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Chemical Limestones
Some limestones form by direct precipitation of calcium carbonate from marine or fresh water. Limestones formed this way are chemical sedimentary rocks. They are thought to be less abundant than biological limestones. Most biological limestones contain significant amounts of directly precipitated calcium carbonate. After the biological grains have accumulated and are buried, water that is…
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Biological Limestones
Most limestones form in calm, clear, warm, shallow marine waters. That type of environment is where organisms capable of forming calcium carbonate shells and skeletons can thrive and easily extract the needed ingredients from ocean water. When these animals die, their shell and skeletal debris accumulate as a sediment that might be lithified into limestone.…
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What is Limestone?
Limestone is a sedimentary rock composed primarily of calcite, a calcium carbonate mineral with a chemical composition of CaCO3. It usually forms in clear, calm, warm, shallow marine waters. Limestone is usually a biological sedimentary rock, forming from the accumulation of shell, coral, algal, fecal, and other organic debris. It can also form by chemical sedimentary processes, such as the precipitation…
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What is Iron Ore Used For?
The primary use of iron ore is in the production of iron. Most of the iron produced is then used to make steel. Steel is used to make automobiles, locomotives, ships, beams used in buildings, furniture, paper clips, tools, reinforcing rods for concrete, bicycles, and thousands of other items. It is the most-used metal by…
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ow Does Iron Ore Form?
Nearly all of Earth’s major iron ore deposits are in rocks that formed over 1.8 billion years ago. At that time Earth’s oceans contained abundant dissolved iron and almost no dissolved oxygen. The iron ore deposits began forming when the first organisms capable of photosynthesis began releasing oxygen into the waters. This oxygen immediately combined…
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What is Iron Ore?
Earth’s most important iron ore deposits are found in sedimentary rocks. They formed from chemical reactions that combined iron and oxygen in marine and fresh waters. The two most important minerals in these deposits are iron oxides: hematite (Fe2O3) and magnetite (Fe3O4). These iron ores have been mined to produce almost every iron and steel object that we use today – from…
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Gems in Skarn Deposits
A variety of gemstones have been found in skarn. Garnet, ruby, and sapphire are the gems most commonly found in skarn. Here are a few examples: Demantoid garnet and topazolite have been mined from the Antetezambato skarns near Ambanja, northern Madagascar. Sapphires are mined from skarn in the Andranondambo Region of Madagascar. Yellow scapolite has been mined from a skarn…
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Minerals Found in Skarn
Skarns often contain a diverse assemblage of metamorphic minerals. The mineral assemblage in a skarn is determined by the lithology of the invaded rock, the chemistry of the invading liquid, and the temperature of the rock environment. Metamorphic minerals that characterize the skarn environment include a wide range of calc-silicates, many types of garnet, and a…
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Other Skarn Environments
In the example described above, skarn formed in a carbonate rock unit adjacent to a magma intrusion. There are numerous other geological situations where skarn can be formed. These include: Skarn can form with various water inputs. These include include:
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Skarn as a Complex Rock Mass
Skarns can form on both sides of the boundary between a magma body and its surrounding rock mass. Those formed on the igneous side of the contact are known as endoskarns. Those formed on the country-rock side of the contact are known as exoskarns. Exoskarns form when the original chemistry of a rock mass is altered –…