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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.
Crustose coralline algae play a variety of important roles in many marine ecosystems, particularly coral reefs. We call them coral reefs because their three-dimensional structures are built by stony coral animals, which produce limestone as they grow ever upwards towards the sun. Corals may create the major structuresâthe bricks of the reef, if you willâbut no brick wall will stand strong without cement. Enter our unassuming friends the crustose coralline algae. They grow as a crust over and between the fragments and gaps in coral reefs and essentially cement the coral bricks together.