sedimentary
Chalk (Limestone variant)
Calcium Carbonate (CaCO3)
Hardness: 2.5-3 (Mohs scale), Color: White to light grey or tan, Luster: Dull/Earthy, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal or irregular fracture), Specific Gravity: 2.3-2.6
- Hardness
- 2
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 2.5-3 (Mohs scale), Color: White to light grey or tan, Luster: Dull/Earthy, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal or irregular fracture), Specific Gravity: 2.3-2.6
Formation & geological history
Formed from the accumulation of microscopic marine organisms like coccolithophores over millions of years. Most large deposits date back to the Cretaceous period (approx. 145 to 66 million years ago).
Uses & applications
Used in agriculture to raise soil pH, in the construction of cement and mortar, and historically as a writing medium and blackboard chalk.
Geological facts
The 'White Cliffs of Dover' in England are the most famous geological example of chalk. It is highly porous and can act as a massive natural carbon sink.
Field identification & locations
Identify in the field by its soft, powdery texture that easily rubs off on fingers and it will vigorously effervesce (bubble) when a drop of diluted hydrochloric acid is applied. Commonly found in coastal bluffs and quarries.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary
Sandstone (with man-made markings)
Arenite (composed primarily of Quartz, SiO2)
sedimentary