Fig. 1.

Sketch of the calculation procedure for forsterite as an example. The following reaction was investigated: 2MgO + SiO₂ = Mg₂SiO₄. Its reaction energy at 0 K (Δ_RU^0K) was calculated by the DFT method and it is assumed that the reaction enthalpy at 0 K (Δ_RH^0K) is identical. Adding the formation enthalpies of the oxides at 0 K (Δ_fH^0K_ox) to this Δ_RH^0K value results in the formation enthalpy of forsterite from the elements at 0 K (Δ_fH^0K_fo). The heat content from 0 to 298.15 K (= ∫C_P^fo dT) is than added to this value yielding finally the standard enthalpy of formation of forsterite from the elements (Δ_fH^298.15_fo)