# Microstructural Characterization of the Mn Lepidolite Distribution in Dark Red Clay Soils
## 3 Results
### Microstructural Characterization  
Scanning electron microscopy (SEM) allows an advanced microstructural investigation of the soil samples before and after the washing separation process. It presents the benefit of coupling with the elemental distribution maps that relate the samples' elemental composition to the mineral particles. Thus, the microstructural aspect of the initial samples, Figure 7, confirms the MOM observations. Quartz particles have boulder-like aspects with irregular margins caused by the blunting induced by prolonged friction with other sandy particles during the geological eras. Clay particles have a tabular-lamellar aspect, but it is difficult to distinguish which are lepidolite and which are kaolinite only by morphological observation. Furthermore, MOM observation reveals that goethite particles appear in both forms as rounded boulder microparticles of about 2-25 \(\upmu\)m and fine microdispersion, which is difficult to distinguish among other fine details in Figure 6.  
Elemental mapping helps to detect each mineral by its specific composition. Thus, quartz, having the chemical formula SiO\({}_{2}\), appears as a bright green color due to the overlapping of the O turquoise label and Si green label. Kaolinite has the chemical formula Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\), and it became very distinct as pale green due to the overlapping of the O and Si characteristic color with yellow labeled for Al. The difference between kaolinite and lepidolite is caused by the presence of manganese, labeled in deep blue. Iron (orange labeled) makes observing its distribution easier within the sample's morphology. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mineral**} & \multicolumn{3}{c}{**Crystallite Size, nm**} \\ \cline{2-4}  & **A** & **B** & **C** \\ \hline Lepidolite & 57 & 59 & - \\ Kaolinite & 42 & 37 & 39 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Crystallite size calculated with the Scherrer formula. The sandy texture of Sample A, Figure 7a, is formed by large quartz particles associated with some bigger kaolinite particles. These coarse components are surrounded by a dense mixture of fine particles, mainly lepidolite and kaolinite. Iron particles are predominantly distributed within the finest fractions. Sample B is formed predominantly by the finest fractions of lepidolite with traces of kaolinite mixed up with quartz particles, Figure 7b. The blue areas within the elemental map evidence significant clustering of lepidolite particles, sustaining the MOM observations. Goethite particles appear to cluster near larger kaolinite particles, a phenomenon previously observed in ceramic slurries [44]. This arrangement of iron hydroxide particles makes the sample more stable and could be useful for its removal through washing separation. In Sample C, Figure 7c, the finest kaolinite fractions dominate the microstructure, closely enveloping the quartz particles. Goethite particles are uniformly spread within the kaolinite, making it very difficult to separate them. However, there appear to be some concentrated clusters of goethite, which are prone to sedimentation during the primary washing separation process. Elemental compositions of the initial soil samples are presented in Table 4. The dominant elements are O, Si, Al, and K, related to quartz and clay particles. These have a significant participation in all initial samples. Manganese has a significant presence of 1.9 at.% in Sample A because of lepidolite particles observed in the elemental map in Figure 6a. It increases to about 2.1 at.% in sample B proportionally to the lepidolite particles observed in the elemental map and completely disappears in Sample C, which contains huge amounts of iron compared with  
Figure 7: SEM images of the initial soil: (**a**) Sample A, (**b**) Sample B, and (**c**) Sample C. The corresponding elemental map and EDX spectrum are presented on the right side of each SEM image.