<Tr> <Th_colspan="2"> Composition </Th> </Tr> <Tr> <Td_colspan="2"> Calcite (calcium carbonate) </Td> </Tr> <P> Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite . Calcite is an ionic salt called calcium carbonate or CaCO . It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores . Flint (a type of chert) is very common as bands parallel to the bedding or as nodules embedded in chalk . It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction . Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint). </P> <P> Chalk as seen in Cretaceous deposits of Western Europe is unusual among sedimentary limestones in the thickness of the beds . Most cliffs of chalk have very few obvious bedding planes unlike most thick sequences of limestone such as the Carboniferous Limestone or the Jurassic oolitic limestones . This presumably indicates very stable conditions over tens of millions of years . </P>

Where does chalk come from and how is it made
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