The key difference between limestone and chalk is that the limestone contains both minerals, calcite, and aragonite whereas chalk is a form of limestone which contains calcite.
Limestone is a type of sedimentary rock. It mainly contains different crystal forms of calcium carbonate. Therefore this mineral is highly alkaline. Chalk is a form of limestone. It has many favorable properties, which we will discuss below. Although it is a form of limestone, 99% of this mineral contains calcite type crystal form.
CONTENTS
1. Overview and Key Difference
2. What is Limestone
3. What is Chalk
4. Side by Side Comparison – Limestone vs Chalk in Tabular Form
5. Summary
What is Limestone?
Limestone is a sedimentary rock, which contains two major types of minerals; calcite and aragonite. These minerals are different crystal forms of calcium carbonate (CaCO3). This sedimentary rock forms as a result of the deposition of skeletal fragments of marine organisms such as coral, forams and molluscs. Therefore, this mineral mostly forms in clear, warm and shallow marine waters. However, this sedimentary rock can form by the precipitation of calcium carbonate from lake or ocean water as well.
The reason why these rocks form in clear, warm and shallow marine waters is that this type of environment allows the marine organisms to form calcium carbonate shells. Moreover, their skeletons can easily extract the necessary ingredients form ocean water. then when these organisms die, their skeletal debris accumulates as sediment. This sediment later lithify into limestone. In addition to that, the waste products of these organisms also contribute to the formation of this sedimentary rock.
Figure 01: The Great Pyramid of Giza
There are several varieties of limestone such as chalk, coquina, fossiliferous limestone, lithographic limestone, tufa, etc. They differ from each other according to the formation, appearance and composition as well.
There are many uses of limestone. It is common in architecture; there are many landmarks that were made from this rock. Ex: The Great Pyramid. Moreover, it is the major ingredient in making quicklime, slaked lime, cement, etc. Apart from that, we can use this mineral as a soil conditioner to neutralize acidic soils.
What is Chalk?
Chalk is a form of limestone containing calcite as the major mineral; about 99%. Therefore, it is also a sedimentary carbonate rock. This type of rocks forms under deep marine conditions, from the accumulation of calcite shells. These calcite shells come from the microorganism coccolithophores. Moreover, chalk has a common type of embedded nodules (or as bands) named Flint.
Figure 02: A Chalk Pit
Chalk has many favorable properties. For example, it has a great resistance to weathering, slumping than the associated clays. Therefore, this mineral forms tall, steep cliffs. We can mine chalk from chalk deposits. These deposits may occur both above ground and underground.
There are many uses of this mineral. Among all, the most common application is the manufacture of blackboard chalk. In addition to that, we can use it as a source of quicklime (via thermal decomposition reactions) or as a source of slaked lime (via quenching quicklime with water). Moreover, we can use chalk to fit the woodworking joints.
What is the Difference Between Limestone and Chalk?
Limestone is a sedimentary rock which contains two major types of minerals; calcite and aragonite. This is because it forms as a result of the deposition of skeletal fragments of marine organisms such as coral, forams and molluscs. Whereas, chalk is a form of limestone containing calcite as the major mineral; about 99%. This is because it forms from the accumulation of calcite shells of the microorganism coccolithophores. Moreover, chalk contains flints. The below infographic presents more details on the difference between limestone and chalk.
Summary – Limestone vs Chalk
Limestone is a sedimentary carbonate rock. Chalk is a type of limestone. The key difference between limestone and chalk is that the limestone contains both minerals, calcite, and aragonite whereas chalk is a form of limestone which contains calcite.
Reference:
1. “Limestone.” Geology. Available here
2. Britannica, The Editors of Encyclopaedia. “Chalk.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 22 Oct. 2013. Available here
Image Courtesy:
1.’Gizeh Cheops BW 1″By Berthold Werner – Own work, (CC BY-SA 3.0) via Commons Wikimedia
2.”Seale Chalk Pit 02″By Lewis Hulbert – Own work, (CC BY-SA 3.0) via Commons Wikimedia