results also show that IIb orthochlorites, apparently structurally similar but having different Fe2+ contents may react differently under the same weathering conditions. For example the acid oxidation treatment altered a IIb chlorite (diabantite) directly into a vermiculite, whereas the same treatment altered another IIb chlorite (brunsvigite) into a regularly interstratified chlorite—vermiculite. Detailed investigations of the chlorite structures are probably necessary to determine whether or not these alteration processes are structurally controlled.