Chalk - a kind of limestone , sedimentary rock of organic (zoogenic) origin, consisting of cryptocrystalline calcite . It has a white color, soft and crumbly, soluble in water. The refractive index is 1.55.
Composition
The basis of the chemical composition of chalk is calcium carbonate with a small amount of magnesium carbonate , but usually there is also a non-carbonate part, mainly metal oxides . An insignificant admixture of the finest quartz grains and microscopic pseudomorphosis of calcite in fossil marine organisms ( radiolarians , etc.) are usually found in the chalk. Large fossils of the Cretaceous period are often found: belemnites , ammonites , etc.
The use of chalk and its analogs
Chalk - a necessary component of coated paper used in the printing industry for printing high-quality illustrated editions. Ground chalk is widely used as a cheap material ( pigment ) for whitewashing, painting fences, walls, borders, to protect tree trunks from sunburn.
Chalk is not used in the paint industry (white pigment is usually a compound of magnesium, zinc) because of the characteristic structure of chalk particles, but used in rubber, paper, sugar industry - to clean beet juice, for the production of binders ( lime , Portland cement ), in the glass industry, for the production of matches. In these cases, usually use the so-called. Chalk precipitated , obtained by chemical means from calcium-containing minerals . Mel found wide application as a dispersed filler for polymer compositions (polypropylene and polyethylene). Chalk is used for writing on large boards for general viewing (for example, in schools ) (molded school chalk contains 40% chalk ( calcium carbonate ) and 60% gypsum ( calcium sulfate ). In animal husbandry, chalk is used as a feed additive, providing The required amount of calcium in the diet of birds and livestock. Chalk as an abrasive is a part of many toothpastes. In the pharmaceutical industry, chalk is used as an auxiliary substance (filler) in medicines. With a lack of calcium, medical chalk can be prescribed food supplement. It should be noted that in X-ray studies of the gastrointestinal tract is used a substance which is sometimes mistaken for chalk. In fact this slurry as the water-insoluble barium sulphate .
The chalk islands of Needles off the west coast of the Isle of Wight
Red chalk rock
White chalk rock
Links
- Chalk // Encyclopedic dictionary of Brockhaus and Efron : in 86 tons (82 tons and 4 extra). - SPb. , 1890-1907.