# Diatomaceous Soils and Advances in Geotechnical Engineering--Part I
## 2 Evolution of Concepts Surrounding Diatomaceous Soils
### Diatoms, Diatomaceous Soils, or Diatomaceous Earth  
The \"ornamentations\" or characteristic patterns of the frustules are due to the differential arrangement or fixation of silica in the cell wall. These are notable for having a definite organization and are helpful for classification. The main ornamentations include striae, pores, areoles, spines, raphe, and nodules [58]. From these patterns is possible to differentiate for example, centric and pennate species [59]. Figure 1: Representation of recent and fossilized freshwater and saltwater diatoms [52]. Figure 3: Morphology and classification of diatoms [50]. **(A)** Basic morphology of a diatom frustule. **(B)** Cross section of a typical double-layered pseudoloculated frustule. **(C)** Cross section of a single silica layer. **(D)** Three basic shapes of frustules: centric circular, centric multipolar (bipolar, triangular, star-shaped), and lateral pennate. Figure 2: Characteristic ornamentations and components of some frustules [58]. Above: **(a)** centric or radial species **(b)** “trelisiode” species **(c)** “gonoide” species **(d)** pennate species. Below: typical distribution of pores and striae for a rapid species. To date, more than 200 genera of diatoms are known, and it is estimated that there are about 100,000 extinct species; see Figure 1. The concentration of silica is mostly influenced by the species and not so much by environmental factors [60,61]. More than 10,000 species of living diatoms are recognized [8], and some authors specify a number between 12,000 and 60,000 [51], or even 100,000, of which between 1400 and 1800 are marine planktonic [50]. These magnitudes indicate a broad uncertainty and a sizeable genetic repertoire [52], as well as a great capacity for adaptation and evolution. The fossil record indicates that the global diversity of diatoms increased in the Cenozoic period (65 million years-today), especially in the Neogene (23.3-5.33 million years) [50]. The fossil record indicates that they evolved from the Cretaceous; fossil diatom deposits from the end of this period are known as diatomaceous earth [58]. Some authors define diatoms as soil deposits formed from fossilized algae which have settled in seas and lakes and are currently obtained by mining. Diatoms can be found whole or as the remains of opalized algal cells incorporated within clay, organic, siltstone, or quartz matrices [61]. The deposits result from the accumulation of amorphous silica in oceans or freshwater bodies from the walls (frustules) of diatoms or dead algae [62]. After their death and organic decomposition, the frustules are deposited on the bottom of oceans or lakes, forming DCs [42,47,48]. Ref. [63] refers to several authors and explains that, according to [PERSON] and [PERSON], the first use of the word diatomic dates back to the 19 th century from deep water deposits called diatom ooze. They also refer to [PERSON], who described \"pure diatomaceous earth\" as a material that reached a concentration between 95 and 98% silica. They expand on the work of [PERSON], who defined diatom ooze as a loose, unconsolidated sediment containing mainly diatoms. In the non-scientific colloquial literature, the term diatomaceous earth is used as a name for ground diatomic and diatom ooze, creating ambiguity in the use of diatomaceous earth as a definition. For this reason, Ref. [63] exclude \"diatomaceous earth\" from a proposed new classification system, as the term should be intended for use in the scientific literature and not in the public domain. Unconsolidated sediments containing diatom frustules in a proportion more significant than 80% of the total weight shall be termed diatomaceous ooze. Sediments with fractions between 50% and 80% of frustules, whose remaining component is clay and silt, will be termed claycy or silty diatomaceous ooze. When the fraction of frustules is less than 50%,  
Figure 4: Representation of centric and pennate species frustule [59] cited by [60]. **(A)** The dark gray discs represent the valves.