# Microstructural Characterization of the Mn Lepidolite Distribution in Dark Red Clay Soils
## 1 Introduction  
Lepidolite is a clay mineral belonging to the mica group. Its standard chemical formula is K(Li, Al, Rb)2(Al, Si)4O10(F, OH)2. The crystal structure is composed of intricate sheets constructed from tetrahedral layers containing Si and Al ions, which are coordinated withfour oxygen atoms. The tetrahedral layers are stacked together through the middle octahedral layers containing mainly Li and F associated with minor traces of other metals like Mg, Fe, or Mn. This structure, commonly referred to in the literature as TOT, is characteristic of all mica minerals. The complex sheets are bonded to one another through the K\({}^{+}\) ions. This crystal plan is highly susceptible to cleavage, ensuring the easy detachment of sheets containing valuable metals like Li and Rb [1; 2]. On the other hand, lepidolite is one of the richest minerals in lithium content, which occurs on Earth's crust along with polylithionite and trilithionite, creating a mineral series with variable amounts of Li. The literature refers to the Li:Al ratio, which is about 2:1 in polylithionite, decreasing to 1.5:1.5 in trilithionite [2; 3]. Lepidolite is typically pink or light purple in color, but it can also appear rose red or violet gray due to the presence of trace elements like manganese [4]. [PERSON] and collaborators revealed the rose-red color of the manganese mineral [5], in good agreement with other findings confirming that lepidolite's red or violet color is caused by the presence of Mn [6; 7]. Minerals like lepidolite, polylithionite, and trilithionite are found crystallized within pegmatite bodies along with quartz, spodumene, and feldspar-bearing rocks that are suitable for lithium ore extraction. There are well-established technological procedures regarding ore grinding and separation through flotation [8; 9]. The ore enrichment also might imply the finest lepidolite fraction extraction using sodium oleate, trapping the lepidolite fine particles that had been previously activated with Mg\({}^{2+}\) ions [10]. The flotation characteristics must be adapted to the ore characteristics to remove sterile minerals such as quartz or iron occurrences [11; 12]. The importance of lepidolite ore exceeds its particle distribution in clay soils, a fact less investigated in the literature. The alteration of volcanic rocks facilitates the formation of thick deposits of clay soils merely based on kaolinite and muscovite [13; 14]. Lepidolite soils may form similarly through the breakdown of lamellar slabs. There is less data in the literature regarding lepidolite soils, perhaps due to their formerly lower industrial interest. It was noticed in the clay mixture sterile in bauxite, bringing lithium content to the red mud [15], and is now being targeted for advanced lithium recovery [16; 17]. It is also renowned as a gemstone for jewelry because of the nice pink-to-deep red-violet nuances of higher-quality stones [18]. Some literature data reveal significant levels of lithium in the karst-related spring waters in the Dobrogea and Banat regions in Romania, categorized as concentrated brine [19]. The literature reports rich lithium brines as an alternative source to lepidolite extraction [20; 21]. [PERSON] found significant levels of lithium in the waters from the Beius area [22]. It is a strange finding since the metal infiltration into the rivers affected by former mining activities contains mainly Cd, Pb, and Zn [23; 24], not lithium. Some random observations of the forest soils around the Beius Basin and Apuseni Mountains reveal interesting soil with a clay texture and a dark red color with a violet appearance. It justifies the assumption that lepidolite might cause such color by its presence in detectable quantities. For instance, manganese oxide is used to produce a red-violet color (e.g., deep ruby red) in glass artwork [25; 26] and for ceramic glazes [27]. This color is difficult to obtain in mineral matter such as soils.