Gypsum is an evaporite mineral most commonly found in layered sedimentary deposits in association with halite, anhydrite, sulfur, calcite, and dolomite. Gypsum (CaSO4.2 H2O) is really similar to Anhydrite (CaSO4). The chemical difference is that gypsum includes two waters and anhydrite is without water. Gypsum is the most common sulfate mineral.
Occurrence of Gypsum
Gypsum is present in a variety of types and is of great economical value. It is colorless or white but can be tinted light brown, gray, yellow, green, or orange due to the presence of impurities. Single, well-developed crystals can be blocky with a slanted parallelogram outline, tabular, or blade. Twinned crystals are common and regularly form characteristic “fishtail”.
Many transparent, swordlike selenite gypsum crystals 61 ⁄ 2 ft (2 m) or more long can be found at the Cavern of Swords, Chihuahua, Mexico, one of the world’s most spectacular mineral deposits. Gypsumoccurs in substantial beds formed by the evaporation of ocean saltwater. It likewise occurs as a modification item of sulfides in ore deposits and as volcanic deposits.
Properties of Gypsum
|Color||Colorless to white; might be yellow, tan, blue, pink, brown, reddish-brown, or gray due to impurities.
|Luster||Vitreous, Sub-Vitreous, Silky, Pearly, Dull.
|Transparency||Transparent, Translucent, Opaque
|Hardness||2 on Mohs scale
|Density||2.312 – 2.322 g/cm3 (Measured) 2.308 g/cm3 (Calculated).
Uses of Gypsum
Gypsum uses include the manufacture of wallboard, cement, plaster of Paris, soil conditioning, a solidifying retarder in Portland cement. Varieties of gypsum called “satin spar” and “alabaster” are utilized for a variety of ornamental functions; however, their low hardness limits their sturdiness.
(a) Role of Gypsum in Agriculture
Gypsum, a hydrated calcium sulfate, is a mineral that occurs in large deposits throughout the world. Gypsum is applied to the soil as a source of calcium and sulfur. The calcium supplied by plaster in fertilizers is of value in crop production in areas where soils go through extensive seeping.
Sulfur has been recognized as a vital constituent of plants. For centuries, sulfur substances had been applied to soils because of their observed helpful impact on plant development. Aside from functioning as a constituent of protein and numerous other substances in plants, sulfur has an impact on chlorophyll development in plant leaves.
Although not a constituent of chlorophyll plants deficient in sulfur exhibits a pale green color. The root system of numerous plants has been observed to be significantly bigger by the application of sulfur. It has actually been reported that excellent crops are produced by the application of sulfur consisting of products such as gypsum.
(b) Role of Gypsum in Industries
When gypsum is heated under thoroughly regulated conditions, it loses 3 quarters of the water of formation. The resulting product is called Plaster of Paris. Plaster must not be heated up too strongly as the anhydrous salt is then formed which takes in water gradually. Such plaster is called ‘Dead burnt.
Plaster of Paris when mixed with half of its weight of water, forms a plastic-type thick mass and then sets to a hard permeable mass. This process is finished within 10 to 15 minutes. Throughout the procedure expansion of about 1% in volume also occurs, which fills the moulds entirely, and thus a sharp impression is attained. Plaster of Paris is used for making plaster walls, casts of statuary, coins, and so on. It is utilized in surgical treatment; Plaster of Paris bandages are used for holding in location fractured bones after they have actually been set.
Special plasters contain plaster of Paris and other ingredients which differ with the demands of the use to which they are to be put. Two varieties of plasters are made.
(1) Cement Plaster
It is plaster of Paris to which generally glue or other oils have actually been included as retarders to prolong the time of setting.
(2) Hard Finish Plasters
These are made by the calcination of the anhydrous sulfate with alum or borax. These plasters are set very slowly however offer a difficult finish. When mixed with wood pulp and enabled to embed in the form of boards, it forms a material, much used in the construction of buildings as wallboards and partitions. Plaster is likewise utilized as a filler in paper markets.
Portland cement is made by highly heating a finely powdered mix of clay and limestone. The final product, known as clinker, is cooled and after that ground into a very fine powder. During the grinding, there is added about 2% of gypsum which prevents the cement from solidifying too rapidly. The addition of gypsum increases the setting time of cement.