# Diatomaceous Soils and Advances in Geotechnical Engineering--Part I
## 2 Evolution of Concepts Surrounding Diatomaceous Soils
### Diatomites  
_Diatomites_ are sedimentary rocks composed primarily of fossilized remains of freshwater and/or saltwater unicellular photosynthetic algae with silica skeletons, known as diatoms [38; 65]. DSs are derived from the progressive sedimentation of soil layers with microfossils [48]. Diatomites present diversity in rock formation processes, mineralogy, and even impurities; therefore, their chemical and physical-mechanical properties and fields of application will depend on their sedimentation conditions and the geological time at which this occurred [66]. Diatoms can accumulate in large numbers and remain long enough to form sediments (diatomites), composed chiefly of frustules [54]. When the diatoms die, they sink into the aqueous medium, forming a sediment of organic composition; the decomposition of the organic residue gives way to the accumulation of siliceous shells, which are compacted to become potential diatomic deposits [64]. In the \"sedimentological literature\", diatomic is a friable, light-colored sedimentary rock with a diatomaceous content of at least 50%. However, in the \"quaternary scientific literature\", diatomic is commonly used to describe a sediment type containing many diatom frustules without precisely defining such abundance [63]. Soils formed from microfossils (except foraminiferal skeletons) are called oozes when their calculated concentration by weight is higher than 50% [8]. The environments in which these microfossils originate are diverse, but diatom oozes are generally found on aqueous surfaces in polar latitudes. There are records of diatom oozes from large areas in the southern Pacific Ocean, on both sides of the 60\({}^{\circ}\) parallel, which are up to 1500 km wide. Ref. [63] suggests that sediment of which more than 50% by weight is composed of diatoms (SiO2) and with a high porosity (>70%) can be called diatomaceous ooze if it is unconsolidated and diatomite if it is consolidated. Diatomites are rocks, and diatoms are their constituents. Diatomite is a white to cream-colored rock, soft to the touch, and chalk-like in appearance, with little or no conductivity. It is inert to most chemicals and gases and has a high porosity (pores <l \(\upmu\)m) and low thermal conductivity, as well as a high absorptive capacity and low bulk density (<l g/cm\({}^{3}\)), which allows it to float on water when not saturated. Additionally, it has a high permeability and high specific surface area. The purer the diatomic is, the whiter its color will be. Any yellowish tones are due to iron oxides [38; 63; 64]. Diatomite is composed almost exclusively of diatomic fossils. It records two types of pores: skeletal pores and interparticle pores. Diatomite samples from Oita (Japan) report skeletal pores with diameters generally less than 1 \(\upmu\)m. Centric diatoms of Cyclotella species have variable diameters between 20 \(\upmu\)m and 40 \(\upmu\)m. Studies on the clay soils of Osaka, Japan, report variable skeletal pore diameters between 0.02 \(\upmu\)m and 1 \(\upmu\)m [24]. As presented in Figure 5, the designation \"diatomic\" will apply strictly to consolidated sediments made up of more than 80% frustules. Consolidated sediments made up of less than 50% frustules by weight will be named based on the principal components--e.g., diatomaceous claystone or diatomaceous siltstone, or diatomaceous mudstone. The porosity of these consolidated sediments must be less than 70%. Diatomite and diatomaceous sediments are formed at low temperatures and pressures (<50 \({}^{\circ}\)C, <600 m depth) [63]. Diatomite has a mild abrasive ability, high thermal resistance, a melting point between 1400 \({}^{\circ}\)C and 1750 \({}^{\circ}\)C, and a hardness of 7/9, and is chemically inert [58]. Mineralogically, diatomites are composed of opal and accessory minerals, such as quartz, clays (kaolinite, illite, smectite), hematite, limonite, and calcite, among others. These compositions vary from one deposit to another [64].