# Microstructural Characterization of the Mn Lepidolite Distribution in Dark Red Clay Soils
## 4 Discussion
The strong pH dependence on the goethite amount is caused by its fine distribution within kaolinite particles, a fact observed by SEM microscopy, especially in Sample C. Sample B has an alkaline pH during the separation process, keeping about the same value at 50 and 23 \({}^{\circ}\)C proving a good stability induced by the higher clay amount and less goethite. The separation process proves to be quite effective for Samples B and C, yielding extraction rates of 20.69 wt.% and 18.60 wt.%, respectively, whereas Sample A shows a significantly lower rate of only 5.77 wt.%. This proves that clayey soils have a better separation yield than sandy soils. Unfortunately, Sample C did not contain lepidolite; however, the lepidolite amount increased to 29 wt.% in Sample A and to 57 wt.% in Sample B. Since kaolinite particles are almost the same size as lepidolite and are similar in nature, they are difficult to separate by simple washing. This highlights the significance of the lepidolite-to-kaolinite ratio, which is 0.76 in Sample A and increases to 3.16 in Sample B. In consequence, Sample B has better characteristics for potential further industrial processing. However, [PERSON] and collaborators reported the successful use of lithium clay slag, including fine kaolinite fractions as pozzolanic matter, to replace part of cement amount in concrete without affecting its mechanical properties [54]. It would be a good approach for further investigations regarding the sandy lepidolite in Sample A. Atomic force microscopy agrees with the XRD observations that clay fractions have a large number of nanoparticles of about 60 nm for lepidolite and 40 nm for kaolinite. This makes the soil texture very fine and soft and also ensures washing efficiency. The similar sizes of the nanoparticles make it nearly impossible to achieve effective separation between lepidolite and kaolinite. This would require advanced flotation processes, implying collector and frother agents [9; 10]. This fact reduces the applicability of Sample A but keeps promising aspects for Sample B. Advanced flotation methods can separate lepidolite particles from the trace minerals if their amount is similar to the one observed in Sample B [55]. Iron oxide removal might be critical for the further utilization of the floated lepidolite in the lithium extraction processes [56; 57]; therefore, the lower goethite observed in the separated Sample B is a good point achieved by the present research. The complex microstructural investigation clearly shows that Sample B might have industrial interest in further processing for lithium extraction, but this depends on the available quantities. As shown in Figure 1, Layer B is relatively thin compared to the underlying yellow ground and does not alone justify the initiation of industrial operations. However, it holds promise for further exploration to locate thicker lepidolite-rich layers similar to that observed in Sample B. On the other hand, the soil samples investigated have very interesting colors that might be useful in the pottery industry for colored ceramics or natural glazes. Their thermal behavior and consolidation potential should be investigated in further research, particularly for soils such as Samples A and C.  
The present research has several limitations partly caused by the sample collection. Samples were taken from the observation site without additional prospection in the area that should be further organized according to the current findings. The finest particle recovery rate strongly depends on the sedimentation process, which can be improved by a proper adjustment of the temperature and sedimentation time. The other limitations depend on the methods used, which were unable to directly measure the lithium and rubidium content within the identified lepidolite. Further research in this direction should include a detailed chemical analysis of the samples to establish the exact available quantity of lithium and rubidium.