Saturday, 14 March 2015

Limestone

Limestone

Limestone is a sedimentary rock that is primarily composed of Calcium carbonate in the form of calcite mineral. It forms mostly in the clear, warm and shallow marine water accumulates in the form of organic debris as coral, shells, algal or fecal deposits. It can also be formed by the precipitation of calcium carbonate dissolved in the water running in the ground water. 

Composition of limestone

Limetone as the name indicates is mainly composed of calcium carbonate, greater than 50%. A few percentage of other minerals are also present as all limestone contains them. The other minerals can be quartz, siderite, pyrite, clay minerals feldspar and other minerals. It also includes large nodules of chert, pyrite or siderite. The calcium carbonate can easily be identified when treated with acid, it reacts. 

Environment of deposition

Marine

The limestone is mainly deposited in calm, warm marine water at shallow depths where organism are capable of generating calcium carbonate shells and skeleton. These organism when die accumulate in the area to form limestone deposits. The waste product of these organism also contribute to sediment mass which in turn when lithifies produces limestone. The limestone produce from this type are biological sedimentary rocks. 
Other limestone is formed by direct precipitation from the marine or fresh water and this type is called chemical sedimentary rock. 

 Evaporite

The limestone formed by evaporation are often formed in the caves where water droplet enters the fracture and when reaching cave ceiling evaporates. When water evaporates the calcium carbonate present precipitates to form limestone. 

Varieties of limestone

There are many varieties of limestone based on the composition of limestone.
  • Chalk A soft limestone that is white or grey in colour which is mainly formed from calcareous shell remains of microscopic marine organisms.
  • Coquina A poorly cemented limestone that is composed mainly of broken shell debris.
  • Fossiliferous limestone A limestone with prominent and abundant fossils.
  • Oolitic limestone Limestone composed mainly of calcium carbonate oolites, spheres concentric precipitates of calcium carbonate.
  • Travertine Limestone formed by precipitation method mainly in caves.
  • Tufa Precipitation of limestone at hot springs or lake shore.

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