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Diatomite The term ‘Diatomite’ is used to refer to biogenic, siliceous deposits composed mainly of the frustules of diatoms with varying amounts of detrital material, mainly clay & silt. (It is sometimes called 'Tripolite' or 'kieselguhr'.)
The diatoms, which are pelagic phyto-plankton, are composed of amorphous colloidal silica, in the so-called Opal A phase of silica mineralogy. The diatoms sink to the bottom of the ocean or lake to form an ooze that gradually becomes lithified as the water is squeezed out. At about 125ºF, due to increasing depth of burial, the Opal A becomes unstable and changes diagenetically into Opal CT. (Opal CT is sometimes called 'porcelanite' because it has a porcelain-like texture.) Dissolution and recrystallization of the Opal A results in the minerals cristobalite & tridymite, which are more stable forms of silica. These minerals are abbreviated as Opal CT. With still higher temperatures of about 190ºF, the Opal CT in turn changes to microcrystalline quartz which is the final and most stable form of silica.
The temperature and therefore depth for these changes is also dependant upon the amount of clay minerals present. Increasing clay content increases the temperature required for the conversion of Opal A to Opal CT by as much as 15ºF which deepens the Opal A to CT transition depth; increasing clay content also decreases the temperature required for the conversion of Opal CT to quartz by up to 20ºF, which decreases the Opal CT to quartz transition depth.
Initial porosity of the diatomaceous ooze is about 90%. By the time the diatoms are buried to about 1000 ft (320 m) the porosity has reduced to about 75%. Typical porosities in the cleaner, clay-poor Opal A phase are 50 to 70% with air permeabilities around 0.1 to 2 md. Typical porosities in the cleaner, clay-poor Opal CT phase are 30 to 45% with air permeabilities around 0.01 to 2 md. Typical porosities in the cleaner,microcrystalline quartz are 5 to 10% with air permeabilities around 0.01 to 0.1 md. However, natural fracturing can contribute significantly to the permeabilities, especially in the quartz phase rocks.
Diatomite has various uses. It is mined and ground into a powder caled 'diatomaceous earth' that is an excellent filtering medium (e.g., used for water purification and swimming pool filters) and has various uses in the food industry (e.g., bread, toothpaste). In the upstream oil industry, diatomite is an important but rare reservoir and an excellent source rock. Key areas for diatomite to occur are around the Pacific Rim (especially in California, Japan, and Siberia) and in India where they are lake deposits.
Noteworthy papers in OnePetro
http://www.nobelprize.org/alfred_nobel/biographical/articles/krummel/kieselguhr.html - Alfred Nobel used diatomaceous earth to stabilize nirtoglycerine, made his fortune and founded the Nobel prizes.