Chalk

Chalk is a pretty common type of rock – probably one of the more commonly encountered. Yup – the every day household ‘chalk’ was originally made of regular chalk. These days, other sources of material are sometimes used – such as gypsum.

Chalk is a form of Limestone, essentially formed by layers and layers of microorganisms shedding their shells over time. Fine examples of Chalk cliffs can be found around the UK – notably the white cliffs of Dover and the Hunstanton chalk formation.

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Appearance, Uses and History

Chalk is an extremely soft sedimentary rock, formed by the layering of Calcite, clay, and silt – largely from the accumulation of tiny Calcite shells and the remains of marine algae. It is an opaque rock, which is fine grained and very porous.

While it is primarily made up of these tiny pieces, it often contains larger fossils including bivalves and belemnites. This is especially obvious in the Hunstanton Red Chalk, which is extremely fossiliferous.

Flint and Chert often occurs in Chalk in seams and fractures, sometimes as replacements for the fossil inclusions.

It is probably best known for one of its past usages – as the material used for drawing on blackboards. The mineral crumbles easily when rubbed against a hard surface, which left a very clear mark. Modern blackboard chalk is no longer made from the mineral Chalk, though.

Chalk does have some other important uses, as it can be used as a source of Quicklime, a widely used chemical. It is also used for its properties as a chemical base. Chalk is alkaline, and can be used for raising pH in soils.

It is also used as a very mild abrasive and cleaning agent and is sometimes used for fine polishing of softer metals. Historically, it was used as a building substance, but this was mostly pre-Victorian.

In England, chalk was often used to carve hill figures, known as geoglyphs. The most famous of these are the Cerne Abbas Giant, and the Uffington White Horse.

 

Locales

The Chalk Group of Europe is one of the best known locales for Chalk, which forms the White Cliffs of Dover, the Cap Blanc Nez, and some amazing cliffs in Jasmund National Park, and Mons Klint.

Chalk does, however, have a worldwide distribution – I just write from a European perspective.

Mineralogy

Chemistry
Calcium Carbonate – CaCO3.
Colours and Variations
Usually white or grey, with some red outcrops.
Streak
White
Luster
Dull
Fracture
Earthy or crumbly
Crystal habit
Mohs hardness
1 – 3
Specific Gravity
2.3 – 2.4
Easiest testing method
Chalk is relatively easy to test. A simple hardness test can be performed, as Chalk is very soft. It may be fossiliferous, so check with a loupe. It will also strongly react with dilute acids, which will help to differentiate from Gypsum.
Common Treatments
N/A.

Hazards and Warnings

 

Almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis.
When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.

Translations

Arabic:

  • طباشير

Hindi:

  • चाक

Portuguese:

  • o giz

Bengali:

  • খড়ি

Indonesian:

  • kapur tulis

Punjabi:

English:

Italian:

  • gessetti

Russian:

  • мелки

French:

  • craies

Japanese:

  • チョーク

Spanish:

  • tizas

German:

  • Kreide

Korean:

Thai:

  • ชอล์ก

Gujurati:

Mandarin and Traditional Chinese:

  • 粉笔
  • 和以白堊

Urdu:

Further Reading / External Links