The criteria to denominate a calcareous rock as chalk are as follows:

1. Texture: soft, earthy, friable, fine-grained

2. Structure: almost structureless/ massive (internally porous)

3. Composition mineralogical: 90 to 99 % calcite with chert (amorphous to crystalline SiO2) nodules and pyrite cubes

4.Rock color: white to whitish gray

5.Component organic: calcareous tests of microorganism mainly of forams, algae, (e.g., coccoliths, rhabdoliths) broken apart and washed together in a shallow-marine environment

6. Post-depositional alteration: bioturbation by bottom-dwelling organism, e.g., echinoderms

See as reference the Late Cretaceous chalky limestones around Dover and Calais along the Channel.

H.G.Dill

Dear Ali,

Chalk consists almost wholly (90 - 99 %) of calcite, with the chemical formula CaCO3 => calcium carbonate

Best regards,

Guenter Grundmann

Guenter Grundmann is totally right. I just wanted to add that other components may include clay minerals, SiO2 (crystalline or amorphous), and other carbonates.

There are many available papers on the subject:

https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=chalk+composition&btnG=&oq=chalk+composition

The criteria to denominate a calcareous rock as chalk are as follows:

1. Texture: soft, earthy, friable, fine-grained

2. Structure: almost structureless/ massive (internally porous)

3. Composition mineralogical: 90 to 99 % calcite with chert (amorphous to crystalline SiO2) nodules and pyrite cubes

4.Rock color: white to whitish gray

5.Component organic: calcareous tests of microorganism mainly of forams, algae, (e.g., coccoliths, rhabdoliths) broken apart and washed together in a shallow-marine environment

6. Post-depositional alteration: bioturbation by bottom-dwelling organism, e.g., echinoderms

See as reference the Late Cretaceous chalky limestones around Dover and Calais along the Channel.

H.G.Dill

Chalk consists mainly (90 - 99 %) CaCo3