SELENIUM MINERALS

There are numerous synthetic and natural compounds called antioxidants which regulate or block oxidative reactions by quenching free radicals or by preventing free-radical formation. Vitamins A, C, and E and the mineral selenium are common antioxidants occurring naturally in foods (104,105). A broad range of flavonoid or phenoHc compounds have been found to be functional antioxidants in numerous test systems (106—108). The antioxidant properties of tea flavonoids have been characterized using models of chemical and biological oxidation reactions. 

Among the solution-growth methods, crystallization from aqeous solution is well known. Materials with low water solubility may be brought into solution by the use of complexing agents (mineralizers). Selenium, for instance, can be grown from aqueous sulphide solutions, by making use of the reaction... 

Since the publication of the previous version of ATSDR s Toxicological Profile for Selenium in 1996, several events have occurred that focused attention on the mineral selenium, its role in maintaining optimal human health, and any risk it may present to those exposed to excessive amounts of this metallic chemical element. 

Antioxidant mineral Selenium Garlic, whole cereal grains Nutrient... 

Most important mineral Selenium is present as a trace element in some sulfide ores. It is won as byproduct in copper manufacturing. A few selenium minerals are known. One is berzelianite, a silver-white mineral Cu Se. 

Extra vitamin E is required to prevent the formation of toxic peroxides in the body when the dietary level of polyunsaturated fats is raised. However, diets containing ample amounts of the essential trace mineral selenium, which is a part of the enzyme that breaks down the peroxides, may also help to offset this danger. The formation of peroxides may contribute to the cocarcinogenic activity observed in some animal studies. 

Peroxide formation and the breakdown of polyunsaturated fatty acids in the body may be prevented by vitamin E (synthetic antioxidants such as BHA and BHT do not act in the cells of the body). Also, the essential trace mineral selenium is part of the enzyme which breaks down peroxides once they have been formed. Therefore, adequate dietary intakes of both vitamin E and selenium are the best forms of insurance against toxicity from peroxides, when the handling of vegetable oils leaves much to be desired. 

Areas in which the soil is high in the mineral selenium. (Also see SELENIUM.)... 

Selenium is found in a few rare minerals such as crooksite and clausthalite. In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode metal from electrolytic copper refineries now provide the source of most of the world s selenium. Selenium is recovered by roasting the muds with soda or sulfuric acid, or by smelting them with soda and niter. 

SELENIUMANDSELENIUMCOMPOUNDS] (Vol21) -from selenium deficiency [MINERAL NUTRIENTS] (Vol 16)... 

Cobalt, copper, molybdenum, iodine, iron, manganese, nickel, selenium, and zinc are sometimes provided to mminants. Mineral deficiency or toxicity in sheep, especially copper and selenium, is a common example of dietary mineral imbalance (21). Other elements may be required for optimal mminant performance (22). ExceUent reviews of trace elements are available (5,22). 

Under unusual circumstances, toxicity may arise from ingestion of excess amounts of minerals. This is uncommon except in the cases of fluorine, molybdenum, selenium, copper, iron, vanadium, and arsenic. Toxicosis may also result from exposure to industrial compounds containing various chemical forms of some of the minerals. Aspects of toxicity of essential elements have been pubhshed (161). 

Efficient homeostatic controls of mammalians generally prevent serious toxicity from ingestion of the mineral nutrients. Toxicity may occur under conditions far removed from those of nutritional significance or for individuals suffering from some pathological conditions. Because of very low concentrations in foods, the trace elements are not toxic under normal nutritional conditions. Exceptions are selenium and iron (162). 

Indicator plants generally have an offensive odor, which varies with the selenium concentration. Other vegetable matter grown on seleniferous soils may have a sufficiently high selenium content to be toxic when ingested by animals or humans. Apart from appearance in these seleniferous plants, selenium has been considered as a variable contaminant. Selenium is a necessary micronutrient in living organisms, needed by humans as well as animals (see Mineral NUTHiENTs). 

Deposits. Selenium forms natural compounds with 16 other elements. It is a main constituent of 39 mineral species and a minor component of 37 others, chiefly sulfides. The minerals are finely disseminated and do not form a selenium ore. Because there are no deposits that can be worked for selenium recovery alone, there are no mine reserves. Nevertheless, the 1995 world reserves, chiefly in nonferrous metals sulfide deposits, are ca 70,000 metric tons and total resources are ca 130,000 t (24). The principal resources of the world are in the base metal sulfide deposits that are mined primarily for copper, zinc, nickel, and silver, and to a lesser extent, lead and mercury, where selenium recovery is secondary. 

Selenium plays a dual role in a living organism, depending on the compound and the amount adsorbed. Controlled small doses of some compounds are used in medicine and as diet supplements, for example, ca 0.1 ppm of diet dry matter for Hvestock (see Feed additives Mineral nutrients). Larger amounts can be toxic. 

In mineral technology, sulfur dioxide and sulfites are used as flotation depressants for sulfide ores. In electrowinning of copper from leach solutions from ores containing iron, sulfur dioxide prereduces ferric to ferrous ions to improve current efficiency and copper cathode quaHty. Sulfur dioxide also initiates precipitation of metallic selenium from selenous acid, a by-product of copper metallurgy (326). 

Like selenium, tellurium minerals, although widely disseminated, do not form ore bodies. Hence, there are no deposits that can be mined for tellurium alone, and there are no formally stated reserves. Large resources however, are present in the base-metal sulfide deposits mined for copper, nickel, gold, silver, and lead, where the recovery of tellurium, like that of selenium, is incidental. 

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

Commercial elemental sulfur is usually of bright-yellow color at 20 °C [36]. Pure orthorhombic a-Ss is, however, of greenish-yellow color at 20 °C but totally colorless at 77 K while commercial sulfur often remains pale-yellow at this temperature [59]. The reasons for this different behavior are twofold. Commercial samples are never pure Ss but besides traces of organic impurities they always contain Sy in concentrations of between 0.1 and 0.5% [59]. Sulfur found as a mineral in Nature sometimes also contains Sy but in addition traces of selenium are quite often present (up to 680 ppm Se, probably as SySe molecules) [60]. These minor components influence the color of the samples at ambient and low temperatures in the sense that a more orange-type of yellow ( egg-yellow ) is recognized. 

The most important minerals of sulfur The most abundant minerals of selenium and tellurium... 

Minerals Yearbook, http //minerals.usgs.gov/minerals/pubs/commodity/selenium http //www.matweb.com/search/MateiialGroupSearch.aspx... 

The /02 of ore fluids responsible for the epithermal base-metal veins might have been in the predominance field of reduced sulfur species because (1) pyrrhotite is occasionally found in these deposits, (2) selenium content of argentite is very low and (3) H2S is dominant in the present-day epithermal base-metal fluids. Implication of selenium content of sulfides will be considered later. Barite is sometimes found in the late-stage of mineralization. Thus, it is likely that /oj of barite stage lies in the predominance field of oxidized sulfur species. 

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