What is Limestone?
Limestone is a sedimentary rock consisting of more than 50% calcium carbonate (calcite – CaCO3). There are many different types of limestone formed through a variety of processes. Limestone can be precipitated from water (non-clastic, chemical or inorganic limestone), secreted by marine organisms such as algae and coral (biochemical limestone), or can form from the shells of dead sea creatures (bioclastic limestone). Some limestones form from the cementation of sand and / or mud by calcite (clastic limestone), and these often have the appearance of sandstone or mudstone. As calcite is the principle mineral component of limestone, it will fizz in dilute hydrochloric acid.
How is it formed?
Limestone is formed in two ways. It can be formed with the help of living organisms and by evaporation. Ocean-dwelling organisms such as oysters, clams, mussels and coral use calcium carbonate (CaCO3) found in seawater to create their shells and bones. As these organisms die, their shells and bones are broken down by waves and settle on the ocean floor where they are compacted over millions of years, creating limestone from the sediments and the pressure of the ocean water.
The second way limestone is formed is when water containing particles of calcium carbonate evaporate, leaving behind the sediment deposit. The water pressure compacts the sediment, creating limestone.
What does it look like?
Because limestone is often formed from shells and bones, it is a light colour like white, tan, or grey. The colour of the limestone depends on the other sediments in the mixture besides the mineral calcite, which is white; impurities such as sand, clay, and organic material are also present in limestone and affect the colour.
There are a few ways to recognize limestone. First of all, it is a soft stone and when it is scratched with a sharp object, it becomes a white powder. When limestone comes in contact with an acid like vinegar or hydrochloric acid (HCl), the stone will actually bubble and deteriorate and then neutralize the acid.
Limestone is a sedimentary rock, which means it was formed from small particles of rock or stone that have been compacted by pressure. Sedimentary rock is important because it often contains fossils and gives clues about what type of rock was on the Earth long ago. Just like a tree’s rings tell a lot about its environment, layers found in sedimentary rock can tell about important changes in the environment.
