# Microstructural Characterization of the Mn Lepidolite Distribution in Dark Red Clay Soils
## 4 Discussion  
The continuous increase in lithium demand required by the battery industry brings to light minerals like lepidolite. It belongs to the mica family, which includes muscovite, illite, and biotite. They are formed by the geological degradation of volcanic rocks. In general, the presence of these clay minerals in certain soil regions indicates an underlying volcanic origin. The Apuseni Mountains are part of the Western Carpathians in Romania, which were formed by the folding of Earth's crust plates and not volcanic activity [48]. However, the local soil is clearly influenced by volcanic emissions from the Ciomodul Mare Volcano, located in the Eastern Carpathians of Romania, which is known for widespread volcanic deposits throughout the Transylvanian Basin. For instance, volcanic tuff deposits generated by its eruption are locally known as Dej tuff formations. They are found near Cluj-Napoca in the proximity of the Apuseni Mountains, as well as in Racos (Brasov County, Romania). This volcanic deposit contains many clay minerals such as kaolinite, muscovite, chlorite, and zeolites like clinoptilolite [49; 50]. Other studies on forest soils from the Apuseni Mountains reveal the presence of kaolinite and muscovite clays alongside quartz particles [45; 46]. The presence of dark red, clay-textured soil interlayered with other clay deposits in the Apuseni Mountains forest, along with significant lithium levels detected in rivers originating from this region, raises the question of whether lepidolite might be present despite Romania not being known for such ore deposits. Usually, red soils are known for their iron oxide content [51; 52], but the collected samples have a dark red color resembling perfectly manganese lepidolite and do not correspond to the iron red soils. Figure 1 shows that the upper dark red layer (Sample A) has a sandy texture, followed by a thicker dark red clayey (Sample B), with deeper layers composed primarily of yellow clays. We collected \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 & 60 & 60 & - \\ Kaolinite & 45 & 40 & 40 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Nanoparticle size observed by AFM. yellow clay from the most representative layer in Figure 1 as (Sample C), without suspicions of lepidolite content. Thus, these samples were subjected to a complex microstructural investigation to determine if they contain lepidolite and at what distribution. In Sample A, XRD analysis combined with mineralogical optical microscopy (MOM) detected 24 wt.% lepidolite, along with coarse fractions of quartz and kaolinite, and 11 wt.% iron hydroxide present as crystallized goethite. The lepidolite amount increases in Sample B at 35 wt.% with less quartz and fine kaolinite particles in addition to a low amount of goethite. Sample C has no lepidolite, but it is dominated by a large amount of kaolinite mixed with 13 wt.% goethite, explaining the yellow color of the layer. Elemental analysis identifies manganese content as a distinctive marker confirming the presence of lepidolite, which aligns well with [47]. SEM morphological observations sustain the MOM observations that both lepidolite and kaolinite have very fine fractions with sizes < 10 \(\upmu\)m.  
Clay particles are less dense than quartz [53], and the observed quartz particles are coarser than the finest clay particles. This satisfies the fundamental criteria for primary washing separation, where clay particles are mobilized into the aqueous dispersion, while the coarser quartz and goethite fractions settle at the bottom of the container under the influence of gravity. The notably elevated goethite content in Samples A and C results in an acidic pH during dispersion at 50 \({}^{\circ}\)C, which slightly increases at 23 \({}^{\circ}\)C but remains within the acidic range.