Anhydrite addition

CaSO4-0-5HiO, prepared by heating gypsum at 130 "C, is used as plaster of Paris. Gypsum and anhydrite are used for H2SO4 production and gypsum is used as a soil additive and as an inert additive to pharmaceuticals and insecticides. 

P-Hemihydrate. The dehydration of gypsum, commonly referred to as calcination in the gypsum industry, is used to prepare hemihydrate, or anhydrite. Hemihydrate is generally called stucco in North America and plaster in many other continents. In North America, plaster is differentiated from hemihydrate or stucco by the inclusion of additives to control intended use properties, eg, rehydration time, density, coverage, strength, and viscosity. 

Anhydrite. In addition to ketde calcination (Fig. 1), soluble anhydrite is commercially manufactured in a variety of forms, from fine powders to granules 4.76 mm (4 mesh) in size, by low temperature dehydration of gypsum. 

In the three simulations, the sulfate minerals form at mixing ratios related to their solubilities. Barite, the least soluble, forms early, when small amounts of seawater are added. The more soluble celestite forms only after the addition of somewhat larger quantities of seawater. Anhydrite, the most soluble of the minerals, forms from the Amethyst fluid at still higher ratios of seawater to formation fluid. 

Luminescence of trivalent Eu was not detected in natural barite yet, but the possible emission maybe seen on artificially activated BaS04 (Eig. 5.15). The Sr addition generates the appearance of an additional Eu " " luminescence center compared with BaS04. The luminescence of Eu " " and Eu " is well known in anhydrite (Tarashchan 1978 Gaft et al. 1985 Baumer et al. 1997). They are also found in time-resolved spectra under excitation at 266 nm (Eig. 4.17). 

Luminescence of in synthetic alkaline earth sulfates is well known (Folk-erts et al. 1995). In this study, CaS04 Pb shows an emission band with a maximum at 235 nm at 300 K, while the excitation maximum is at 220 nm. The decay curve of the emission is single exponential with a decay time of 570 ps at 4.2 K. The emission spectrum of BaS04 Pb demonstrates a broad band peaking at 340 nm with an excitation maximum at 220 nm, while in SrS04 Pb the luminescence band has a maximum at 380 nm. hi natural barite and anhydrite samples we detected several narrow UV bands, which may be connected with Pb emission, but for confident conclusion additional study is needed. In any case, Pb participation in natural sulfates liuninescence has to be taken into consideration. 

If soluble anhydrite is desired, firing is maintained until a second boil occurs accompanied by a second temperature plateau at about 190°C. Virtually all the water of crystallization has been removed at 215°C. Soluble salts are impurities that increase the vapor pressure within the kettle. Aridized stucco refers to ketde-calcined hemihydrate that has been made with the intentional addition of 0.55—1.1 kilograms of NaCl or CaCl per metric ton of land plaster. The stucco characteristic of lower water demand permits higher density and higher strength casts. The hygroscopic nature of the chlorides prevents the use of aridized stucco for some applications. 

Although the number of samples analyzed is limited, the inescapable conclusion to be drawn from our data is that some of the Nile silt formations can be differentiated on the basis of trace element contents and REE distribution patterns. In addition, we conclude that the Early Predynastic (Amratian) pottery was made from clays found in the immediate vicinity of the kiln. Older shales that are found layered with the much earlier sandstone deposits that were cut by the Great Wadi were also analyzed. The REE patterns as well as the scandium, chromium, and iron concentrations are different enough to suggest that these materials were not used to produce pottery (21), In a previous paper, we discussed the possibility that some of this shale, or the white salt found associated with it (anhydrite-CaSOj, was mixed with the local clay to produce the finer, harder plum red ware (21). This addition could account for the slight difference in the average composition of the sherds from localities 11, 39, and 59 and the Masmas silt. 

Natural anhydrite is extracted by open cast or underground mining. A hardenable product is obtained by fine grinding to < 0.2 mm particles and the addition of activators (ca. 2% heavy metal or alkali sulfates, or calcium hydroxide) to natural anhydrite. 

System I was comprised of calcium and sodium acetates, kaolinite, and pyrite to see if sulfur-containing aluminosilicates would be formed. As expected, anhydrite (CaS04) was formed by pyritic sulfur fixation by calcium. At 1000 C not only did the anhydrite persist but a sulfur containing aluminosilicate, hauyne, was formed in addition to gehlenite. 

Gay (1965) concluded from crystallographic measurements and phase-transformation experiments that there are only two additional solid phases in the system CaSO4—H2O at 25°C and 0.101-MPa (1-atm.) pressure besides gypsum and anhydrite, i.e. soluble anhydrite and bassanite, CaSO4" 0.5 H2O. Soluble anhydrite has never been reported in nature, and forms in the laboratory by dehydration of bassanite, but not in experiments involving liquid water. Heard and Rubey (1966) cited preliminary experiments of... 

In addition to ion exchange with rock surfaces, alkali can react directly with specific rock minerals. When divalents, Ca and Mg ", exist, alkali will react with them and precipitation can occur. One example is the incongruent dissolution of anhydrite or gypsum in the rock to produce the less soluble calcium hydroxide (CaS04(s) -F NaOH Ca(OH)2(s) + Na2S04). Another simple example is Ca -F COs " CaC03(s). Alkali can also dissolve other minerals from a rock, for example, silica. These reactions could cause plugging. 

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