the availability of silica from volcanic ash may be necessary for thick diatomite deposits. Diatomaceous earth is sometimes found on desert surfaces. Research has shown that the erosion of diatomaceous earth in such areas (such as the Bodélé Depression in the Sahara) is one of the most important sources of climate-affecting dust in the atmosphere. The siliceous frustules of diatoms accumulate in fresh and brackish wetlands and lakes. Some peats and mucks contain a sufficient abundance of frustules such that they can be mined. Most of Florida's diatomaceous earths have been found in the muck of wetlands or lakes. The American Diatomite Corporation, from 1935 to 1946, refined a maximum of 145 tons per year from their processing plant near Clermont, Florida. Muck from several locations in Lake County, Florida was dried and burned (calcined) to produce the diatomaceous earth. It was formerly extracted from Lake Mývatn in Iceland. The commercial deposits of diatomite are restricted to Tertiary or Quaternary periods. Older deposits from as early as the Cretaceous Period are known, but are of low quality. Diatomite deposits rich in fossils have been located in New Zealand, but mining of the Foulden Maar deposits on an industrial scale, for conversion to animal feed, has drawn strong opposition. Commercial form Diatomaceous earth is available commercially in several formats: granulated diatomaceous earth is a raw material simply crushed for convenient packaging milled or micronized diatomaceous earth is especially fine (10 μm to 50 μm) and used for insecticides. calcined diatomaceous earth is heat-treated and activated for filters. Usages Explosives In 1866, Alfred Nobel discovered that nitroglycerin could be made much more stable if absorbed in diatomite (kieselguhr). This allowed a much safer transport and handling than pure nitroglycerin under the liquid form. Nobel patented this mixture as dynamite in 1867; the mixture is also called guhr dynamite by reference to the German term kieselguhr. Filtration The Celle engineer, Wilhelm Berkefeld, recognized the ability of the diatomaceous earth to filter and developed tubular filters (known as filter candles) fired from diatomaceous earth. During the cholera epidemic in Hamburg in 1892, these Berkefeld filters were used successfully. One form of diatomaceous earth is used as a filter medium, especially for swimming pools. It has a high porosity because it is composed of microscopically small, hollow particles. Diatomaceous earth (sometimes referred to by trademarked brand names such as Celite) is used in chemistry as