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Calcite: calcite forms and is stable at the Earth's surface. Some calcite is formed from precipitation, but much of the calcite in sedimentary rocks is formed by biological activity. Many marine organisms secrete skeletal material (shells) of calcite.In addition to these three most abundant minerals, some sedimentary rocks also contain the iron oxides hematite and limonite, and small amounts of feldspars and micas.any marine organisms secrete skeletal material (shells) of calcite. In addition to these three most abundant minerals, some sedimentary rocks also contain the iron oxides hematite and limonite, and small amounts of feldspars and micas.
Corals are marine organisms of class Anthozoa (Kingdom Animalia, Phylum Cnidaria, Class Anthozoa) that typically live in large colonies of identical individuals. They are important reef builders in tropical seas, and are secretors of calcium carbonate (CaCO3) that forms their hard skeleton.nthozoans do not have a medusa stage in their development as do other cnidarians. Rather, they release sperm and eggs that form a planula that attaches to a surface on which they grow. A coral head contains thousands of genetically identical polyps, each typically a few millimeters in diameter.