When the reaction proceeds further, after 120 h
degradation, the pseudomorphosis zones which
appeared as yellowish spots in Fig. 22 are now
principally black spots in Fig. 24. These spots stay
dark under crossed polars (Fig. 25). This optical
characteristic change shows an amorphization of the
flint aggregate. Furthermore, the grain size has greatly
decreased and their outlines are very irregular. The
heterogeneity of the reaction is observed in Fig. 25 with
a few grains which retained their initial optical properties
(grains stay light on optical microscopy with
crossed polars)