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The end members used in MELTS are spinel (MgAl₂O₄), hercynite (FeAl₂O₄), magnetite (Fe₃O₄), chromite (FeCr₂O₄), and ulvöspinel (Fe₂TiO₄). Dependent end members -- magnesiochromite (MgCr₂O₄), magnesioferrite (MgFe₂O₄), and qandilite (Mg₂TiO₄) -- may also be input. All molar proportions will be converted to the MELTS independent end members on submission.

It is assumed that Ti⁴⁺ and Cr³⁺ cations occupy only the octahedral site, so the FeCr₂O₄ and Fe₂TiO₄ end members are always normally ordered. The standard state ordering state in the volume model is normal for MgAl₂O₄ and FeAl₂O₄, and inverse for Fe₃O₄. The ordering state may be input as site occupancy or calculated using the MELTS ordering model. Alternatively, the ordering state may assumed to be normal or inverse, to the extent allowed by the stoichiometry; where appropriate equipartition will be assumed for isovalent cations (i.e. Mg²⁺ and Fe²⁺; Al³⁺ and Fe³⁺).

As formulated, V_total = V_ideal + V_excess where V_ideal is a function of the conditions at which the volume is measured (P_v, T_v). On the other hand, V_excess depends on the equilibrium ordering state, which may reflect different conditions (P_s, T_s). Output will be as follows:

• If the model is used to calculate the volume of spinels at crustal or mantle conditions (i.e. the MELTS ordering model is used with P_v = P_s, and T_v = T_s) then derivatives of V_ideal, V_excess and V_total will be displayed.
• If the site occupancy is input or assumed (i.e. the MELTS ordering model is not used) then P_s and T_s are unknown; in this case only derivatives of V_ideal will be output.
• If, for example, the model is used to calculate the volume at ambient conditions of a spinel that was previously annealed (i.e. the MELTS ordering model is used but P_v ≠ P_s or T_v ≠ T_s) then derivates of V_ideal will be with respect to P_v and T_v and derivates of V_excess will be with respect to P_s and T_s.