It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. įlint (a type of chert) is very common as bands parallel to the bedding or as nodules in seams, or linings to fractures, embedded in chalk. Chalk is also the only form of limestone that commonly shows signs of compaction. Because this conversion process is responsible for the early cementing of limestone, chalk lacked early cementing, which partially accounts for its high porosity. This is in contrast with most other limestones, which formed from high-magnesium calcite or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition.
![black chalk black chalk](https://www.thepaintedbench.ca/wp-content/uploads/2018/12/Black-Chalk-Paint-Wax-Group-500ml-and-120ml-open-with-brush.jpg)
The coccolithophores, foraminifera, and other microscopic organisms from which the chalk came mostly form low-magnesium calcite skeletons, so the sediments were already in the form of highly stable low-magnesium calcite when deposited. The lack of nearby erosion explains the high purity of chalk. 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. Formation Ĭhalk was formed in the Cretaceous, between 99 and 65 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.
![black chalk black chalk](https://mir-s3-cdn-cf.behance.net/project_modules/disp/ff52f09190217.560ca148bb932.jpg)
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.