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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.
The earth is covered with layers of sedimentary rock, much of it containing microscopic fossils such as plankton, pollen, and spores. The entire record of visible fossils consists mainly of marine invertebrates (animals without a backbone), including clams, jellyfish, and coral. What is surprising is that these ocean creatures are found primarily on the continents and rarely in the deep ocean basins.