Minerals anhydrite

Both the anhydrous calcium sulfate and dihydrate occur in nature, the former as the mineral, anhydrite, and the latter as gypsum. Gypsum is widely distributed in nature. It has been known since ancient times. 

II Anhydrite 11 insoluble anhydrite inactive anhydrite dead-burned gypsum chemical anhydrite mineral anhydrite Produced hy calcining at 250-1.000 C. Relatively inert. Reactivity depends upon calcining-time-lemperaiure relationship and particle size. 

FIG. 17-3. The Frasch process for mining sulfur. (The mineral anhydrite which lies below the sulfur-calcite layer is anhydrous calcium sulfate, CaS04.)... 

Anhydrous calcium sulfate occurs naturally as the mineral anhydrite. The naturally occurring rock gypsum may be crushed and ground for use as the dihydrate or calcined at 150°C to produce the hemihydrate. A purer variety of calcium sulfate may also be obtained chemically by reacting calcium carbonate with sulfuric acid or by precipitation from calcium chloride and a soluble sulfate. 

Both the anhydrous and dihydrate forms of calcium sulfate occur naturally in the form of the minerals anhydrite, angelite, muriacite, and karstenite (CaS04) and gypsum (CaS04-2H20). These minerals have been known to humans and used by them for thousands of years. The method for converting natural gypsum to the hemihydrate (plaster of... 

Cogenetic sulphate minerals (anhydrite) are in equilibrium with seawater sulphate. 

Complex Sulfate Ores- Complex mixtures of potash ores may contain any or all of the foQowing minerals anhydrite, epsomite, halite, kainite, kieserite, langbeinite, polyhalite, and sylvite along with clays. Extraction of potassium salts from such ores becomes quite complicated because the mineralogical forms of the various components can be unstable. Whenever potassium sulfate is the desired product, free sylvite and kainite must be present in the same molecular proportions. Ideally the reaction is as follows ... 

The analysis of the primary minerals remains constituting the impurities in the secondary crystals enables determination of the diagenetic processes taking place in the evaporite deposits (including the mineral precursor for the secondary crystal), and the direction and cause of diagenetic transformations (e.g. anhydrite gypsification piitnaiy mineral -anhydrite, cause - presence of fresh or low-mineralized water in the deposit, e.g. as a result of tectonic uplift and exposition to the activity of shallow underground water). 

Blanc P, Baumer A, Ceshron E, Ohnenstetter D, Panczer G, Remond G (2000) Systematic cathodoluminescence spectral analysis of synthetic doped minerals anhydrite, apatite, calcite. 

Sulfur constitutes about 0.052 wt % of the earth s cmst. The forms in which it is ordinarily found include elemental or native sulfur in unconsohdated volcanic rocks, in anhydrite over salt-dome stmctures, and in bedded anhydrite or gypsum evaporate basin formations combined sulfur in metal sulfide ores and mineral sulfates hydrogen sulfide in natural gas organic sulfur compounds in petroleum and tar sands and a combination of both pyritic and organic sulfur compounds in coal (qv). 

Calcium sulfate [7778-18-9J, CaSO, ia mineral form is commonly called gypsum and occurs abundandy ia many areas of the wodd. In natural deposits, the main form is the dihydrate. Some anhydrite is also present ia most areas, although to a lesser extent. Mineral composition can be found ia Table 1. 

Anhydrite is formed as a result of tire HF produetion proeess, when fluorspar mineral (CaF,) reaets with H SO (ee). However, before using tliis subproduet, anhydrite must be neutralized with CaO resulting in a maximum amount of 1.5 % CaO. The neutralization reaetion generates water, and then a part of CaO is hydrated. 

Fluoride production consists on reacting fluorspar mineral (CaF ) with H SO (cc) to form HF and calcium sulphate. However, reactions do not achieve 100 % yield, so fluorogypsum or anhydrite (CaSO ) obtained contains small amounts of fluoride as CaF in its stmcture. The application of this kind of subproduct is well-known as constmction material, so that the flouride content has to be controlled. 

Hydration is the incorporation of water mole-cule(s) into a mineral, which results in a structural as well as chemical change. This can drastically weaken the stability of a mineral, and make it very susceptible to other forms of chemical weathering. For example, hydration of anhydrite results in the formation of gypsum ... 

Gypsum is a relatively soluble mineral and can undergo dissolution vhereas anhydrite is less soluble. 

Of the principal minerals of Ca listed in Table 1, the most important ores are the various deposits of CaCOj, especially limestones, which occur as immense sedimentary beds over extensive parts of the earth s surface. Extraction of Ca from CaCOj is a simple and relatively inexpensive process. Although the other Ca-bearing minerals are rarely considered as potential Ca sources, they are widely distributed and extensively mined fluorite and apatite for their fluoride and phosphate content, gypsum and anhydrite for their use in construction. 

Dominant gangue minerals in Kuroko deposits are quartz, barite, anhydrite, gypsum, chlorite, sericite, and sericite/smectite. Morphology of quartz changes from euhedral in the centre to the irregular in the margin of the deposits (Urabe, 1978). No amorphous silica and cristobalite have been found. 

Kuroko deposits are characterized by large amounts of sulfate minerals (barite, anhydrite, and gypsum). Estimated total amount of barite and sekko (gypsum + anhydrite) from individual deposit is shown in Table 1.4. Sr contents of gypsum, anhydrite and barite... 

Isotopic compositions of minerals and fluid inclusions can be used to estimate those of Kuroko ore fluids. Estimated isotopic compositions of Kuroko ore fluids are given in Table 1.10. All these data indicate that the isotopic compositions lie between seawater value and igneous value. For instance, Sr/ Sr of ore fluids responsible for barite and anhydrite precipitations is 0.7069-0.7087, and 0.7082-0.7087, respectively which are between present-day. seawater value (0.7091) and igneous value (0.704-0.705). From these data, Shikazono et al. (1983), Farrell and Holland (1983) and Kusakabe and Chiba (1983) thought that barite and anhydrite precipitated by the mixing of hydrothermal solution with low Sr/ Sr and seawater with high Sr/ Sr. 

---