Controls on the Formation and Stability of Siderite (FeCO3) and Chukanovite (Fe2(CO3)(OH)2) in Reducing Environment

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.