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Composition
Because limestone is a sedimentary rock, different limestones are composed of different minerals. All limestones are primarily composed of calcium carbonate, which is one of the most abundant minerals on the planet. Calcium carbonate makes up at least 50 percent of all limestones in the form of calcite. Some limestones are almost pure calcite, while most limestones contain sand and other substances such as quartz, pyrite and clay minerals. Limestone is the result of the weight of layers of calcite and different minerals combined with the chemical reaction created by the heat and pressure.
Where is Limestone Formed?
Limestone is primarily formed in ocean waters where marine life containing calcium carbonate shells and skeletons are nourished from the ingredients in the water. Their shells and skeletons become sediments found in the limestone. When limestones are formed from marine life debris, they are known as biological sedimentary rocks and are the most common types of limestones. Chemical sedimentary rocks are developed by the direct precipitation of calcium carbonate by marine or fresh water. The third way for limestone to form is through evaporation. This can occur in caves where water might seep through cracks. If the water droplets evaporate before falling to the floor, the calcium carbonate that was in the water is left on the ceiling. The sediments will then grow downward, hanging like icicles. If the droplets fall to the floor first before evaporating, the calcium carbonate is left on the floor and the sediments grow upward.
Properties
Limestone is known to be impervious, which means that liquid or gas cannot pass through it. Limestone is also very hard and dense. Limestone's chemical makeup contains mostly calcium carbonate. Limestone is also chemically comprised of small amounts of alumina, ferric, and alkaline oxides. Limestone is known to be acid-sensitive; it's easily dissolved with acid. This property of limestone is often used in identifying it. When a cold 5 percent solution of hydrochloric acid is applied to limestone, it fizzes or bubbles.
Types
After their initial compaction, all limestones experience some sort of alteration process. Cementation, recrystalization, and compaction are just a couple of examples of the changes that could occur. Consequently, there are many varieties of limestones, whose names are designated based upon some of its properties. One type of limestone is chalk. Chalk is a limestone with a soft texture and is usually found in marine waters resulting from the remains of microorganisms containing calcium carbonate or from types of marine algae. Coquina is another type of limestone. Coquina is composed mainly of broken shell sediments. Travertine is formed by evaporative precipitation and is often found in caves.
Uses
Some types of limestones are strong and dense without many pores, allowing them to endure freezing and thawing and enabling its use in many construction projects. Most often, limestone is crushed and is used in road base, and to make concrete and cement. Limestone is also used as an animal feed filler because chickens need the calcium carbonate in it to help produce stronger eggshells. It also helps replenish calcium levels in cows that have lost large amounts when they are milked. Many of us have used it as chalk.