## 4 Results and Discussion  
### Sample Characterization  
The studied cobalt-rich ferromanganese crusts are dark brown to black in color, with a botryoidal to current-smoothed surface. The botryoids vary in size from millimetric to centimetric, characteristic of hydrogenetic growth, which occurs through slow precipitation from cold seawater. Cross sections of the samples reveal finely laminated growth, forming two to three distinct layers (Figure 3A). Polished thin sections examined under a reflected light microscope show two primary internal structures within the ferromanganese crusts: dendritic to mottled (Figure 3B) and columnar to densely parallel (Figure 3C). The crusts' mineralogy is dominated by poorly crystalline Fe and Mn oxyhydroxides, with Fe-vernadite ((Mn\({}^{4+}\), Fe\({}^{3+}\), Ca, Na)(O, OH)\({}_{2}\) \(\cdot\) nH\({}_{2}\)O) being the primary mineral, constituting 70-90% \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & **Sample** & **Drago 0511/DR04-15 [S1]** & & **Subvent 1/DA06-4 [S2]** \\ \hline \multirow{2}{*}{**Test**} & \multirow{2}{*}{**Time (min)**} & **Ferromanganese Crust/Fe(res)** & & **Mn Extraction (\%)** & **Co Extraction (\%)** \\  & & & & & \\ \hline
1 & \(-1\) & \(-1\) & 74.15 & 31 & 73.95 & 29 \\
2 & \(-1\) & 0 & 69.5 & 28 & 71.25 & 27.5 \\
3 & \(-1\) & 1 & 55 & 20.55 & 56.5 & 17.6 \\
4 & 0 & \(-1\) & 72.45 & 32.8 & 73.45 & 31 \\
5 & 0 & 0 & 74.21 & 31.5 & 74.21 & 30 \\
6 & 0 & 1 & 67.8 & 27.25 & 66.7 & 26.55 \\
7 & 1 & \(-1\) & 83 & 35.95 & 84.7 & 33 \\
8 & 1 & 0 & 81.85 & 34.65 & 82 & 32.5 \\
9 & 1 & 1 & 73.55 & 31.4 & 71.8 & 30.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experimental configuration and Cu and Co extraction. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters [Variable]/Values** & **Low** & **Medium** & **High** \\ \hline \multicolumn{3}{c}{Time (min) \([x_{1}]\)} & 10 & 20 & 30 \\ \multicolumn{3}{c}{Ferromanganese Crust/Fe(res) \([x_{2}]\)} & \(1/3\) & \(1/2\) & \(1/1\) \\ \multicolumn{3}{c}{Cotifications} & \(-1\) & 0 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental conditions. of the composition. Vernadite, a poorly crystalline mineral, is identified primarily by its reflections at 2.45 and 1.72 A. Other manganese minerals, present in minor quantities, include birnessite and as-bolane, while Fe minerals are represented by the goethite group. Detrital and authigenic minerals are also present, such as quartz, feldspar, and clays, with calcite typically originating from bioclastic accumulation. These minerals form light-colored laminae interspersed with oxyhydroxoxide minerals. The geochemistry of the samples studied, Drago 0511/DR04-15 and Subvent 1/DA06-4, aligns with the mineralogical results, showing the highest contents of Fe and Mn (averaging 19.6 wt.% and 19.5 wt.% respectively) among the major elements. The aluminum-silicate elements have a total average of 19 wt.%, with the highest content observed in the deepest sample (SUBVENT 1/DA06-4), which also exhibits a significant input of detrital minerals, as confirmed by the XRD analysis (Table 5). In contrast, the sample DRAGO 0511/DR04-15 shows higher concentrations of CaO and P\({}_{2}\)O\({}_{5}\) (6.05 wt.% and 2.47 wt.% respectively), linked to the presence of phosphates (CFA), also identifiable in the mineralogical analysis. Fe-Mn crusts, due to their genetic processes and mineralogy, concentrate high contents of critical and strategic elements. Between them the most enriched are Co, Ni, Cu, V, Mo, and REY, with an average of 3300, 2600, 900, 800, 380, and 2300 \(\upmu\)g/g, respectively (See Table 6)  
Figure 3: **(A)** Crosscut section of Co-rich ferromanganese crust SUBVENT 1/DA06-4. **(B,C)** Photomicrographs of characteristic internal textures—dendritic-mottled and columnar-dense parallel, respectively. Canary Islands Seamounts Province.