Impact of CO2 on iron-argillite experimental interactions at 90 deg. C

Document available in extended abstract form only. The present study shows the mineralogical transformations observed after an experimental interaction between metallic iron and an argillite from the Callovo-Oxfordian formation of the Paris Basin (Bure, France, borehole EST 26457 FOR1118). One of the objectives of this work was to experimentally test the impact of CO2 on precipitation of new phases like siderite. The experiment was performed during 14 weeks at 90 deg. C under saturated vapour pressures (water/argillite mass ratio was equal to 1). A plate of metallic iron and iron powder were reacted together with the argillite (Fe0 powder/argillite mass ratio = 1). A partial pressure of CO2 ranging between 10 and 100 times atmospheric pressure was fixed at the beginning of the experiment by adding sodium bicarbonate in the experimental system. Sodium bicarbonate decomposes into sodium carbonate, water and CO2 with the increase of temperature (60 deg. C). The total pressure measured in the autoclave quickly increased to reach 6 bar after three days. Then a logarithmic increase occurs to reach 50 bar at the end of the experiment. The gases produced during the experiment were analyzed by Raman spectrometry. The gas phase was mainly made of H2; CO2 was under the limit of detection.The main mineralogical evolution was the formation of iron-rich 7 Angstrom clay with a low degree of crystallinity (low intensity of the X-ray patterns) which probably issued from the starting illite-smectite mixed-layer clays. The dissolution of quartz and the alteration of pyrite as well as the crystallization of pyrrhotite on the surface of the iron plate were also observed. The precipitation of iron carbonate (perhaps siderite) in very small amount was suspected considering the results from Moessbauer and infrared spectroscopies. Water was totally consumed by the oxidation of metallic iron and to form iron-rich 7 Angstrom clays. The following reactions are proposed as examples of transformation in the experimental system: 4 H2O + 3 Fe0 to Fe3O4 + 4 H2 Montmorillonite Si4(Al2-x,R2+x)O10(OH)2M+x + 6Fe0 + 12H2O to Greenalite Si4Fe6O10(OH)8 + (2-x)Al3+ +6OH- + 6H2 + R2+x + M+x; Muscovite Si3Al(Al2)O10(OH)2K + 6Fe0 + 12H2O + SiO2aq + 2H2O to Greenalite Si4Fe6O10(OH)8 + 3Al3+ +10OH- +6H2+ K+ These reactions induce an increase of pH of the experimental solution favouring quartz dissolution. The processes involved in the evolution of illites and mixed-layer clays (I-S) of argillite towards iron-rich 7 Angstrom clay may be solid-state transformations or processes of dissolution of several layers of the starting clays (mechanism already suggested by Wilson et al., 2006a; 2006b). Fe replaces Al and Mg in octahedral sheets, Al substitutes Si in tetrahedral sheets to form berthierine-like minerals. In this work these transformations were probably limited by the small quantity of water in the experimental system (low water/rock mass ratio). Ferric iron can also replace Si in tetrahedral sheets. This last chemical evolution is favoured when water/rock ratio is > 1 in the experimental system. Thus a mechanism of dissolution-crystallisation can be envisaged to form Si-Fe silicates like cronstedtite which is more stable at lower temperature (< 80 deg. C; Pignatelli et al., 2012). As the experiment presented here was carried out under a weak pressure of CO2 (<30 mbar), the influence of CO2 on mineral transformations was difficult to estimate, the gaseous phase of the experimental system being widely dominated by H2 produced during the oxidation of metallic iron. Nevertheless, it appears that the main transformations were the formation of a few iron carbonates on the surface of the iron plate together with iron-rich 7 Angstrom clay and pyrrhotite. (authors)