Two arbitrarily defined types of microstructure were identified in the binder phase: (1) domains, which were regions of common basal plane align­ment extending over linear dimensions >100 pm and (2) mosaics, which were regions of small ran­domly oriented pseudocrystallites with linear dimen­sions of common basal plane orientation of less than about 10 pm. Cleavage of domains occurred at stres­ses well below the fracture stress, and such regions acted as sites for crack initiation, particularly when in the vicinity of pores. Fracture of mosaic regions was usually observed only at stresses close to the fracture stress. At lower stresses, propagating cracks that encountered such regions were arrested or deflected.