Diatomite, or diatomaceous earth, is a very distinctive sedimentary rock. It is whitish, powdery, and very lightweight. It seems quite soft, but the individual particles making up the rock are siliceous (opaline silica), having a hardness around 6 on the Mohs Scale. This makes diatomaceous earth a wonderful mild abrasive. It is mined for use in a wide variety of products. Most people encounter diatomites everyday as one of the ingredients in toothpaste (the opaline silica scrapes away foreign material from tooth surfaces).
Diatomites form by the accumulation of billions of diatom skeletons in lake or marine settings. Diatoms are very small, unicellular, photosynthetic organisms (Kingdom Protista, Phylum Bacillariophyta). Some call them "algae", but they're not. Some call them "plants", but they're not. They're protists. Diatoms make a two-part skeleton composed of opal (opaline silica, SiO2ĚnH2O). The diatom skeleton is called a frustule (see example photo). Diatom frustules are either rounded or elongated, and the two parts of the skeleton nest into each other, like a large petri dish over a slightly smaller petri dish. Careful examination of fossil diatoms typically requires use of a scanning electron microscope (SEM). Diatomites contain immense numbers of many different species of fossil diatoms.
The appearance of diatomite closely resembles chalk. Chalk is calcareous, and will bubble in acid - diatomite won't do that. Chalk is also noticeably heavier than diaomite. Diatomite and chalk also resemble kaolinite, a clay mineral. Kaolinite will not bubble in acid. It also has an earthy feel and an earthy smell, especially when wet. Kaolinite becomes noticeably sticky when wet.
Diatomite (diatomaceous earth) (6.8 cm across) from the Miocene-aged Monterey Formation at a diatomite quarry just south of Lompoc, Santa Barbara County, southern California, USA.