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Bryozoan Archimedes on matrix
The mineral Calcite consists of calcium carbonate (CaCO3). It is formed when water soluble Calcium ions (Ca2+) and carbon dioxide (CO2) react. Many organisms actively produce calcite to form some sort of skeleton e.g. echinoderms.
Calcite has the same chemical composition as Aragonite, it does however have a different crystalline rearrangement. Aragonite is a less stable form of calcium carbonate and easier to dissolve. The ‘hard’ parts of fossil mollusks (shells) and scleratarians (corals) originally where made up from Aragonite. The Aragonite in certain depositions is often replaced by calcite during diagenesis.
There have been periods with either more Calcite or more Aragonite deposition depending on conditions and chemical composition of the ocean water and the diversity of species at the time these sediments where deposited.
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