Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal, and fecal debris. It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.
Limestone is by definition a rock that contains at least 50% calcium carbonate in the form of calcite by weight. All limestones contain at least a few percent other materials. These can be small particles of quartz, feldspar, clay minerals,pyrite, siderite, and other minerals. It can also contain large nodules of chert, pyrite, or siderite.
The calcium carbonate content of limestone gives it a property that
is often used in rock identification – it effervesces in contact with a
cold solution of 5% hydrochloric acid.