ORIENTATION, LOCATION, AND TOTAL-ENERGY OF HYDRATION OF CHANNEL H2O IN CORDIERITE INVESTIGATED BY AB-INITIO TOTAL-ENERGY CALCULATIONS

Ab initio total energy calculations, based on the local density approximation, have been performed for hydrated Mg end-member cordierite to elucidate the location, orientation, and total energy of hydration in the ground state. In contrast to diffraction studies, but consonant with the interpretation of infrared experiments, the calculations demonstrate that the energetically most stable orientation of the H2O molecule in alkali-free Mg end-member cordierite is with the proton-proton vector parallel to [001]. The H2O molecule in cordierite is calculated to be nearly undistorted, and there is only very weak H bonding, again in contrast to the structural studies and a recent interpretation of a calorimetric study, but in agreement with the interpretation of spectroscopic and quasi-elastic neutron scattering experiments. Furthermore, the calculations show that the H2O molecule is slightly displaced relative to the center of the cavity in (001). The calculated displacement of the O atom in H2O is 0.04 angstrom to 0,0,1/4, in agreement with results obtained by quasi-elastic incoherent neutron scattering experiments. The total energy of hydration for cordierite with one H2O molecule per primitive unit cell with fixed unit-cell parameters has been calculated to be almost-equal-to 0.4 eV. Local distortions in the framework caused by the incorporation of the H2O molecule are very small, which indicates ideal mixing between H2O and anhydrous Mg end-member cordierite.