Chalky

Chalk is a soft, white, porous, sedimentary carbonate rock.

It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor.

Chalk is common throughout Western Europe, where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as the Dover cliffs on the Kent coast of the English Channel.

Chalk is mined for use in industry, such as for quicklime, bricks and builder's putty, and in agriculture, for raising pH in soils with high acidity.

It is also used for "blackboard chalk" for writing and drawing on various types of surfaces, although these can also be manufactured from other carbonate-based minerals, or gypsum.

Chalk is a fine-textured, earthy type of limestone distinguished by its light colour, softness, and high porosity.

It is composed mostly of tiny fragments of the calcite shells or skeletons of plankton, such as foraminifera or coccolithophores.

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.

The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such as molluscs, echinoderms, or bryozoans.

Chalk is typically almost pure calcite, CaCO3, with just 2% to 4% of other minerals.

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.

In some chalk beds, the calcite has been converted to dolomite, CaMg(CO3)2, and in a few cases the dolomitized chalk has been dedolomitized back to calcite.

Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent.

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).

In Western Europe, chalk was formed in the Late Cretaceous Epoch and the early Palaeocene Epoch (between 100 and 61 million years ago).

It was deposited on extensive continental shelves at depths between 100 and 600 m, during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock.

The lack of nearby erosion explains the high purity of chalk.

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.

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, resulting in the early cementation of such limestones.

In chalk, absence of this calcium carbonate conversion process prevented early cementation, which partially accounts for chalk's high porosity.

Chalk is also the only form of limestone that commonly shows signs of compaction.

Flint (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.

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).

Chalk is so common in Cretaceous marine beds that the Cretaceous Period was named for these deposits.

The name Cretaceous was derived from Latin creta, meaning chalk.

Some deposits of chalk were formed after the Cretaceous.

The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period.

It forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait.

The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage.

Some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Møns Klint in Denmark.

Chalk deposits are also found in Cretaceous beds on other continents, such as the Austin Chalk, Selma Group, and Niobrara Formations of the North American interior.

Chalk is also found in western Egypt (Khoman Formation) and western Australia (Miria Formation).

Chalk of Oligocene to Neogene age has been found in drill cores of rock under the Pacific Ocean at Stewart Arch in the Solomon Islands.

There are layers of chalk, containing Globorotalia, in the Nicosia Formation of Cyprus, which formed during the Pliocene.

Chalk is mined from chalk deposits both above ground and underground.

Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks.

Most people first encounter chalk in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased.

Blackboard chalk manufacturers now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate).

While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the developing world, use of carbonate-based chalk produces larger particles and thus less dust, and it is marketed as "dustless chalk".