Abstract
The formation of ferrous carbonate mineral is a significant geochemical reaction linked to iron and carbon cycling in the sedimentary environment. However, knowledge of the controlling factors and conditions for the mineral formation is limited. Two types of ferrous carbonate mineral, siderite (FeCO3) and chukanovite (Fe2(CO3)(OH)2) were synthesized under a FeCl2–NaHCO3 system with various concentration ranges (10–100 mmolal) and ratios (Fe:Dissolved inorganic carbon (DIC) = 1:1, X:50, and 50:X) to verify the concentration limit and control species for the formation of those minerals. The mineralogy of filtered precipitates at the reaction time of 1 week and 1 month were identified by X-ray diffraction (XRD), and scanning/transmission electron microscopic (S/TEM) analyses were applied for direct identification of the synthesized siderite and chukanovite at various conditions. A semi-quantitative calculation to estimate siderite proportion (siderite/[siderite + chukanovite]) in the precipitates was carried out using peak intensity ratios of siderite (d104 [2θ = 32.02°]) and chukanovite (d211 [2θ = 33.98°]) from XRD profiles. The framboids or trigonal-rhombohedron crystals and flaky rosette-shaped minerals were identified in SEM analysis. In addition, the chemical compositions of Fe and C of framboid (Fe:C = 1:1.01) and flaky mineral (1:2.04) were identified as siderite and chukanovite, respectively. The formation of siderite was predominated over chukanovite in 50 mmolal (both Fe and DIC) or higher conditions (siderite proportion = 49–100%). The estimated siderite proportion increased (27–100%) as DIC concentration increased (15–100 mmolal) in conditions of varying ratios of iron and DIC (50:X), indicating that DIC is a decisive factor in siderite formation. The increase in the reaction time promotes the siderite proportion increase, so that chukanovite may be dissolved and re-precipitated as siderite for the long-term reaction, except in the enriched DIC condition (Fe:DIC = 15:50). This study demonstrates that various conditions, not limited to the concentrations or reaction time, may affect the geochemical pathways of carbonate mineral formations.
Highlights
Siderite (FeCO3 ) is a common ferrous mineral in oxygen-free environmental conditions including lakes, rivers, and marine sediments [1,2,3], and even in extraterrestrial materials such as meteorites [4,5,6].Ferrous iron is unstable in aqueous solutions because it reacts readily with oxygen, oxidizing to ferric iron and precipitating as iron oxides and/or oxyhydroxides, including goethite, hematite, magnetite, maghemite, and ferrihydrite [7,8,9]
Compared to Experiment 1, in the second experiment set with aqueous iron, only chukanovite was observed under a low-concentration condition, but at a concentration of 50 mmolal, dominant siderite was observed even in the 1-week reaction samples (Figure 2A)
These results suggest that chukanovite could be formed with siderite even if the dissolved inorganic carbon (DIC) concentration is high in a short-term reaction
Summary
Siderite (FeCO3 ) is a common ferrous mineral in oxygen-free environmental conditions including lakes, rivers, and marine sediments [1,2,3], and even in extraterrestrial materials such as meteorites [4,5,6].Ferrous iron is unstable in aqueous solutions because it reacts readily with (dissolved) oxygen, oxidizing to ferric iron and precipitating as iron oxides and/or oxyhydroxides, including goethite, hematite, magnetite, maghemite, and ferrihydrite [7,8,9]. Siderite (FeCO3 ) is a common ferrous mineral in oxygen-free environmental conditions including lakes, rivers, and marine sediments [1,2,3], and even in extraterrestrial materials such as meteorites [4,5,6]. Carbon dioxide and hydrocarbons are formed when siderite reacts with water to form magnetite, which could be used as a tracer for tracking life activities in extraterrestrial conditions [12,13,14,15]. Minerals 2020, 10, 156 produce carbonate and/or carbon dioxide and, as a consequence, enrich the Fe(II) in supernatant or pore water [16,17] that could enhance the formation of secondary phase mineral. The formation of siderite has been identified in lab experiments associated with microbial Fe(III) reduction of Fe-oxide (akaganeite: [18]; magnetite: [19]; ferrihydrite: [20]) and Fe-rich clay minerals (nontronite: [21]), no detailed conditions or controlling factors on biotic siderite formation have been studied
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