The experimental results of the title reaction were modeled as a nucleation and growth process. Nucleation is restricted to the lateral faces of the platelets of the parent hydrate (space group Cc), and growth is anisotropic, two rate constants (for lateral and normal growth) being introduced. The results are compared with the empirical description given by Avrami’s equation, and shown to rationalize the data adequately. In particular, the influence of water vapor pressure is discussed.