High-temperature and high-pressure behavior of carbonates in the ternary diagram CaCO₃-MgCO₃-FeCO₃

Abstract

We report the thermal expansion and the compressibility of carbonates in the ternary compositional diagram CaCO3-MgCO3-FeCO3, determined by in situ X-ray powder and single-crystal diffraction. High-temperature experiments were performed by high-resolution X-ray synchrotron powder diffraction from ambient to decarbonation temperatures (25-850 °C). Single-crystal synchrotron X ray diffraction experiments were performed in a variable pressure range (0-100 GPa), depending on the stability field of the rhombohedral structure at ambient temperature, which is a function of the carbonate composition. The thermal expansion increases from calcite, CaCO3, α0 = 4.10(7) ×10-5 K-1, to magnesite, MgCO3, α0 = 7.04(2) ×10-5 K-1. In the magnesite-siderite (FeCO3) join, the thermal expansion decreases as iron content increases, with an experimental value of α0 = 6.44(4) ×10-5 K-1 for siderite. The compressibility in the ternary join is higher (i.e., lower bulk modulus) in calcite and Mg-calcite [K0 = 77(3) GPa for Ca0.91Mg0.06Fe0.03(CO3)] than in magnesite, K0 = 113(1) GPa, and siderite, K0 = 125(1) GPa. The analysis of thermal expansion and compressibility variation in calcite-magnesite and calcite-iron-magnesite joins clearly shows that the structural changes associated to the order-disorder transitions [i.e., R3c calcite-type structure vs. R3 CaMg(CO3)2 dolomite-type structure] do not affect significantly the thermal expansion and compressibility of carbonate. On the contrary, the chemical compositions of carbonates play a major role on their thermo-elastic properties. Finally, we use our P-V-T equation of state data to calculate the unit-cell volume of a natural ternary carbonate, and we compare the calculated volumes to experimental observations, measured in situ at elevated pressure and temperatures, using a multi-anvil device. The experimental and calculated data are in good agreement demonstrating that the equation of state here reported can describe the volume behavior with the accuracy needed, for example, for a direct chemical estimation of carbonates based on experimental unit-cell volume data of carbonates at high pressures and temperatures

Melting relations in the system FeCO3–MgCO3 and thermodynamic modelling of Fe–Mg carbonate melts

“…are fromMerlini et al (2016) and this study. We used our melt solution model termed LIQ(NK) combined with the solid solution model for ternary Ca-Mg-Fe carbonates termed dis(EF) fromFranzolin et al (2010).…”

“…For these calculations, the thermodynamic properties of the endmembers siderite (Table 2) and siderite melt (FeCO 3 L) are taken from Kang et al (2015). The caloric properties G 0 and S 0 of magnesite were adopted from (Holland and Powell 2011), whereas the volumetric data (V 0 , α 0 , K 0 and K') are from Merlini et al (2016). Note that the volume data of Merlini et al (2016) are consistent with those on siderite and magnesite by Litasov et al (2008Litasov et al ( , 2013 It is assumed that the temperature dependence of the thermal expansion (α T ) and bulk modulus (K T ) of all endmembers are adequately represented by the empirical relations…”

Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth’s deep mantle

“…Magnesian calcite of the present composition has not been examined but it has been established that the product of bulk modulus and volume of Ca-and Mg-carbonates in the calcite-and dolomite-series are constant within uncertainties (48,49). Thus, we used the mixing volume at ambient pressure (50) as reference volume and the interpolated bulk modulus and it's pressure derivative of the solid solution (49) to determine the pressure of magnesian calcite.…”

“…Thus, we used the mixing volume at ambient pressure (50) as reference volume and the interpolated bulk modulus and it's pressure derivative of the solid solution (49) to determine the pressure of magnesian calcite. We used an ambient pressure volume of 246 Å 3 , K = 85 GPa and K0' of 3 (49)…”

The X-ray diffraction beamline MCX at Elettra: a case study of non-destructive analysis on stained glass

“…Also in situ diffraction experiments designed for following reaction mechanisms or kinetics can be performed (see e.g. [6], [7]). …”