Limestones chemical limestone formed from the precipitation of CaCO3 from sea water biological limestone formed from organic remains such as shell fragments Most limestones contains more than 90% of CaCO3, usually in the form of calcite, so observing its mineral composition is a good method of identification. Modern limestone is composed of aragonite, a form of CaCO3 that is unstable in rocks and changes to calcite. Calcite reacts vigorously with dilute HCl and has two cleavage directions, not at right angles, making it easy to identify.Oolitic LimestoneThese are chemically formed limestones containing sub-spherical sand sized grains about 1mm in diameter, called ooliths. In cross-section, ooliths show concentric layers of calcium carbonate surrounding a nucleus, which may be a grain of sand, shell fragment or pellet. The ooliths are surrounded either by a fine grained calcite mud matrix (micrite) or a crystalline calcite cement (sparite). They form in a tropical or sub-tropical sea, in shallow water agitated by high energy waves.Fossiliferous LimestonesThese biologically formed limestones are composed of fossils or fragments of fossil fragments, but they are not clastic. They may have a micrite matrix or sparite cement. One common type is a crinoidal limestone that is made up of stem sections or single ossicles with rare plates formed from the calyx or arms of a crinoid. Reef limestones contain fragments of corals as well as many brachiopod or biovalve shells. Many other limestones can be described as shelly, made almost entirely from bivalve or gastropod shells. Fossil rich limestones can form in a range of environments from low energy freshwater lakes or lagoons, such as the gastropod Viviparus limestone of the Jurassic, to marine beds, such as the Ostrea bed full of bivalves, also of the Jurassic. If the fossils are whole then the rock is likely to have been formed in a low energy environment, while broken fossils suggest high energy.ChalkThis is a biologically formed limestone composed of coccoliths, the calcareous disc or oval shaped platelets that form part of the skeletons of single-celled algae. The chalk is white as it is pure calcium. Chalk is commonly found in low energy, deep water shelf environments.