PREDICTION OF ENTHALPY OF FORMATION BASED ON REFINED CRYSTAL-STRUCTURES OF MULTISITE COMPOUNDS .1. THEORIES AND EXAMPLES

The method of prediction of enthalpy of formation is based on the parameter Delta HO2- cation (comp). The calculation of this parameter requires the knowledge of crystallographical and optical properties of minerals: refractive index, molecular volume, average distances, and shortest bond lengths of cation-oxygen, in all sites involved in the compounds. From the initial work on the two oxides minerals and extended to three oxide and three site bearing compounds valid for sites occupied by a single cation, the generalisation of the method of prediction to multisite compounds has been undertaken by proposing some new considerations in the calculation of Delta H(f,oxides)degrees which are the following: 1) make the distinction between sites occupied either by one either by more than one cation; 2) use a new formulation of the parameter Delta HO2- cation (comp) of sites occupied by more than one cation; 3) consider the existence in some polyhedra, of extra long bond length cation oxygen; 4) take into consideration the nonoxygen bridges between two nearest neighbor polyhedra, necessary to the calculation of the enthalpy of formation from the constituent oxides; 5) introduce an evaluation of the prediction error related to the error standard deviation of measured bond lengths. This method has been tested on a great number of minerals and artificial compounds belonging to the system Li2O-Na2O-K2O-BeO-MgO-CaO-MnO-FeO-Fe2O3-Al2O3-SiO2-H2O. An example of calculation of Delta H(f,oxides)degrees and Delta H-f degrees has been given in detail with a new evaluation of the error of prediction on a well-known mineral, muscovite.