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Diatomite

Pictures of Sedimentary Rock Types

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Diatomite ("die-AT-amite") is an unusual and useful rock made up of the microscopic shells of diatoms. It is a sign of special conditions in the geologic past. (more below)
Made of microscopic shells
Photo (c) 2011 Andrew Alden, licensed to About.com
Diatomite may resemble chalk or fine-grained volcanic ash beds. But pure diatomite is white or nearly white and quite soft, easy to scratch with a fingernail. When crumbled in water it may or may not turn gritty, but unlike degraded volcanic ash it doesn't turn slippery like clay. When tested with acid it will not fizz, unlike chalk. It is very lightweight and may even float on water. It can be dark if there is enough organic matter in it.

Diatoms are one-celled plants that secrete shells out of silica that they extract from the water around them. The shells, called frustules, are intricate and beautiful glassy cages made of opal. Most diatom species live in shallow water, either fresh or salt.

Diatomite is very useful because silica is strong and chemically inert. It's widely used to filter water and other industrial liquids including foods. It makes excellent fireproof lining and insulation for things like smelters and refiners. It's a very common filler material in paints, foods, plastics, cosmetics, papers and much more. Diatomite is part of many concrete blends and other building materials. In powdered form it's called diatomaceous earth or DE, which you can buy as a safe insecticide—the microscopic shells injure insects but are harmless to pets and people.

It takes special conditions to yield a sediment that is nearly pure diatom shells, usually cold water or alkaline conditions that do not favor carbonate-shelled microorganisms (like forams), plus abundant available silica, often from volcanic activity. That means polar seas and high inland lakes in places like Nevada, South America and Australia—or where similar conditions existed in the past, as in Europe, Africa and Asia. Diatoms are not known from rocks older than the Early Cretaceous, and most diatomite mines are in much younger rocks of Miocene and Pliocene age (25 to 2 million years ago).

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