# Micromechanical Characterization of Diatom Frustules of Multiple Origin
## 1 Introduction  
Diatoms are unicellular algae with a siliceous skeleton called a frustule [1, 2], and they are present in almost all aquatic environments (fresh and salty) [3]. After their death and organic decomposition, the frustules are deposited at the bottom of water bodies, forming Diatomaceous Soils (DS) [4, 5, 6]. These fossils can be found whole or as fragments within the clay, organic, siltstone or quartz matrices [7]. Particles are characterized by having different pore patterns, shapes and sizes. These Silica structures are solid residues of metabolic processes [8, 9]. Diatoms have not become extinct due to their frustules' protective action, which presents supernormal mechanical characteristics, such as a high elastic modulus, which gives them restorative properties when subjected to loads and deformations [10, 11]. The surface properties of diatoms and their frustules should be analyzed at the macro scale, corresponding to the total shape of the shells, and at the nanoscale, related to pore networks and wall thickness. The frustules support external and internal mechanical loads due to predation, turgor and reproductive processes [9]. Sediments rich in diatoms presentdifferent physical properties from those categorized as organic and inorganic. Conventional geotechnical methods cannot completely characterize it [12; 13]. Scanning Electron Microscopy (SEM) is a small-scale visualization technique [14]. It provides information on the observed particles' topography, morphology, shape, size, composition and crystallographic structure [15; 16]. The behavior of some soils (silts, clays, DS) is only sometimes understood from the results of physical and mechanical tests. In such cases, it is necessary to assess the microstructure and fabric through SEM. Traditional characterization and mechanical evaluation tests need to provide more information about DSs [17]. From SEM tests, it has been possible to conclude that Diatomaceous Soils are mostly pure biogenic opal, with some mechanical alteration or fragmentation level. The quantity of fractured or intact diatoms inside another type of soil matrices depends on environmental factors [18]. This condition (breakup and rearrangement) of particles is corroborated through microscopy techniques [19]. Atomic Force Microscopy (AFM) allows for obtaining high-resolution 3D images from the change in surface height of the observed particle. It is applied to advanced studies of deformation, adhesion, Young's modulus, and nano and micromechanical properties, among others [20; 21; 22]. The AFM consists of a micromachined elastic cantilever with a sharp tip perpendicular to the cantilever. When a force is applied to the tip, the cantilever deflects. From AFM, stress and strain records can be obtained, even on the scale of single molecules [23]. About diatoms, AFM has been implemented for the total assessment of the cell through the progressive application of loads on its frustule, up to the point of reaching rupture, as well as for the evaluation of local properties, i.e., the strength of the different elements that make up the structure. The--Cs--swelling index obtained in a one-dimensional consolidation test on diatomaceous soil is up to twice as high as those recorded for kaolin-type soil [24]. This phenomenon can be associated with the elastic properties of diatom frustules. Diatoms are not entirely rigid and show considerable elastic deformation before breaking [11]. Diatom-rich sediments can, therefore, \"remember\" the stress regime in situ. However, these soils are not cohesive and can quickly lose or acquire water, which generally disturbs the samples and leads to the loss of stress memory [24]. After reviewing state-of-the-art regarding the application of microscopy techniques in diatomaceous fossils, the present research aimed to physically, chemically and mechanically characterize, by SEM and AFM observation, diatom frustules of three morphologically distinguishable species (Peruvian, Mexican and Colombian origin).