Structure anhydrite

Data for illite (not shown in Fig. 9) indicate a slight expansion in the temperature range 450-800°C followed by a rapid continuous shrinkage. PO] Expansion is attributed to the development of the anhydrite structure, and the shrinkage to loss of structure of the phase. The high rate of shrinkage correlates with the relatively lower development of high temperature crystalline phases. 

Figure 4.34 A clinogmphic view of the anhydrite structure. The density for anhydrite is about 2.3 g/cc. 

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 ... 

Where fluids discharge from hot springs and mix with seawater, they cool quickly and precipitate clouds of fine-grained minerals. The clouds are commonly black with metal sulfides, giving rise to the term black smokers. Some vents give off clouds of white anhydrite these are known as white smokers. Structures composed of chemical precipitates tend to form at the vents, where the hot fluids discharge into the ocean. The structures can extend upward into the ocean for several meters or more, and are composed largely of anhydrite and, in some cases, sulfide minerals. 

Other inorganic crystals studied by Mark and his collaborators, sometimes leading to complete structure determinations, include strontium chloride, zinc hydroxide, tin tetraiodide, potassium chlorate, potassium permanganage, and ammonium ferrocyanide. Minerals investigated by them include CaSO (anhydrite), BaSO (barite), PbSO, Fe2TiO[j (pseudobrookite), and three forms of Al2Si05 (cyanite, andalusite, and sillimanite). 

The Carboniferous sediments of the Maritimes Basin were originally deposited as red-green interstratified continental to marginal siliciclastics, marine limestone, dolostone, gypsum, anhydrite, halite, and locally, potash. The salt deposits vary from stratified, with only minor structural complications, to those that have been tectonized into pillows, anticlines e.g., Penobsquis Deposit) and diapirs or domes. In the latter cases, structural complexities make the stratigraphic position of many of these deposits uncertain. 

The supersaturation is too low in all experiments to be measured accurately, but it seems reasonable to assume that the effect of residence time is imposed through the kinetics. Another observation is that the D-value for cadmium uptake in anhydrite is about ten times higher than in HH or DH. An explanation for this higher D seems to be related to the crystal structures of the calcium sulfates. Only the AH structure matches with an anhydrous CdSO phase, while no hemi- or dihydrate phase of CdS04 exists. 

Anhydrite is a calcium sulfate, CaS04, with an orthorhombic structure (mmm) where sulfur atoms, which are at the centers of tetrahedra of oxygens, and calcium atoms he on the lines of intersection of mirror planes (100) and (010). In nature, REE and Mn can easily substitute for Ca, becoming luminescence... 

At 77 K the relative intensity of the band at 605 nm with an even longer decay time of 25 ms is much stronger. Such bands are usually connected with Mn +, but Mn " " in barite has different luminescence. Mn " in anhydrite CaS04 is characterized by a narrow band peaking at 505 nm. Thus such a band in barite may be connected with Ca impurity and anhydrite type local structure. 

Sulfur constitutes about 0.052 wt % of the earth s crust. The forms in which it is ordinarily found include elemental or native sulfur in unconsolidated volcanic rocks, in anhydrite over salt-dome structures, 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). 

Evaponte Basin Sulfur Deposits. Elemental sulfur occurs in another type of subsurface deposit similar to the salt-dome structures in that the sulfur is associated with anhydrite or gypsum. The deposits are sedimentary, however, and occur in huge evaporite basins. It is believed that the sulfur in these deposits, like that in the Gulf Coast salt domes, was derived by hydrocarbon reduction of the sulfate material and assisted by anaerobic bacteria. The sulfur deposits in Italy (Sicily), Poland, Iraq, the CIS, and the United States (western Texas) are included in this category. 

Effect of Impurities on CaS04 Transformation. The transition from gypsum to orthorhombic anhydrite is slow but occurs even at ambient temperatures (44). The relatively large concentration of finely divided sodium chloride present in the plaster in the tomb of Nefertari may have facilitated the dehydration process. The presence of a hygroscopic material, such as sodium chloride, can help promote dehydration reactions. Also, impurities within the lattice of a crystalline structure can weaken the lattice (46, 47) and thereby accelerate thermodynamically favored reactions. These points suggest a strong correlation between the extent to which the dehydration reaction proceeds and sodium chloride concentration, but they do not exclude the possibility that dehydration can take place in the absence of salt. 

Sleep N. H. (1991) Hydrothermal circulation, anhydrite precipitation, and thermal structure at ridge axes. J. Geophys. Res. 96, 2375-2387. 

After the induction period (initial solution of hemihydrate t>r anhydrite), nuclcation of dihydrale occurs. Further crystal growth leads to a close-grained structure of dihydratc crystals which accounts for the rigidity of plaster Excess water is rcmove<.l by drying... 

Where undiluted hydrothermal fluids mix with cold seawater at and near the seafloor, minerals precipitate out of solution to form chimney structures and other deposits. These deposits are mineralogically complex, containing sulphides (e.g. pyrite, marcasite, chalcopyrite, sphalerite, wurtzite, galena), sulphates (e.g. anhydrite, barite), silica and oxyhydroxides (e.g. Hannington etal., 1995 ... 

The exceptionally light carbonates (5 C= i8.9%o) in the Medjounes structure result from sulfate reduction by sulfate-reducing bacteria in the presence of hydrocarbons, leading eventually to the formation of elemental (native) sulfur. In view of the presence of thick layers of gypsum (CaSO x zH O) and anhydrite (CaSO ) in the succession at the above-mentioned site (sample 27a), a potential for the presence of a native sulfur deposit is indicated. 

We continue our investigation of the affect of the anion on structure with several sulfates— anhydrite (CaS04) and barite (BaS04). The tetrahedral sulfate ion is shown in Figure 71 (a) sitting on its base (a face) and oriented on an edge in Figure 71 (b). 

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