DETERMINATION OF THE ORIGIN AND MAGNITUDE OF AL SI ORDERING ENTHALPY IN FRAMEWORK ALUMINOSILICATES FROM AB-INITIO CALCULATIONS/

Ab initio total energy calculations based on a new optimised oxygen ps eudopotential has been used to determine the enthalpy of disorder for the exchange of Al and Si in tetrahedral coordination in simple deriva tive aluminosilicate structures based on the high temperature tridymit e structure. The problem has been studied as a function of defect inte raction, and defect concentration, and the results indicate that the e nergy for Al/Al neighbouring tetrahedra can be assigned primarily to t wo effects, the first, a coulombic effect, associated with the disturb ed charge distribution. and the second associated with the strain rela ted to misfit due to the very different dimensions of the Si and Al co ntaining tetrahedra. In practice each of these effects contributes app roximately 0.2 eV per Al-Al neighbour to the overal disorder enthalpy. These simple results were obtained after a careful study of possible chemical interaction between adjacent Al/Si containing tetrahedra whic h showed that chemical interaction was effectively absent. Since indiv idual Al/Si tetrahedra proved to be discrete entities that an individu ally heavily screened by the shared oxygens it follows that coulombic and strain effects in disorder effectively account for the whole of th e disorder enthalpy. The complete set of results have been used to est ablish new criteria for the structure and disorder enthalpies of the f eldspar group of minerals and their long period derivatives.