Diatomite

Diatomite, also called diatomaceous earth or kieselguhr, is a soft, fine-grained, porous sedimentary rock composed almost entirely of the fossilized silica skeletons (frustules) of diatoms, single-celled algae. It is extremely lightweight and chalky, often crumbling between the fingers.

The rock is usually white to pale cream or grey and feels powdery and dry. Under the microscope its intricate, lacy diatom shells are visible, giving the material its enormous surface area and porosity.

This porosity and chemical inertness make diatomite valuable far beyond its modest appearance, supporting many industrial applications.

Diatomite accumulates on the floors of lakes, lagoons and shallow seas where diatoms bloom in great numbers in silica- and nutrient-rich, often cool waters. When the diatoms die, their opaline silica shells sink and build up as a fine biogenic ooze.

Over time, with low input of other sediment, these shells accumulate into thick beds and lithify into diatomite. Marine deposits form along upwelling-influenced coasts, while freshwater diatomite forms in lakes. Major deposits occur in California, Nevada, Spain, France, Denmark and Mexico, many associated with volcanic regions that supplied dissolved silica.

Diatomite is recognized by being remarkably light, soft and chalky, often white or cream, and able to be scratched with a fingernail or crushed to powder. It is dry to the touch and highly absorbent, sometimes sticking slightly to the tongue.

It resembles chalk and certain volcanic ash (tuff) but, unlike chalk, does not fizz in acid because it is silica rather than carbonate. Compared to ordinary clay it is much lighter and less plastic. Confirmation is microscopic: the presence of countless intricate diatom frustules is definitive. Its low weight and non-effervescence in acid are the key field tests.

Diatomite is a versatile industrial mineral. Its porous structure makes it an excellent filtration medium for beer, wine, water, sugar syrups and swimming pools. It is also used as a mild abrasive, an absorbent (cat litter, spill cleanup), and a functional filler in paints, plastics and rubber.

It serves as an insulating material and was historically used to stabilize nitroglycerin in dynamite. Food-grade diatomaceous earth is sold as a natural insecticide and anti-caking agent. Caution is warranted with dust inhalation, as crystalline silica forms can be hazardous.