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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.
Figure 10f-10: Gypsum. Several types of sedimentary rocks are formed from the lithification of once living organisms. Limestone deposits can be formed by the direct lithification of coral reefs, marine organism shells, or marine organism skeletons.igure 10f-10: Gypsum. Several types of sedimentary rocks are formed from the lithification of once living organisms. Limestone deposits can be formed by the direct lithification of coral reefs, marine organism shells, or marine organism skeletons.