Diatomaceous earth or diatomite is a light-colored sedimentary rock composed chiefly of siliceous shells (frustules) of diatoms. Diatomaceous earth is a soft and friable rock. It leaves hands dusty if touched and has a fragile feel as if it has a delicate and light-weight internal structure. This feeling is not misleading. Diatomite is composed of many unicellular algae with a hollow opaline test. Diatomite often floats in water just like pumice, at least initially before the pores are filled with water. The porous nature gives it many useful properties. Diatomaceous earth is widely used in industry for many different applications.

Pure diatomite is white as this sample from Armenia. Its width is 7 cm. Diatomite is friable and fragile very light-weight rock.

Diatomaceous earth is usually known as diatomite to geologists but for the general public diatomaceous earth is much more frequently used term. Geologically speaking, “diatomaceous earth” (often abbreviated DE or D.E.) is more appropriate term for unconsolidated sediment and diatomite for a consolidated rock. People who have experience with diatomaceous earth probably know it as a white powder, not as a rock type. However, this powder is directly manufactured from a naturally occurring rock.

There are more terms than diatomite and diatomaceous earth when referring to this material. German word kieselgur (or kieselguhr) has been used since the 19th century because first industrial scale mining of diatomaceous earth took place in Germany. This diatomite was used by Alfred Nobel as an absorbent and stabilizer for nitroglycerine. His invention which made the use of nitroglycerine much less hazardous is known as dynamite. It was the beginning of a large-scale diatomite use for industrial purposes although the material itself was known well before. Diatomite was used in antiquity by the Greeks as an abrasive and in making lightweight building brick and blocks. Blocks of diatomite were used for the dome of the Hagia Sophia church in Constantinople (now Istanbul in Turkey). This architectural wonder (its diameter is 30 meters) was much easier to construct with light-weight building blocks of diatomite¹.

Another term that may be encountered is tripolite. Originally it referred to diatomaceous earth deposits near Tripoli (capital of Libya) but in many cases the term has been applied to deposits far away from Libya. Hence, tripolite is just a synonym of diatomite. An impure (up to 30% clay) Danish variety of diatomite is called moler¹.

Diatomite deposits do exist in many places and are extensively mined nowadays because of its highly useful properties. The main producing countries are USA and China¹.

Diatomaceous earth is composed of diatom frustules. Diatoms are a large group of (mostly) unicellular algae. They live in both oceans and lakes and are one of the most common types of phytoplankton. Majority of diatoms use sunlight to photosynthesize, so they are producers in the food chain. Their frustules are almost bilaterally symmetrical, hence the name (Greek diatomos means ‘cut in two’). They are not entirely symmetrical because one half of their frustule valves is slightly smaller than the other to fit inside the edge of it. Frustules are made of amorphous opaline silica (SiO₂·nH₂O). The majority of frustules are between 5 and 200 micrometers in diameter³.

The most important thing for geology is the fact that they build their frustules out of silica that is in solution in sea or lake water. This silica comes from weathered silicate rocks which are very abundant in the crust. Diatoms are not the only ones who use silica. Some sponges for example secrete silica to make internal spicules that look like three-armed Mercedes stars. However, these spicules are never abundant enough to provide material for a distinct sediment and rock type. Another matter is with radiolarians (amoeboid zooplankton) who also use silica. Radiolarian and diatom shells fall to the bottom of the water body after its inhabitants die and form there radiolarian (radiolarite as a rock type) and diatomaceous ooze, respectively.

Diatomite deposits are mostly geologically young. Diatoms themselves have become abundant only in the past 50 Ma². Since the Miocene (it began 23 million years ago) diatoms have been major producers of lacustrine (related to lakes) sediments³. Most commercially exploited diatomaceous earth deposits are lacustrine as well although the largest producing deposit that outcrops near Lompoc in California is a marine deposit of Miocene age¹.

Impure diatomite from Georgia (country in the Caucasus). Common impurities in diatomaceous earth are clay minerals and iron oxides. Width of sample is 10 cm.

Diatoms can be really abundant if the supply of nutrients and dissolved silica are both high. Diatom blooms are often seasonal which can lead to varved sediments at the bottom of lakes. This is the case at some areas of Lake Malawi in Africa where diatom-rich layers form in dry windy season when turbulent mixing brings nutrients up to the surface but runoff from the land is at a minimum. Layers rich in silicate clastic material and organic debris form during rainy season³.

Most commercially exploited deposits of diatomaceous earth are lacustrine but diatoms are even more abundant or at least cover much larger areas of seafloor, especially at high latitudes. Most pelagic oozes are foraminiferal (47%). Siliceous ooze covers 15%. It is mostly diatomaceous ooze around Antarctica. Diatoms are also abundant in the extreme Northern Pacific north of the Aleutians. Radiolarians are common in equatorial waters. The rest of the abyssal plain (38%) is covered with abyssal brown clay. Sedimentation rate of diatomaceous ooze is 2-10 mm in thousand years⁴.

Siliceous oozes are a source material of siliceous rocks. Not only diatomite and radiolarite but also very hard cryptocrystalline siliceous rock known as chert or flint. Chert is of course very different material. It is dense and very hard without any pores at all but the material it is made of must be buried siliceous ooze. This is another reason why diatomaceous earth deposits tend to be young — older material is buried and compacted to form chert.

Diatomite from Russia. Width of sample is 12 cm.

Diatomaceous earth is used as a filter in wine and beer industry among many others. It is added to toothpaste because of its mildly abrasive properties. Diatomite is used as an absorbent in dynamite as mentioned earlier. It is a filler in rubber and plastics. It is also used in cat litter because it absorbs moisture. Porosity makes it a good thermal insulator. Diatomaceous earth has also find use as a natural insecticide. This list is not exhaustive. Some clever chaps are even selling it as a food additive that is supposed to do a lot of amazing things to our health. Whether it is effective or not is mostly a matter of belief in my opinion.

References

1. Dolley, Thomas P. (2000). Diatomite. US Geological Survey Minerals Yearbook. PDF-document.
2. Wignall, P. B. (2005). Anoxic Environments. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 495-501
3. Talbot, M. R. (2005). Lake Processes and Deposits. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 550-561
4. Piper, D. J. W. (2005). Deep Water Processes and Deposits. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 641-649