C2/c is not preferred over single cation carbonates with 12.5% Fe substitution at any pressure, and is preferred over single cation carbonates at about 38-45 and 33-49 GPa for 50 and 100% Fe substitution, respectively. With an internally consistent U, ferrodolomite C2/c is preferred over single-cation carbonates at about 41-44, 35-48, and 31-53 GPa for 12.5, 50 and 100% Fe substitution. With either choice of U, a higher iron concentration decreases the enthalpy of dolomite C2/c with respect to decomposition products, effectively increasing its stability field.
Figure 7.6: Enthalpy differences with respect to high-spin (Mg,Fe)Ca(CO3)2 with the C2/c space group with varying concentrations of iron, calculated using a fixed U of 4 eV in (a) to (c) and an internally consistent U in (d) to (e). In (a) and (d), 12.5% of the Mg sites are replaced with Fe; in (b) and (e), 50% of the Mg sites are replaced with Fe; and in (c) and (f), 100% of the Mg sites are replaced with Fe. Two-phase assemblages are indicated by solid lines and three-phase assemblages are indicated by dashed lines. Enthalpy difference between HS and LS (Mg,Fe)Ca(CO3)2 is not shown for clarity (see Figure 7.4 for spin transition pressures). The phases are abbreviated as follows: aragonite – a, post-aragonite – p, magnesite – m, HS ferromagnesite – Hf, LS ferromagnesite – Lf, HS siderite – Hs, and LS siderite – Ls. Grey regions indicate pressure ranges where high-spin (Mg,Fe)Ca(CO3)2 has a lower enthalpy than decomposition products.
In (Mg,Fe)SiO3 perovskite, theory predicts a compositional dependence opposite to that observed in (Mg,Fe)O: that the spin transition pressure of ferrous iron decreases with increasing iron concentration [Bengtson et al., 2008; Umemoto et al., 2008]. The interpretation has been that larger site distortions are allowed in the perovskite structure compared to the B1 structure of ferropericlase, demonstrated by the remarkable difference in the spin transition pressure between a fully relaxed FeSiO3 perovskite (77 GPa) and a perfectly cubic FeSiO3 perovskite (900 GPa) [Bengtson et al., 2008]. In this context, we note that the FeO6 octahedra in the carbonates are less distorted than in FeSiO3 perovskite and, at the same time, more distorted than in (Mg,Fe)O. It may be that chemical pressure on iron induced by magnesium is negligible in carbonates due to the ability to distort the iron site to accommodate the stresses. On the other hand, the octahedra are not able to distort to the extent possible in the perovskite structure, and so the effect of site distortion with increasing iron concentration is not observed. Thus, the crystal field splitting energy remains essentially unchanged, resulting in an insensitivity of the carbonate spin transition pressure to the iron concentration.
7.4.2 Monoclinic Dolomite
We find that monoclinic ferrodolomite with space group C2/c is preferred over single-cation carbonates at a maximum pressure width of about 32-52 GPa, which corresponds to an approximate depth of 850-1300 km within Earth’s lower mantle.
Although the spin transition is overestimated with an internally consistent U, the relative enthalpies may be more accurate. Experiments have annealed high-pressure polymorphs of dolomite (referred to as “dolomite III”) and did not observe decomposition into single- cation carbonates. Mao et al. [2011] annealed dolomite III with 8 mol% FeCa(CO3)2 at 36 to 83 GPa at 1500 K and found that it did not decompose into single-cation carbonates. Similarly, Merlini et al. [2012] annealed dolomite III with 40 mol%
FeCa(CO3)2 at 46, 55 and 72 GPa at 2100-2200 K and also did not observe decomposition. The dolomite III structure observed in Merlini et al. [2012] is not the
same structure as dolomite C2/c, but it has a higher enthalpy (less stable) at all pressures than dolomite C2/c [Solomatova and Asimow, 2017] and therefore the stability field of dolomite C2/c should be even larger than that for dolomite III. Thus, the experimental results suggest that our calculations underestimate the stability field of iron-bearing dolomite C2/c.
Given that the stability field of endmember rhombohedral dolomite increases with increasing temperature at constant pressure [Martinez et al., 1996; Buob et al., 2006], it may be that a similar trend is true for high-pressure iron-bearing dolomite polymorphs.
Although the effect of cation ordering and temperature on the behavior of iron-free rhombohedral dolomite I and triclinic dolomite II has been investigated up to about 30 GPa [Hammouda et al., 2011; Zucchini et al., 2017], additional experiments and calculations are needed on iron-bearing dolomite polymorphs at high temperatures and pressures.
So far, we have only discussed the stability of various carbonates with respect to one another. However, other decomposition routes are possible, for example to oxides or silicates and diamond or CO2. Dorogokupets et al. [2007] calculated that MgCO3 will not decompose into MgO + CO2 up to at least 130 GPa and 3000 K based on the extrapolation of thermodynamic functions of MgCO3, MgO and CO2. Likewise, Oganov et al. [2008] calculated that high-pressure polymorphs of MgCO3 and CaCO3 are energetically favorable over oxides and will not react with SiO2 to produce CO2 up to core-mantle-boundary pressures. In reduced regions of the mantle, Oganov et al. [2008]
calculated that MgCO3 should decompose only above 50 GPa. Given that (Mg,Fe)CO3
and CaCO3 are energetically favorable with respect to oxides [Dorogokupets et al., 2007;
Oganov et al., 2008; Lin et al., 2014], monoclinic (Mg,Fe)Ca(CO3)2 is expected to be stable with respect to simple oxides in at least the same pressure range for which it is energetically favorable over (Mg,Fe)CO3 and CaCO3.