Ankerite

About: Ankerite is a(n) research topic. Over the lifetime, 859 publication(s) have been published within this topic receiving 23960 citation(s).

Development of a sequential extraction procedure for iron: implications for iron partitioning in continentally derived particulates

Abstract: The development of a sequential extraction procedure for iron in modern and ancient sediments is presented. The scheme recognizes seven operationally derived iron pools: (1) carbonate associated Fe (Fe carb ), including siderite and ankerite; (2) easily reducible oxides (Fe ox1 ), including ferrihydrite and lepidocrocite; (3) reducible oxides (Fe ox2 ), including goethite, hematite and akaganeite; (4) magnetite (Fe mag ); (5) poorly reactive sheet silicate Fe (Fe PRS ); (6) pyrite Fe (Fe py ); and (7) unreactive silicate Fe (Fe U ). As such, this is the first extraction scheme specifically developed to allow the separate identification of magnetite, and the first to allow a complete evaluation of Fe carbonate phases such as siderite and ankerite. The scheme was developed following tests on pure mineral phases to evaluate the minerals solubilized by each technique and to determine optimum extraction times. Further tests on mixtures of pure minerals and on grain-size separated sediments from two major US rivers and two glacial meltwaters validate the specificity of the scheme for different pools of iron minerals, and demonstrate a high degree of reproducibility for each analytical stage. The data obtained for the riverine and glacial sediments are additionally discussed in relation to the dominant modes of transport of different iron minerals in fine-grained continental sediments.

Clay Diagenesis in Wilcox Sandstones of Southwest Texas: Implications of Smectite Diagenesis on Sandstone Cementation

Abstract: Sandstones and shales of the Wilcox Group (lower Eocene) in southwest Texas were examined by X-ray powder diffraction, electron microprobe, and petrographically to interpret their diagenetic history. Samples analyzed are from depths of 975 to 4650 m, representing a temperature range of 55°C to 210°C. No consistent trend of depositional environments is recognized with increasing depth, and mineralogic changes observed are interpreted as diagenetic. Major mineral distribution patterns are (1) disappearance of discrete smectite at temperatures >70°C, (2) gradation of mixed-layer illite/smectite to less expandable (more illitic) illite/smectite over the entire temperature range, (3) disappearance of kaolinite from 150-200°C accompanied by an increase in chlorite, and (4) replacement of calcite cement at about 117 120°C by ankerite. Calculations based on data of Hower and others (1976) indicate that the stability of smectite layers may be a function of composition. Smectites with high ratios of octahedral (Fe + Mg)/Al appear to resist conversion to illite until temperatures high enough to produce ordering are attained. A diagenetic model is proposed which involves the breakdown of detrital K-feldspar and of some smectite layers in illite/smectite to convert other smectite layers to illite. Silica and calcium released by the illitization of smectite is transferred from shales to sandstones to produce quartz overgrowths and calcite cements at temperatures as low as 60°C. Iron and magnesium released by the illitization reaction are transferred from shales to sandstones at temperatures >100°C and react with kaolinite to produce high-alumina chlorite and/or with calcite to produce ankerite.

An isotopic study of siderites, dolomites and ankerites at high temperatures☆

Abstract: Siderite, dolomite and ankerite were reacted with “>103°” phosphoric acid at temperatures up to 150°C with >99° yields achieved in less than two hours, using a modification of the McCrea (1950) technique. The oxygen fractionation factors, α, between the δ18O of the carbonate and that of the acid-extracted CO2 are: Siderite Dolomite Ankerite 100°C 1.00881 1.00913 1.00901 150°C 1.00771 - - Full-size table Table options View in workspace Download as CSV An empirically derived relationship between the chemical composition of a carbonate in the CaCO3-(Ca, Mg)(CO3)2-FeCO3system and the function 103 In α at 100°C is 103 In α = 8.94XCaCO3 + 9.29XMgCO3 + 8.77XFeCO3 where Xi is the mole percent of component i in the carbonate.

Numerical simulation of CO2 disposal by mineral trapping in deep aquifers

Abstract: Carbon dioxide disposal into deep aquifers is a potential means whereby atmospheric emissions of greenhouse gases may be reduced. However, our knowledge of the geohydrology, geochemistry, geophysics, and geomechanics of CO2 disposal must be refined if this technology is to be implemented safely, efficiently, and predictably. As a prelude to a fully coupled treatment of physical and chemical effects of CO2 injection, the authors have analyzed the impact of CO2 immobilization through carbonate mineral precipitation. Batch reaction modeling of the geochemical evolution of 3 different aquifer mineral compositions in the presence of CO2 at high pressure were performed. The modeling considered the following important factors affecting CO2 sequestration: (1) the kinetics of chemical interactions between the host rock minerals and the aqueous phase, (2) CO2 solubility dependence on pressure, temperature and salinity of the system, and (3) redox processes that could be important in deep subsurface environments. The geochemical evolution under CO2 injection conditions was evaluated. In addition, changes in porosity were monitored during the simulations. Results indicate that CO2 sequestration by matrix minerals varies considerably with rock type. Under favorable conditions the amount of CO2 that may be sequestered by precipitation of secondary carbonates is comparable with and can be larger than the effect of CO2 dissolution in pore waters. The precipitation of ankerite and siderite is sensitive to the rate of reduction of Fe(III) mineral precursors such as goethite or glauconite. The accumulation of carbonates in the rock matrix leads to a considerable decrease in porosity. This in turn adversely affects permeability and fluid flow in the aquifer. The numerical experiments described here provide useful insight into sequestration mechanisms, and their controlling geochemical conditions and parameters.

Mineral sequestration of carbon dioxide in a sandstone–shale system

Abstract: A conceptual model of CO2 injection in bedded sandstone–shale sequences has been developed using hydrogeologic properties and mineral compositions commonly encountered in Gulf Coast sediments. Numerical simulations were performed with the reactive fluid flow and geochemical transport code TOUGHREACT to analyze mass transfer between sandstone and shale layers and CO2 immobilization through carbonate precipitation. Results indicate that most CO2 sequestration occurs in the sandstone. The major CO2 trapping minerals are dawsonite and ankerite. The CO2 mineral-trapping capacity after 100,000 years reaches about 90 kg/m3 of the medium. The CO2 trapping capacity depends on primary mineral composition. Precipitation of siderite and ankerite requires Fe+2 supplied mainly by chlorite and some by hematite dissolution and reduction. Precipitation of dawsonite requires Na+ provided by oligoclase dissolution. The initial abundance of chlorite and oligoclase therefore affects the CO2 mineral-trapping capacity. The sequestration time required depends on the kinetic rate of mineral dissolution and precipitation. Dawsonite reaction kinetics is not well understood, and sensitivity regarding the precipitation rate was examined. The addition of CO2 as secondary carbonates results in decreased porosity. The leaching of chemical constituents from the interior of the shale causes slightly increased porosity. The limited information currently available for the mineralogy of natural high-pressure CO2 gas reservoirs is also generally consistent with our simulation. The “numerical experiments” give a detailed understanding of the dynamic evolution of a sandstone–shale geochemical system.