## 3 Results
Iron cations (Fe\({}^{3+}\)) must have come from dissolved phases in an acid medium, such as iron-rich smectites (nontronite or beidellite type), as well as from the pseudo-decomposition of illite layers in the interstratified. Figure 4: Comparison of XRD patterns (14–19\({}^{\circ}\) 20 region) of the original blue marl and the original blue marl oriented and ethylene-glycol original blue marl, after 120 and 240 h of sc-CO\({}_{2}\) interaction. The increase in carbonate phases was confirmed by the increase in weight loss observed by thermogravimetric analysis. In these tests, the original sample (untreated) had a weight loss in the temperature range 450-900 \({}^{\circ}\)C of 16.9 wt% compared to the samples treated at 120 h and 240 h with losses of 20.3 wt% and 21.7 wt%, respectively (Figure 5). In general, the alteration mechanism takes place in two steps (Figure 6):  
1. The mechanism of dissolution of smectite would be equivalent to that observed in other studies on the dissolution/alteration of swelling clay minerals (such as smectite and mixed-layers) by inorganic acid [42; 43; 44; 45; 46; 47]. 2. Leading of Al, Mg, and Fe from the octahedral and tetrahedral sheets. As a result, the process could result in illitization of the smectite structure into the illite structure, by loss of the exchangeable cations and modification of the octahedral layer. On the other hand, in the studies described previously [42; 43; 44; 45; 46; 47], different inorganic acids were introduced into the reaction as an activating agent of the swelling clay mineral activating agent; however, in the present study, the acid is carbonic acid. The initial reaction involved the dissolution of CO\({}_{2}\) in water to form carbonic acid, which dissociated to form (HCO\({}_{3}\))\({}^{-}\) and (CO\({}_{3}\))\({}^{-}\) ions. The increase in Ca, Na, and Mg ions obtained by ICP-OES (Table 2) as a function of time with the original marl would be related to the exchange of interlayer cations from smectite to illite, as observed by the reduction of K (typical interlayer cation of illite). However, Ca also comes from a slight dissolution of the calcite in the marl. Figure 5: Thermogravimetric analysis of the original sample (solid line) and treated samples at 120 h (dash line) and 240 h (dotted line). The leaching of the octahedral cations caused a relative increase in the Fe and Mg ion content in the reaction solutions. The results were non-conclusive for Al (below the quantification limit). However, authors previously referred to noted that leaching of the octahedral cations occurred in the order Mg, Fe, and Al [42; 43; 45; 47]. After interaction with CO\({}_{2}\) or carbonic acid later, the specific surface area of BET (BET-SSA) and the pore volume increased (Table 3, and more details are in the Supplementary Materials file). After 120 h of reaction with CO\({}_{2}\), a significant increase in BET-SSA was observed, which maintained a slight increase over time until 240 h. The tendency of the pore volume also increased with time. On the other hand, the pore size decreases with the reaction time. Consequently, it appears that the marks in interaction with CO\({}_{2}\) undergo an increase in the specific surface area probably due to acid attack, increasing the pore volume, creating a new porosity of a lower average size than the pre-test porosity. The change in morphology could be caused by the modification of the smectite structure promoted by an acid attack. The reaction was primarily caused by the exchange of interlayer cations during the reaction and structural modification in the octahedral layer by the leaching of ions.