The Limestones

Limestones consist essentially of calcium carbonate, with which there is generally some magnesium carbonate, and siliceous matter such as quaertz grain. The average of over 300 chemichal analyses of limestone showed 92% of  CaCo3 and MgCO3 together, and 5% of SiO2 ; the proportion of magnesium carbonate is small except in dolomite and dolomitisc limestone. The limestones considered here are those mainly by the accumulation of carbonate detritus. They are bedded rocks often containing many fossils; they are readily scratched with a knife, and effervesce on addition of cold dilute hydrochloric acid (except dolomite). The distance between bedding planes in limestones of centimeters or less in thin bedded rocks (such as the Stonesfield ‘Slate’) to over 6 m in some limestones.

Calcium carbonate is present in the form of crystals of calcite or aragonite, as amorphous calcium carbonate and also as the hard parts of organism(fossils) such as shells and calcareous skeletons, or their broken fragmens. Thus, a consolidates shell sand is a limestone by virtue of the calcium carbonate of which the shell are made. On the other hand, chemically deposited calcium carbonate builds limestone under conditions where water of high alkalinity has a restricted circulation, as in a shalloe sea or lake. Non calcareous constituents commonly present in limestones include clay, silica in colloidal form or a quartz grains or as parts of siliceous  organims, and other hard detrital grains. Though usually grey or white in colour, the rock may be tnted by iron compound or finely divide carbon, or by bitumen. common type of limestone are now described.

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(Source : A Geology for Engineers, Blyth & Freitas)