The gypsum is a Minéral composed of Sulfate hydrated Calcium of formula CaSO₄, 2 (H₂O) as well as a evaporitic Roche . The name comes from Latin gypsum, borrowed from the Greek γύψος, gypsos , of the same direction.

The gypsum is a very common mineral of the sedimentary sequences and can form mineral Roches mono .

Properties

• crystalline System: monoclinical

• hardness Mohs: 2-3
• unit weight: 2,3-2,4 (according to the impurities)
• glare: vitreous or silky
• Color: colorless, white, yellowish, of gray with black, chestnut to brown, pink with dark red…),

Varieties

The gypsum can crystallize in very diverse forms. The most known forms are the crystals in foot of lark, or the saccharoid gypsum (with sugar aspect). The Albâtre is a variety of massive gypsum with fine grains; it is translucent. She is opposed to the selenite ( stone of the moon ), also called “glossed spar”, which is a variety in seams with fibrous structure.

The pink of sands are lenticular gypsum crystallizations of which the provision points out the rose petals. They are formed by infiltrated water evaporation. These crystallizations meet in tender grounds (sand, clay), mainly in the deserts.

Natural layers

The gypsum is one of the sulfate ores among most common. It is formed in general by sedimentation during the evaporation of cut sea water lagoons of the sea, by the crystallization of the salts contained in water. It thus forms part of the Sedimentary rocks.

• when the level of the oceans increases, of the lagoons fills.
• when the level drops, these lagoons are cut sea, its water evaporates and the gypsum settles at the bottom.

The gypsum deposits are covered then by other sediments or subjected to other geological influences. The gypsum can lose the water molecules retained during its crystallization to give rise to the Anhydrite, not hydrated, (CaSO₄), which gypsum retransforme slowly if it comes again into contact with water.

In the layers, according to the conditions, the gypsum crystallizes in various ways, forming more or less large crystals in particular.

In France of important gypsum deposits were formed there is between 100 and 200 million years. 70% of the reserves are in the Paris basin, where the extraction and the transformation of the gypsum employ: 3400 paid and generate a sales turnover of 740 million euros. In 2005,3,1 million tons of ores were extracted from the careers franciliennes whose principal ones are currently with Cormeilles-in-Parisis, Villevaudé, Vaujours and under the Forêt of Montmorency, but the exploitation gypsière marked many other sites, such as the Plateau of Avron, the Ridge-Finch, the hillock of Romainville…

A curiosity: in Mexico, the mine of Naica lodges crisaux gypsum giants which can reach 10 meters length. The phenomenon is explained by particular conditions of crystallization very stable starting from water having dissolves Anhydrite.

The natural gypsum just like the Anhydrite are extracted either with open sky or in underground.

Gypsum of synthesis

The gypsum of synthesis is the product of an industrial chemical reaction. Independent sources of chemical gypsum:

• the manufacture of the phosphoric acid;
• the manufacture of other mineral acids (acid citric…) ;
• the desulphurization of gases.
• the neutralization of acid water (sulphuric) of industries of the titanium dioxide

gypsum of desulphurization In Germany, the law of 1983 concerning protection against the toxic rejections in the atmosphere forces the thermo plants with fossile fuels to be equipped with installations of desulphurization of gases of smoke. It is about a simple process of treatment of smoke, resting on the use of lime wet (production of lime generating nevertheless strong gas emissions with greenhouse effect). The gypsum crystals thus obtained can be used as raw material with the industry of construction materials. A similar process is used for the treatment of acid gases resulting from the incineration of the household refuse.

the Phosphogypsum The phosphoric manufacture of acid led to very important tonnages of phosphogypsum often poured at sea, which is source of pollution. Many studies were undertaken to substitute it for the natural gypsum, in particular in the manufacture of gypsum blocks. The drying of the latter proved to be prohibitory. On the other hand, the manufacture of the " variety; alpha" Hémihydrate of calcium sulfate, obtained by pressure-sealing in the presence of mineral additives makes it possible to obtain crystals of size definitely more important and whose drying is definitely less expensive.

Use

The gypsum, like the Anhydrite, is used for the manufacture of the Plâtre, whose request is increasing because of the boom of real construction current.

By heating, one obtains a hémihydraté sulfate which after crushing forms a binder which gypsum réhydrate in contact with water.

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