An investigation of three minerals in the Ca₂Fe_2-xAl_xO₅ system using a new method in quantum chemistry

Three minerals in the Ca₂Fe_2-xAl_xO₅ system (Ca₂Fe₂O₅(C₂F), Ca₂Fe_1·43Alos_0·57O₅ and Ca₂Fe_1·28Al_0·72O₅) are studied by means of a new method in quantum chemistry, the self:consistent charge-discrete variational X_a (SCCDVX_a) method. By analysing the energy of their molecular orbits, along with net charge, population and covalent order, we explain the improved hydration activity of Ca₂Fe_2-xAl_xO₅ as Al content increases as follows: first, it is easy to accept the lone-pair electrons of water so that the reaction with water can take place because the energy of the lowest unoccupied molecular orbit decreases; second, it is easy to absorb the lone-pair electrons because the net charge of Al is higher than that of Fe; and third, the chemical bonds can be broken easily to bring about a reaction with water because the covalent bond of Al—O is weaker than that of Fe—O.