AB-INITIO TOTAL-ENERGY STUDY OF BRUCITE, DIASPORE AND HYPOTHETICAL HYDROUS WADSLEYITE

Ab initio total energy calculations based on the local density approxi mation (LDA) and the generalised gradient approximation (GGA) of densi ty functional theory have been performed for brucite, Mg(OH)(2), diasp ore, AlOOH and hypothetical hydrous wadsleyite, Mg7Si4O14(OH)(2). The use of a general gradient approximation (GGA) is essential to obtain a good agreement (approximate to 1%) of the calculated lattice paramete rs to diffraction data. The calculated fractional coordinates of bruci te and diaspore are in good agreement (approximate to 1.5%) with exper imental data. The angle of the non-linear hydrogen bond in diaspore is reproduced well, and the calculated Raman active OH stretching freque ncy in brucite is in very good agreement with spectroscopic data. Ther e are no significant differences between the calculated fractional coo rdinates and the second derivative of the energy when GGA is used inst ead of standard LDA. It is concluded that the description of the stati c and the dynamic behavior of the OH groups in these hydroxides is ver y good. It is therefore inferred that the parameter free model is pred ictive and it has been used to evaluate a hypothetical structure of hy drous wadsleyite. The model reproduces the unusual Si-O bond length of 1.7 Angstrom, observed in beta-Mg2SiO4. It predicts an O-H distance o f 0.97 Angstrom, which is significantly shorter than the distance obta ined from earlier model calculations.