# Structural Pattern Analysis in _Patella vulgata_ Shells Using Raman Imaging
## 1 Introduction  
Mollusk shells are complex biomineralized structures composed primarily of calcium carbonate (CaCO\({}_{3}\)) and a minor organic matrix of proteins and polysaccharides [1, 2]. The organic matrix plays a critical role in regulating crystal nucleation and growth by interacting with dissolved ions, thereby directing the formation of specific mineral phases [3]. In addition to their structural and protective functions, mollusk shells serve as valuableenvironmental archives, as their mineralogical composition is influenced by genetic and environmental factors, including seawater temperature and ion availability [4]. Because shell deposition is controlled by both biological and environmental processes, these structures preserve essential information about the conditions at the time of their formation, making them indispensable proxies in fields, such as paleoceanography and archaeology [5; 6]. Among marine mollusks, _Patella vulgaris_ (common limpet) has received particular attention due to its widespread distribution in intertidal zones and frequent occurrence in archaeological sites [7]. The growth of _P. vulgaris_ shells is influenced by seasonal variations in environmental conditions, rendering them useful indicators for reconstructing past climates [8; 9]. Moreover, the distribution of mineral phases and trace elements within these shells provides insight into both their biomineralization processes and the external conditions during their formation [10; 11]. Biogenic calcium carbonate occurs in several polymorphs, with aragonite and calcite being most common in molluscan shells. These minerals exhibit distinct physical and crystallographic properties, which include variations in density, solubility, and mechanical strength [12; 13]. Both polymorphs coexist within a single shell, organized in layers or microstructural units (e.g., narcous or prismatic layers) [12]. The formation pathways leading to aragonite versus calcite depend on biological regulation within the mantle, environmental conditions such as temperature and carbonate ion concentration, and the presence of trace elements that can stabilize one form over the other [14]. In addition to calcium carbonate, mollusk shells contain organic pigments, such as carotenoids, which contribute to their distinctive colors and patterns [15]. The nature and distribution of these pigments can vary within or between species, influenced by both genetic factors and local habitat conditions [16]. These colored macromolecules frequently reside in the organic matrix layers, highlighting the complex interplay between organic and inorganic components during shell formation [17]. In archaeological contexts, shell coloration can degrade over time, but remnants of pigments or their degradation products sometimes persist [18]. Studies on shell pigments have identified variations in spectral signatures linked to different pigment types, allowing the differentiation of species and insights into shell formation processes [17; 19; 20]. Magnesium (Mg) is a common trace element found in marine carbonates and can significantly affect shell formation. The substitution of Mg\({}^{2+}\) for Ca\({}^{2+}\) in calcite modifies the crystal lattice and can favor one calcium carbonate polymorph over another [21]. Environmental factors, such as water temperature, salinity, and Mg/Ca ratios in the local habitat, influence how much Mg is incorporated into the developing shell [21; 22; 23; 24]. Experiments and analyses of synthetic carbonates indicate that Mg incorporation alters calcite growth dynamics [25]. Consequently, mapping the distribution of Mg in mollusk shells can reveal subtle patterns in seasonal growth or environmental change, while also helping to interpret shell-based proxies [22]. Traditional analytical techniques have contributed significantly to the study of shell composition but also present certain limitations. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are complementary techniques for material characterization.