Controls on the Formation and Stability of Siderite (FeCO3) and Chukanovite (Fe2(CO3)(OH)2) in Reducing Environment
Controls on the Formation and Stability of Siderite (FeCO3) and Chukanovite (Fe2(CO3)(OH)2) in Reducing Environment
复制标题
还原环境中菱铁矿(FeCO3)和楚卡诺夫石(Fe2(CO3)(OH)2)的形成及其稳定性的控制
作者：
T. Koo;Jin
碳酸亚铁矿物的形成与沉积环境中的铁和碳循环有关，是一种重要的地球化学反应。然而，对成矿的控制因素和条件的了解有限。在FeCl2-NaHCO3体系中，合成了不同浓度范围(10-100摩尔)和不同比例(Fe：溶解无机碳=1：1、X：50和50：X)的两种碳酸亚铁矿物--菱铁矿(FeCO3)和楚卡诺夫石(Fe2(CO3)(OH)2)，以验证形成这两种矿物的浓度限制和控制物种。用X射线衍射仪对反应1周和1个月过滤后的沉淀物进行了矿物学分析，并用扫描/透射电子显微镜(S/透射电子显微镜)分析了不同条件下合成的菱铁矿和蓝晶石。利用X射线衍射峰强度比d104[2θ=32.02°]和D211[2θ=33.98°]，半定量计算了沉淀物中菱铁矿的比例(菱铁矿/[菱铁矿+楚卡诺夫铁矿])。扫描电子显微镜分析表明，该矿物为框架状或三方菱形晶体和片状玫瑰花结状矿物。此外，方晶石(Fe：C=1：1.01)和片状矿物(1：2.04)中的Fe和C的化学成分分别为菱铁矿和丘卡诺夫石。在50摩尔(Fe和DIC)或更高的条件下(菱铁矿比例为49-100%)，菱铁矿的形成以菱铁矿为主。在不同铁与DIC比例(50：X)的条件下，随着DIC浓度的增加(15-100mmol.)，估算的菱铁矿比例增加(27-100%)，表明DIC是菱铁矿形成的决定性因素。反应时间的延长促进了菱铁矿比例的增加，除在富集型DIC条件下(Fe：DIC=15：50)外，在长期反应中可溶解和再沉淀为菱铁矿。这项研究表明，各种条件，不限于浓度或反应时间，可能会影响碳酸盐矿物形成的地球化学途径。
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.