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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.
Diatoms like this one, Amphiprora sp., are one of several major types of marine phytoplankton. These microorganisms that live near the ocean surface and convert carbon dioxide into organic carbon via photosynthesis. They produce much of the oxygen we breathe and are the base of the marine food chain. They also play an important role in drawing heat-trapping carbon dioxide from the atmosphere into the ocean.