Ca and Mg Incorporation in Siderite at Low Temperatures (< 50° C): Results from Laboratory Experiments

Details Ca and Mg Incorporation in Siderite at Low Temperatures (< 50° C): Results from Laboratory Experiments Ca and Mg Incorporation in Siderite at Low Temperatures (< 50° C): Results from Laboratory Experiments

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p51a1403s&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #P51A-1403

Mathematics

Logic

0419 Biomineralization, 0424 Biosignatures And Proxies

Scientific paper

Siderite (FeCO3) is a common mineral found in modern environments and in ancient rocks produce usually by microbia mediation [1,2]. It usually forms concretions with strongly varying chemical compositions which are governed by both pore-water origin and by microbial influence. In addition, siderite has also been identified in extraterrestrial material such as meteorites and dust particles [3,4]. The geochemical information stored in siderite provides valuable insights into the environmental conditions of mineral formation and the processes by which it is modified over time [5]. To unerstand the inorganic constraints on precipitation relative to natural compositions we undertook free drift experiments under anaerobic conditions at 25, 35 and 45°C with variable concentrations of Fe, Ca and Mg in solution. Samples of solution and solid were withdrawn at different time intervals (15, 21 and 30 days) during time course experiments to determine the composition of the solution and mineral precipitates, and the morphology and mineralogy of the precipitates. After 15 days of incubation a metastable phase was formed, whereas after 21 and 30 days of incubation siderite, Ca-siderite, Mg-siderite Ca-Mg siderite and/or Fe-pokrovskite (a hydrated magnesium hydroxy carbonate) were formed depending on the aqueous Fe, Ca and Mg concentrations in the solution. The Mg and Ca contents in the siderite increased with increasing Mg and Ca concentrations in the medium and with increasing temperature. Siderite precipitates ranged from 1.5 to 50.81 mol percent CaCO3 and from 0.54 to 41.38 mol percent MgCO3. Pokrovskite precipitates ranged from 48.8 to 57.7 mol percent MgCO3 and from 42.34 to 51.17 mol percent FeCO3. The Fe content in the pokrovskite increased with increasing temperature. These inorganic experiments will help to understand the mechanism of Ca-Mg-Fe carbonate formation in natural systems and they are of fundamental importance not only for understanding modern and extraterrestrial environments but also as a window into the geologic past. References [1] Kelts K (1988) Geol. Soc. Spec. Publ. 40, 3-26. [2] Garrels RM, Perry EA, Mckenzie FT (1973) Economic Geology 68, 1173-1179. [3] Romanek CS, Grady MM, Wright IP et al. Nature 372, 655-430. [4] Keller LP, Thomas KL, McKay DS (1994) Meteoritics 29, 480-481. [5] Woods TL, Garrels RM (1992) Geochim. Cosmochim. Acta 56, 3031- 3143.

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