CARBONATE PRECIPITATION FROM FE2+-, CA2+- AND MG2+-BEARING SOLUTIONS AT 25O AND 70OC

The origin of carbonate minerals in some extraterrestrial materials (i.e., Martian meteorite ALH84001) remains controversial, partially because there are few studies regarding the low temperature formation of (Ca, Fe, Mg)CO₃. In this study, twenty two free-drift experiments were carried out at 25^o and 70^oC from Fe²⁺-, Ca²⁺- and Mg²⁺-bearing solutions (Fe²⁺: 0 - 50 mM; Ca²⁺: 0 - 25 mM; Mg²⁺: 0 - 60 mM) to gain a better understanding of the precipitation of mixed cation carbonates.

At 25^oC, siderite was obtained (XRD) in all experiments when (Ca/Fe)_soln £ 1 and (Mg/Fe)_soln £ 20. Chemical analyses of the solid (ICP-OES) showed that at (Ca/Fe)_soln £ 1 and (Mg/Fe)_soln £ 2, Ca²⁺ was incorporated in the crystal structure up to 18 mol %, but Mg²⁺ was not. Siderite crystals that grew at (Ca/Fe)_soln £ 1 and (Mg/Fe)_soln = 20 incorporated both Ca²⁺ (up to 27 mol %) and Mg²⁺ (up to 13 mol %) into the crystal structure. The incorporation of Ca²⁺ was also detected by a shift of some XRD reflection peaks [(108), (110), (113), (024) and (202)] to higher d-spacings while the incorporation of Mg²⁺ induced a shift to lower d-spacings [(104) and (012)]. In all cases the crystals displayed a rhombohedral habit.

At 70^oC, magnesite rhombohedra were obtained (XRD) in all experiments when (Ca/Fe)_soln £ 1 and (Mg/Fe)_soln = 20. Both Ca²⁺ (up to 20 mol %) and Fe²⁺ (up to 24 mol %) were detected in magnesite (ICP-OES), with the subsequent shifting of d-spacings [(018), (110), (202), (113) and (104)] to higher values.

These results suggest that Ca²⁺ is a common constituent of siderite at relatively low temperature (25^oC) and low Mg/Fe ratio, while Mg²⁺ coprecipitates in siderite readily at higher Mg/Fe ratio (= 20). Magnesite can be precipitated from Fe²⁺-, Ca²⁺- and Mg²⁺-bearing solutions at 70^oC, with Ca²⁺ and Fe²⁺ coprecipitation at a relatively low Ca/Mg and Fe/Mg ratio in solution (= 0.05).