Using first-principles calculations, this study evaluates the structure, equation of state, and elasticity three compositions phase D up to 75 GPa: (1) magnesium endmember [MgSi2O4(OH)2], (2) aluminum [Al2SiO4(OH)2], (3) with 50% Al-substitution [AlMg0.5Si1.5O4(OH)2]. We find that Mg-endmember undergoes hydrogen-bond symmetrization is linked a 22% increase in bulk modulus D, agreement previous ...