マントル鉱物中の水素の存在状態 –分子動力学法からのアプローチ–

抄録

Molecular dynamics (MD) simulation was carried out to obtain positions and energy states of hydrogen in nominally anhydrous mantle minerals. An appropriate OH interatomic potential model (Morse-AT) was determined to reproduce vibrational spectrum and bulk modulus of brucite Mg(OH)₂. Pressure gradient of OH stretching peak position in brucite is negative, showing increase of hydrogen bonding energy with pressure. This developed Morse-AT model was applied for hydrous forsterite, wadsleyite and ringwoodite Mg₂SiO₄. Based on the MD simulations, the small amounts of hydrogen in forsterite forms a Si-OH bond, showing lack of the hydrogen bond. In wadsleyite, the O1-site is partially replaced by OH with M3 site magnesium vacancy. In ringwoodite, hydrogen is present at a vacancy of Mg-site and coordinates SiO_4 tetrahedron oxygen. Decreasing O-O interatomic length with an increasing pressure produces a strong hydrogen bond. Hydrogen in the ringwoodite becomes more stable at higher pressure.