The chalk defines itself as:
- A limestone (= rock made of calcium carbonate CaCO3 = calcite weakly magnesian), type biomicrite, texture mudstone to packstone;
- Porous (porosity from 30 to 40 %),
- Crisp and striking (on the blackboard).
- In generally very fine grain.
She is generally pure, to more than 95 % of CaCO3, but she can contain other minerals:
- Clayey minerals (phyllosilicates), giving a marly chalk or a marl;
- Of the glauconie, giving a chalk glauconieuse;
- Some phosphate of calcium, in the form of phosphate tricalcique, in the condensed chalky or on-surface matrix; rare in the Country of Callus, he can train exploitable deposits in the North of France;
- Of the dolomite, giving a chalk dolomitique;
- Some flint, giving a chalk to flint.
Here is the chemical composition of a chalk of Liège (Siwak, on 1994):
Today the chalk settles only in big depth in the oceans, but in the Cretaceous it settled on platforms or certain ponds flooded with continents.
Here are some properties of chalks of the Country of Callus (Interreg IIIa, final report):
|Cénomanien||15,4 - 41,3||0,1 - 13,1||1,6 - 2,1|
|Turonien||26,3 - 47||0,1 - 4,1||1,7 - 2,1|
|Coniacien||22,6 - 38,3||1,1 - 2,6||1,6 - 1,9|
|Santonien||40,2 - 45,6||3,5 - 5,6||1,6 - 1,8|
|Campanien||43,4 - 45||3,8 - 6,5|
The chalk contains cavities, with big pores from 1 to 5 µm and smaller pores. The solid grains are bound by adhesion or coalescence or by bridges of calcite. This porosity is an essential characteristic of the chalk and it can remain in very big depths of burying (the gas fields of The North Sea prove it).
The resistance to monoaxial compression is different depending on whether the chalk is dry or saturated in water (Como, 1971). We note a weakening of more than one half for the saturated chalk. Similarly, in the horizontal direction, this resistance is much weaker :
Compression resistance (bar)
100 - 120
80 - 100
40 - 55
25 - 35
When she is dry, the chalk breaks in a explosive way and he forms of small small columns directed in the sense of the main constraint.