Chalk is commonly formed in low energy, deep water shelf environments. The chalk is white as it is pure calcium carbonate. Chalk This is a biologically formed limestone composed of coccolithophores (coccoliths), the calcareous disc or oval shaped platelets that form part of the skeletons of single-celled algae. 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. 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. Many other limestones can be described as shelly, made almost entirely of bivalve or gastropod shells. Reef limestones contain fragments of corals as well as many brachiopod or bivalve shells. 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. They may have a micrite matrix or sparite cement. They can be described as bioclastic if they are made of fossil fragments. Fossiliferous limestone These biologically formed limestones are composed of fossils or fragments of fossils. They form in tropical or sub-tropical seas, in shallow water agitated by high-energy waves. The ooliths are surrounded either by a fine grained calcite mud matrix ( micrite ) or a crystalline calcite cement ( sparite ). In cross-section, ooliths show concentric layers of calcium carbonate surrounding a nucleus, which may be a grain of sand, shell fragment or pellet. Oolitic limestone These are chemically formed limestones containing sub-spherical sand-sized grains about 1mm in diameter, called ooliths. Calcite reacts vigorously with dilute HCl and has two cleavage directions, not at right angles, making it easy to identify. Modern limestone is composed of aragonite, a form of CaCO 3 that is unstable in rocks over time and changes to calcite. Most limestone contains more than 90% CaCO 3, usually in the form of calcite, so observing its mineral composition is a good method of identification. A rock containing more than 50% calcium carbonate is limestone. Biological limestone formed from organic remains such as shell fragments.Chemical limestone formed from the precipitation of CaCO 3 from sea water.Limestones Limestones fall into two groups: They are also known as limestone, or dolomitic limestone depending on their mineral content. 60 Classification of carbonate rocks Specification area: 2.1.3(d) (ii) (iii) Sedimentary rocks containing a high percentage of the minerals calcite (CaCO 3 ) or dolomite (CaMg(CO 3 ) 2 ) are classed as carbonate rocks.
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