Chalk

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):

Oxide
%
CO2
44
CaO
53,5
SiO2
1,7
MgO
0,3
K2O
Tracks
Fe2O3
0,1
Na2O
Tracks
P2O5
0,2
SO2
Tracks
TiO2
Tracks
Al2O3
0,2

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):

Porosity Permeability Density
% Millidarcy
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 :

Sample Direction
Compression resistance (bar)
Chalk dries Verticalehorizontale

100 - 120

80 - 100

Saturated chalk Verticalehorizontale

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.