What is limestone, where does it form and why is there so much of it?

attention to the shells of marine organisms in limestone. This indicates the biogenic origin of this rock. I decided to study how and where limestone deposits were formed, whether there are many of them on our planet.
It turned out that limestones are widespread in nature. Their huge deposits, formed millions of years ago from the skeletons of marine animals, account for approximately 20% of the total amount of sedimentary rocks. The place and role of limestones in the history of the Earth is evidenced by the fact that in geology an entire epoch of the planet’s development is called the Cretaceous period.
Limestones were formed as a result of not very complex, but long-term geochemical processes. Rivers annually carry out to the sea many millions of tons of lime in suspension and dissolved form. When river water meets salty sea water, a kind of “geochemical barrier” is formed, on which soluble compounds, including lime, precipitate, mixing with silt. Some of the calcium bicarbonate remains in a dissolved state and is gradually absorbed by marine plants and animals.
As a result, over millions of years, a huge number of shells of dead molluscs and corals have formed colossal accumulations of calcium carbonate. Thus, various limestones arose, among which, according to rock-forming organisms, coral, shell, nummulite, bryozoan, algal, etc. are distinguished.
Shell limestone (or shell rock) is a fairly porous rock, consisting almost entirely of whole or fragmented shells of marine organisms. Here you can find shells of mollusks of different types and sizes.
Sometimes the inner surface of the shells is covered with brushes of small transparent calcite crystals. If such a sample is viewed in sunlight, it seems that exquisite jewelry is interspersed into the stone.
All varieties of limestone provide the richest material not only for geology, but also for other sciences that study the history of the Earth: paleontology, paleobotany, etc.
Limestone has universal applications in industry, agriculture and construction. In metallurgy, limestone is used as a flux. In the production of lime and cement, limestone is the main component. Limestone is used in the chemical and food industries: as an auxiliary material in the production of soda, calcium carbide, mineral fertilizers, glass, sugar, paper. It is used in the refining of petroleum products, dry distillation of coal, in the manufacture of paints, putties, rubber, plastics, soaps, medicines, mineral wool, for cleaning fabrics and processing leather, liming soils.

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