The temperature and compositional dependence of disordering in Fe-­bearing dolomites

Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered (R3̄), but transforms with increasing temperature into a disordered higher symmetry structure (R3̄c). To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300-1350 K and 2.6-4.2 GPa. The long-range order parameter s, quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20-30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and X Fe. Complete disorder is attained in dolomite at ∼1240 K, 100-220 K lower than previously thought, and in an ankeritic-dolomite s.s. with an X Fe of 0.43 at temperatures as low as ∼900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ∼470 K. The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ∼800 K favoring an almost straight Clapeyron-slope for this disputed reaction.