File:Lacustrine diatomite with fossil gastropods (Mascall Formation, Middle Miocene; Rt. 21 roadcut, west of Mount Vernon & east of Dayville, Oregon, USA) 1 (19181616162).jpg

Lacustrine diatomite with fossil gastropods from the Miocene of Oregon, USA. (~20.5 cm across at its widest)

Diatomite, or diatomaceous earth, is an uncommon but distinctive biogenic 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 numerous 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, SiO2·nH2O). The diatom skeleton is called a frustule (see example photo - <a href="http://www.astrographics.com/GalleryPrints/Display/GP2131.jpg" rel="nofollow">www.astrographics.com/GalleryPrints/Display/GP2131.jpg</a>). Diatom frustules are usually 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.

The diatomite shown above comes from a Miocene-aged lake deposit exposed in a roadcut in Oregon. The outcrop has off-white colored lacustrine diatomites and macrofossiliferous diatomites with a few thin interbeds of water-laid volcanic tuff. The elongated & coiled structures in the rock are macrofossils of freshwater gastropods (Lymnaea cf. Lymnaea stearnsi). The diatomite itself is reported to be dominated by the centric diatom species Melosira italica.

Stratigraphy: lower Mascall Formation, lower Middle Miocene, 15 Ma