Coupled (H⁺,M³⁺) substitutions in forsterite

Coupled substitutions involving hydrogen plus trivalent elements (Al, Eu, Fe, Ga, Gd, Lu, Mn, Nd, Pu, Sc, Y and Yb) in forsterite (Mg₂SiO₄) are studied using atomistic simulation methods. Incorporation of hydrogen is energetically favourable when included in the forsterite lattice as hydroxyl groups (OH^-) at O3 sites while the trivalent cations replace either magnesium or silicon. Our calculations show a strong dependence on the ionic radius of the impurity species and some variation with pressure. There are also significant structural distortions around the impurity defects. At low pressure (0GPa), the smaller trivalent cations, (e.g. Al, Fe, Mn and Ga) substitute into forsterite by replacing Si as: M_Si ^'+OH_O3 ^• The larger trivalent cations (e.g. Eu, Gd, Lu, Nd, Pu, Y and Yb) however, replace Mg at the M2 site coupled with an Mg1 vacancy as described by V_Mg1 ^″+M_Mg2 ^• +OH_O3 ^• At 12GPa, the large cations are more stable at Mg1 relative to Mg2, but both are predicted to be less stable than configurations associated with Si vacancies. The trivalent ionic radius has a significant effect on the H incorporation mechanism, however, the high formation energy of Si vacancies suggests that the presence of H in forsterite could inhibit incorporation of these elements, particularly at high pressure.