Paper No. 32-6
Presentation Time: 2:30 PM-2:45 PM
JIMENEZ-LOPEZ, Concepcion1, ROMANEK, Christopher S.2, PADDOCK, Lindy1, and JONES, CarriAyne2, (1) Savannah River Ecology Laboratory, Univ of Georgia, Drawer E, Aiken, SC 29802,, (2) Dept. Geology, Univ of Georgia, Athens, GA 30602,

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 25o and 70oC 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 25oC, 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 70oC, 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 (25oC) 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 70oC, with Ca2+ and Fe2+ coprecipitation at a relatively low Ca/Mg and Fe/Mg ratio in solution (= 0.05).

2002 Denver Annual Meeting (October 27-30, 2002)
Session No. 32
Planetary Geology
Colorado Convention Center: C101/103
1:00 PM-3:45 PM, Sunday, October 27, 2002

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