# Diatomaceous Soils and Advances in Geotechnical Engineering--Part I
## 2 Evolution of Concepts Surrounding Diatomaceous Soils
### Diatom Frustules  
Diatom frustules are a clear example of micro- and nano-structured materials available in nature, derived from biomineralization processes. They have four main functions in a diatom: to generate mechanical stability, to separate and filter nutrients from viruses, to regulate the rate of sinking, and to regulate the amount of light entering the cell [68]. They have multiple pores or areoles and are elongated, spherical, or disc-shaped, with variable sizes between 10 and 200 \(\upmu\)m [29]. Frustules are amorphous silica nanospheres deposited in geometric [50] and dimensional patterns specific to each species [68].  
The morphological characteristics of diatoms and their frustules should be analyzed on two scales. The macro corresponds to the shell's total shape, and the nano is related to the pore networks and the thickness of the walls. These constructive principles aim to generate self-supporting structures from the minimum silica consumption. The frustules must be able to withstand mechanical loads, not only external but also internal, due to the effects of turgor and reproductive processes [52].  
The morphological and surface characteristics of the frustules, including size, pores, and stiffness, are utterly different from those of other inorganic particles whose diameters vary between 30 and 100 \(\upmu\)m. Ref. [38] report diatom frustules extracted in Jawornik (Poland) with primarily cylindrical and centric (discoid) shapes, radii between 50 and 60 \(\upmu\)m, and a highly porous surface. Pore sizes range from 100 to 200 nm. No traces of amorphous or dissolved silica were identified within the pores. Conventional geotechnical methods have not been proven to be sufficient to complete the characterization of DSs [69]. Ref. [14] suggest establishing a classification for diatomites based on their engineering properties and the particular biological structure of the frustules.  
The effect of the frustules on the water retention properties of DS and claystones has yet to be fully recorded in the literature. The content of the fossils affects both the fabric and the microstructure of the soils containing them and hence the retention properties [17]. In kaolinite soils artificially dosed with fossils, the suction values and geometry of the \"Water Retention Curve\" (WRC) present important changes as the number of fossils is increased. The WRC is restricted with the addition of diatoms; likewise, the range of mobilized suction becomes smaller [17].  
Silica uptake in diatoms is mainly related to aerobic respiration processes and less so to photosynthesis [60,70]. Silica in aquatic environments is mainly called silicic acid or Si(OH)4 [71]. When acid concentrations are low, cells activate silica transporters that facilitate fixation [60,72]. The extent of the silicification of frustules depends on the rate of silica fixation, either by availability in the environment or by the effectiveness and genomic characteristics of its transporters [73; 74]. Growth depends on factors such as salinity, aeration, pH, and amount of light (natural or artificial). However, there are records of species that can grow in the dark [75].  
In metabolic terms, the frustules' silica can be considered an impermeable and solid excrement. However, evolutionary adaptations have led it to be considered a dominant element in the physiology and morphogenesis of cells [52]. The existence of two porous layers in the structure of diatoms helps to reduce friction between them and the ambient water and increases the ability to resist liquid pressure. Depending on the shape and species, the frustules can act as self-repairing and self-lubricating media [76].