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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.
Spicules of Sponges. 1  Spicules are formed by scleroblast cells present in the mesenchyme. 2  They consists of an axis of organic material around which calcium carbonate or silica is deposited. 3  They are of various shapes. 4  Monaxons-single axis growing in one or more directions. 5  Triaxons-three axes crossing each other and have six rays. 6  Tetraxons-four ...eneral Characteristics of Porifera. 1  Poriferans are commonly called sponges. 2  These are multicellular organisms which are sessile/sedentary in nature. 3  Most of them are marine while a few are fresh water forms. 4  Body is cylindrical, asymmetrical or has radial symmetry.