Structural complexity of uranophane and uranophane-beta: implications for their formation and occurrence

Information-based parameters of structural complexity for two polymorphs of Ca[(UO2)(2)(SiO3OH)(2)] center dot 5H(2)O, uranophane and uranophane-beta, were calculated in order to assess the impact of the structural complexity on their stability and formation. Calculations proved that uranophane-beta is far more complex (I-G,I-total= 691.90 bits/cell) than uranophane (I-G,I-total= 345.95 bits/cell), which indirectly suggests that uranophane may form as a metastable phase during crystallization process. This suggestion is further supported by the lower density of uranophane (3.834 g/cm(3)) compared to that of uranophane-beta (3.983 g/cm(3)). The hypothesis of a metastable character of uranophane is in agreement with Goldsmith's simplexity principle and the Ostwald-Volmer rule and is well supported by the frequency of occurrences of both dimorphic minerals in Nature.