Main Component

• Calcium carbonate is the main component of chalk, which is in the form of calcite that is found in the soft and fine-grained type of limestone. The soft nature of the limestone allows it to be easily ground into dust for chalk. The large size and low surface area makes it useful as a filler for paper production, as it does not negatively affect the strength of the paper as much as other fillers.
  
  

Calcium Carbonate Origins

• Large deposits of chalk and its main component, calcium carbonate, can date back more than 65 million years ago from the Cretaceous Period. According to the Encyclopedia Britannica, chalk's calcium carbonate is formed when generations of small marine organism shells build up deposits, with large deposits found in western Europe, England and parts of the Southeastern U.S. These shelled organisms are often called coccolith and are what separate chalk from other minerals.
  
  

Other Mineral Components

• While calcium carbonate makes up the majority of chalk, other components are also incorporated. Encyclopedia Britannica lists these other components as what contribute silica to even the purest forms of chalk: sponge spicules, shells, detrital grains of quartz and flint. There are also small amounts of clay minerals, glauconite and calcium phosphate. (See Ref 1)
  
  

Uses for Chalk

• While chalk can be used as a filler for making paper, it does easily produce a solid color. Other materials must be added to ensure the paper is not transparent. The most common use of finely ground chalk is to make putty due to its absorption of oil, aging qualities and plasticity. Chalk can also be found in lime, portland cement, fertilizer and pigments.