# Microstructural Characterization of the Mn Lepidolite Distribution in Dark Red Clay Soils
## 3 Results
### Mineral Distribution Assessment
The XRD patterns resulting from the finest fractions indicate a strong decrease in the quartz peak intensities and a strong increase in the clay minerals peaks, Figure 4b. Lepidolite and kaolinite amounts increase in Sample A, and a very intense increase in the lepidolite amount is observed in Sample B. Sample C reveals a strong decrease in the quartz peaks and a significant increase in the kaolinite peaks. The increased amount of lepidolite in Sample B is further supported by a notable rise in intensity at 26.57\({}^{\circ}\), close to the quartz peak at 26.53\({}^{\circ}\), as shown in Figure 5b. The amounts of individual minerals can be calculated through X-ray diffraction based on the specific peak intensities and the corundum factor. This method is referred to in the literature as the Relative Intensity Ratio (RIR), as described in our previous studies [38; 39]. It was applied to the XRD patterns in Figure 4 for both initial samples and samples resulting from the washing separation process, and the resulting amounts of identified minerals are collected in Table 2.  
Mineralogical optical microscopy (MOM) examines samples under cross-polarized light. Each mineral affects the oscillation angle of the polarized light beam, producing illumination peaks and color extinction specific to that mineral due to the birefringence phenomenon. Quartz has a green-gray nuance in thin sections and for small particles, but when bigger particles occur, they often feature a rainbow glitte \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multirow{2}{*}{**Sample**} & \multicolumn{6}{c}{**Mineral Composition, wt.\%**} \\ \cline{3-8}  & & & & **Quartz** & **Lepidolite** & **Kaolinite** & **Goethite** \\ \hline \multirow{6}{*}{**MODEL**} & \multirow{4}{*}{A} & \multirow{2}{*}{A} & Amount, \% & 45 & 24 & 20 & 11 \\  & & & Size, \(\mu\)m & 5–180 & \textless{}10 & \textless{}15 & 2–25 \\  & & & Amount, \% & 41 & 35 & 18 & 6 \\  & & & Size, \(\mu\)m & 5–100 & \textless{}10 & \textless{}5 & 2–20 \\  & & & Amount, \% & 39 & 0 & 48 & 13 \\  & & & Size, \(\mu\)m & 5–75 & - & \textless{}5 & 2–15 \\ \hline \multirow{6}{*}{**MODEL**} & \multirow{4}{*}{A} & Amount, \% & 25 & 29 & 38 & 8 \\  & & & Size, \(\mu\)m & 5–10 & \textless{}5 & \textless{}5 & 2–10 \\ \cline{1-1}  & & & Amount, \% & 20 & 57 & 18 & 4 \\ \cline{1-1}  & & & Size, \(\mu\)m & 5–10 & \textless{}5 & \textless{}5 & 2–10 \\ \cline{1-1}  & & & Amount, \% & 22 & 0 & 72 & 9 \\ \cline{1-1}  & & & Size, \(\mu\)m & 5–10 & - & \textless{}5 & 2–10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mineral composition of the samples investigated. Figure 5: Detail of quartz/lepidolite overlapped peak in Sample B: (**a**) initial and (**b**) after washing separation. Larger and well-defined kaolinite particles have a white-pale blue nuance but turn yellow upon the compact massing of the finest particles, and goethite has a predominantly brown nuance with intense red spots in thin sections; all of these aspects were evidenced on etalon samples in our previous studies [39; 40]. Lepidolite particles feature pink nuances under cross-polarized light, but manganese occurrence turns it into red-orange nuance [41; 42]. The sample observation under cross-polarized light in Figure 5 allows for establishing the size range for each mineral, a fact also featured in Table 2.  
Sample A has a sandy texture given by the abundant large quartz particles. These have a boulder-like aspect, Figure 6a; their size ranges from 5 to about 18 \(\upmu\)m. Quartz particles are accompanied by some large kaolinite slivers up to 15 \(\upmu\)m having a preponderantly white aspect and by a few rounded boulders of goethite ranging from 2 to 25 \(\upmu\)m, which appear to have a brown nuance. All these mineral particulate matter are surrounded by small particles of lepidolite of about 1 \(\upmu\)m and below, with only a few of them reaching 10 \(\upmu\)m. These finest lepidolite particles appear in Figure 4a as a red-orange fog surrounding the other mineral particles.