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Siliceous Rocks Help

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By — McGraw-Hill Professional
Updated on Sep 1, 2011

Siliceous Rocks

This type of sedimentary rock is commonly formed from silica-secreting organisms such as diatoms , radiolarians , or some types of sponges. It is most commonly called diatomaceous earth . Many expert gardeners use high silica containing diatomaceous earth to aerate and balance the acidity in soil.

Siliceous (silica-containing) rocks are sedimentary rocks with high silica (SiO 2 ) content.

Biologic sedimentary rocks form when large numbers of living organisms die, pile up, and are compressed and cemented to form rock. Accumulated and pressurized carbon-rich plant material may form coal. Deposits made mostly of animal shells may form limestone , coquina , or chert .

Diatomite looks like chalk and fizzes easily in acid. It is made up of microscopic plankton (tiny plants) called diatoms . When the silica from diatom remains is dried and powdered, it is used as one of the main ingredients in dynamite .

Chert (also known as flint ) is very different in appearance from diatomite. It is made of hard, extremely fine, microcrystalline quartz and can be dark or light in color. Chert is formed when silica in solution goes through chemical changes within limestone. It often replaces limestone and does not fizz in acid.

Flint was used by early hunters for spear and arrowheads. It was easily formed into points and sharp, cutting edges. Opal is a white or multicolored, less-developed crystalline form of chert used in jewelry. Opal has a high water content.

Coals (organic Sedimentary Rocks)

Organic sedimentary rock is made up of rocks that were originally organic material (like plants). Because of this, they don’t contain inorganic elements and minerals. These organic sedimentary rocks are known as coals .

Coals are usually described in the order of their depth, temperature, and pressure. They are made almost completely of organic carbon from the diagenesis of swamp vegetation. Coals contain the following types of materials:

  • Peat (spongy mass of brown plant bits a lot like peat moss),
  • Lignite (easily broken and black),
  • Bituminous coal (dull to shiny and black; sooty; sometimes with layers), and
  • Anthracite coal (very shiny and black, a bit of a golden gleam; low density; not sooty; could be a metamorphic rock from exposure to high temperatures and pressures).

Coal is formed from peat, which is a collection of rotting plants found in and around swamps. The conversion from peat to coal is called coalification . In the various stages of coalification, peat is changed to lignite , lignite is changed to subbituminous coal , subbituminous coal is changed to bituminous coal , and bituminous coal is changed to anthracite coal .

In the United States, coal is found in areas of eastern and western Kentucky, where it is layered between shales, sandstones, conglomerates, and thin limestones. The time span from approximately 320 million years ago and until about 30 million years is commonly called the Coal Age.

Practice problems of this concept can be found at: Sedimentary Rock Practice test

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