It was deposited on extensive continental shelves at depths between 100 and 600 metres (330 and 1,970 ft), during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock. In Western Europe, chalk was formed in the Late Cretaceous Epoch and the early Palaeocene Epoch (between 100 and 61 million years ago). While it is similar in appearance to both gypsum and diatomite, chalk is identifiable by its hardness, fossil content, and its reaction to acid (it produces effervescence on contact). Ĭhalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent.
In some chalk beds, the calcite has been converted to dolomite, CaMg(CO 3) 2, and in a few cases the dolomitized chalk has been dedolomitized back to calcite. These are usually quartz and clay minerals, though collophane (cryptocrystalline apatite, a phosphate mineral) is also sometimes present, as nodules or as small pellets interpreted as fecal pellets. Ĭhalk is typically almost pure calcite, CaCO 3, with just 2% to 4% of other minerals. The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such as molluscs, echinoderms, or bryozoans. These fragments mostly take the form of calcite plates ranging from 0.5 to 4 microns in size, though about 10% to 25% of a typical chalk is composed of fragments that are 10 to 100 microns in size. It is composed mostly of tiny fragments of the calcite shells or skeletons of plankton, such as foraminifera or coccolithophores. Chalk is a fine-textured, earthy type of limestone distinguished by its light color, softness, and high porosity.
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