Gypsum

Classification：Sedimentary Rock

Collection of gypsum mineral crystals, which belongs to evaporate, usually associated with other evaporite minerals such as halite and anhydrite. In the form of lumpy and fibrous form. It is an important industrial raw material.

Gypsum can generally refer to two minerals: Gypsum and Gypsum. The raw Gypsum is two water calcium sulfate (Ca [SO4]. 2H2O), also known as two water Gypsum, water Gypsum or soft Gypsum, the theoretical component CaO32.6%, SO346.5%, H2O+20.9%, monoclinic, crystal plate, usually dense lump or fibrous, white or gray, red, brown, glass or silk luster, the moth hardness of 2, Jie Liping Line {010} is complete, density is 2.3g/cm3; hard Gypsum is anhydrous calcium sulphate (Ca [SO4]), the theoretical component CaO41.2%, SO358.8%, trapezoid system, crystal plate, usually dense lump or granular, white, gray white, glass luster, 3 to 3.5 moth hardness, cleavage parallel {010} complete, density 2.8 to 3.0g/cm3. The two kinds of Gypsum are often produced and can be transformed into each other under certain geological conditions.

Basic nature:
The natural two water Gypsum (CaSO4. 2H2O) is also known as the raw Gypsum. After calcining and grinding, beta type semi water Gypsum (2CaSO4 H2O) can be obtained, that is, building Gypsum, also known as mature Gypsum and plaster. If the calcination temperature is 190 degrees C, the model Gypsum can be obtained, and its fineness and whiteness are higher than those of building Gypsum. If the raw Gypsum is calcined at 400-500 degree C or higher than 800 degree C, that is, the setting and hardening of the floor Gypsum are slow, but the strength, wear resistance and water resistance after hardening are better than that of the ordinary building Gypsum.
Usually, white, colorless, colorless and transparent crystals are called Gypsum. Sometimes they are gray, light yellow and light brown due to impurities. White streak. Transparent. Glass luster, cleavage surface, pearl luster and fibrous aggregate silk luster. Cleavage is very complete and medium. The cleavage piece is split into 66 and 114 diamonds. It's crisp. The hardness is 1.5~2. There is a slight change in different directions. The relative density is 2.3. Polarizing mirror: colorless. Two axis crystal (+). 2V=58. Ng=1.530, Nm=1.523, Np=1.521. 2V decreases with increasing temperature, and 2V is zero at about 90 C.
During heating, there are 3 stages of discharge crystallization: at 105~180 C, 1 water molecules are discharged first, and then half water molecules are discharged immediately to burn Gypsum Ca[SO4]. 0.5H2O, also known as cooked Gypsum or semi water Gypsum. At 200~220 C, the remaining half of the water molecules were discharged into type III hard GypsumCa[SO4] H2O 0.06< (epsilon <0.11). At about 350 C, it is converted to type II GypsumCa[SO4]. At 1120 C, it is further transformed into type I hard Gypsum. The melting temperature is 1450 degrees centigrade.

Chemical composition:
The theoretical composition (wB%):CaO 32.5, SO3 46.6, H2O+ 20.9. There is little change in the composition. There are often mechanical mixtures such as clay, organic matter and so on. Sometimes it contains SiO2, Al2O3, Fe2O3, MgO, Na2O, CO2, Cl and other impurities.

Structural form:
Monoclinic system, a0=0.568nm, b0=1.518nm, c0=0.629nm, beta =11823', Z=4. The crystal structure is composed of [SO4]2- tetrahedron and Ca2+ connected to (010) bilayer, and the bilayer is connected by H2O molecule. Its complete cleavage occurs in this direction. The coordination number of Ca2+ is 8, which is associated with 6 O2- and 2 H2O molecules in 4 adjacent [SO4] tetrahedra. H2O molecules are linked to hydrogen bonds in O2- in [SO4], and water molecules are linked by molecular bonds.
Trapezoid columnar crystal, C2h-2/m (L2PC). The crystal is often developed into a plate and a granular form. It is usually simple: parallel double sided B, P, trapezoid column m, l, etc., with crystal face and regular longitudinal stripe, sometimes with lentils. The double crystal is common, one is Gary double crystal or swallowtail double crystal based on (100), and the other is the double crystal of Paris or arrowhead double crystal with (101) as the double crystal surface. The aggregate is densely granular or fibrous. The fine-grained mass is called snowflake Gypsum, and the fibrous aggregate is called fiber Gypsum. Rare rosette like aggregates form lentiform crystals. There are also soil - like and flaky aggregates.

Main origin:
The largest Gypsum producer in the world is the United States. In the United States, the Gypsum deposit is distributed in 22 states, with a total of 69 mines, the largest origin in Dodge in Iowa, followed by Canada, and France in the leading position in European Gypsum production; again for Germany, Britain and Spain. China is rich in Gypsum mineral resources. The 23 provinces (regions) of the country have Gypsum mineral resources. There are 169 proven reserves in the mining area, and the total reserves are 576 million tons. From the perspective of regional distribution, Shandong Gypsum mine accounts for the most, accounting for 65% of the total national reserves, followed by Inner Mongolia, Qinghai and Hunan. The main Gypsum mining areas include Shandong Zaozhuang bottom Pavilion, Inner Mongolia e - Chi Flag, Hubei City, Jilin Hun River, Jiangsu Nanjing, Shandong Da Wen mouth, Guangxi Qinzhou, Shanxi Taiyuan, Ningxia middle Wei Gypsum mine and so on.

Metallogenic type:
The Gypsum deposit is mainly hosted by sedimentary deposits, which account for more than 90% of the total reserves. The epigenetic and hydrothermal Gypsum deposits are not very important. Gypsum ore has been produced in all geological ages. The early Cretaceous and third stage sedimentary Gypsum deposits are the most important.
Mainly for the products of chemical deposition, it often forms a huge ore layer or lens, which occurs in limestone, red shale and sandstone, marl and clay rocks, often associated with hard Gypsum and stone salt. The hard Gypsum layer in the near surface area, due to the decrease of the external pressure, is changed to Gypsum:CaSO4+2H2O - CaSO4 / 2H2O by the action of surface water, and the volume increases about 30% at the same time, causing the destruction of the Gypsum layer.

Discriminant characteristics:
Low hardness, a group of completely cleavage, and various forms of morphology can be identified. Dense block Gypsum can be distinguished from carbonate by its low hardness and acid and no foaming.
One of the standard minerals in the classification of hardness.