Chromium Chromium



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Chromium

  • Chromium

  • Universe: 15 ppm (by weight) 

  • Sun: 20 ppm (by weight) 

  • Carbonaceous meteorite: 3100 ppm 

  • Earth's Crust: 100 ppm 

  • Seawater: Atlantic surface: 1.8 x 10-4 ppm    

  • Atlantic deep: 2.3 x 10-4 ppm



In natural terrestrial environments, chromium commonly exists in the hexavalent or trivalent oxidation states. The bulk of chromium in the Earth exists in the trivalent state; hexavalent chromium is restricted to near-surface oxidizing environments. In highly reducing extraterrestrial environments, metal alloys with high chromium contents have been reported from meteorites and divalent chromium has been inferred to substitute isomorphically in olivines from lunar basalts. It usually is a trace to minor component of rock-forming minerals; in Cr-rich environments chromium can be a major constituent of rock-forming minerals such as spinels, pyroxenes, and garnets.

  • In natural terrestrial environments, chromium commonly exists in the hexavalent or trivalent oxidation states. The bulk of chromium in the Earth exists in the trivalent state; hexavalent chromium is restricted to near-surface oxidizing environments. In highly reducing extraterrestrial environments, metal alloys with high chromium contents have been reported from meteorites and divalent chromium has been inferred to substitute isomorphically in olivines from lunar basalts. It usually is a trace to minor component of rock-forming minerals; in Cr-rich environments chromium can be a major constituent of rock-forming minerals such as spinels, pyroxenes, and garnets.



Trivalent chromium has a high octahedral site preference

  • Trivalent chromium has a high octahedral site preference

  • energy and almost exclusively substitutes in the octahedral

  • sites of simple oxides (spinels, than chromite FeCr2O4 and magnesiochromite MgCr2O4) and silicates (garnets, pyroxenes, tourmalines). Because the chemical properties and ionic radius of Cr(III) are similar to those of Fe(III) and Al(III), trivalent chromium is commonly enriched in Fe(III)- and Al(III)-bearing minerals. The highest chromium concentrations are associated with ultramafic rocks and the lowest concentrations are found in granitic rocks. In metamorphic rocks, the granulite facies contain the highest chromium enrichments.





Hexavalent chromium prefers tetrahedral coordination in minerals and in aqueous solution. Uncommon minerals in oxidation zone of Cr-bearing ore deposits are chromates, e.g. crocoite, PbCrO4.

  • Hexavalent chromium prefers tetrahedral coordination in minerals and in aqueous solution. Uncommon minerals in oxidation zone of Cr-bearing ore deposits are chromates, e.g. crocoite, PbCrO4.

  • Chromium is usually concentrated in weathered material relative to the underlying parent rock. In sediments and soils, chromium is strongly associated with the clay mineral fraction. It concentrated by adsorption on clays, or as relict phases in bauxite, laterite. Hexavalent chromium into the environment via anthropogenic activities is a source of concern because of its toxicity and potential for high mobility. Chromium is an essential nutrient at relatively low concentrations, but is toxic at elevated concentrations.



Cr(III) is only sparingly soluble and relatively non-toxic; whereas Cr(VI) is very soluble, toxic and a known carcinogen material. Consequently, both homogeneous and heterogeneous redox reactions are important determinants of the solubility and threat posed by chromium in the environment.

  • Cr(III) is only sparingly soluble and relatively non-toxic; whereas Cr(VI) is very soluble, toxic and a known carcinogen material. Consequently, both homogeneous and heterogeneous redox reactions are important determinants of the solubility and threat posed by chromium in the environment.

  • Reduction of Cr(VI) to Cr(III) can occur via reducing agents such as Fe(II)aq, Fe(II)-containing minerals, organic matter, H2S, and microbial action.

  • Chromium found in soils in 80-200 ppm, mainly oxides. In water it contains 1-10 ppb. In the atmosphere is 0.01-0.03 mg/m3 Cr (it is higher in the air of the towns).



Molybdenum

  • Molybdenum

  • Universe: 0.005 ppm (by weight) 

  • Sun: 0.009 ppm (by weight) 

  • Carbonaceous meteorite: 1.2 ppm 

  • Earth's Crust: 1.5 ppm 

  • Seawater: 0.01 ppm



It can function as a metallic cation with a valence of +4

  • It can function as a metallic cation with a valence of +4

  • (as in MoS2 molybdenite), or as part of the complex molybdate anion, in which it has a valence of +6 (as in CaMoO4 powellite). Molybdenite (MoS2) is the most common molybdenum mineral and is the principal source of molybdenum. Powellite (CaMoO4 ) and scheelite (CaWO4 ) are end-members of a solid solution series and are found in metamorphic veins and skarns. It occurs in porphyry copper ore deposits, in which molybdenite occurs in quartz veins and veinlets, and disseminated particles. It is commonly have associated molybdenite-bearing aplites and pegmatites.



Ferrimolybdite (FeMoO3 • H2O) is a powdery yellow

  • Ferrimolybdite (FeMoO3 • H2O) is a powdery yellow

  • weathering product of molybdenite and pyrite, as is brown

  • limonite (mixture of iron oxides and clays) with adsorbed molybdate ions. Wulfenite (PbMoO4 ) is present in oxidized zones of some Pb ore deposits that contain weathering lead and molybdenum minerals. Ilsemannite (blue molybdenum oxide) such a secondary mineral that is unstable in most weathering environments. Some sandstones and sandstone-hosted uranium deposits also contain molybdenite or ilsemannite, and some coal, shale, and phosphorite contain subeconomic concentrations of molybdenum.



Water that drains pyrite-molybdenite zones has pH 1-3 and

  • Water that drains pyrite-molybdenite zones has pH 1-3 and

  • contains high concentrations of dissolved metals, including

  • iron, aluminum, zinc, copper and uranium, but not molybdenum. At low pH, molybdate anion combines with iron to form ferrimolybdite, and co-precipitates with, and adsorbs on ferric hydroxide. Thus, molybdenum is relatively immobile in acidic surficial environments, and is a good pathfinder element for copper, which tends to be leached from surface outcrops in acidic environments. It is mobile in alkaline surface waters, in which the mobile molybdate anion is stable. Plants that grow on Mo-bearing soils with pH 6.5 or higher tend to take up mobile molybdate ions.



Tungsten

  • Tungsten

  • Universe: 0.0005 ppm (by weight) 

  • Sun: 0.004 ppm (by weight) 

  • Carbonaceous meteorite: 0.12 ppm 

  • Earth's Crust: 1.1 ppm 

  • Seawater: 1.2 x 10-4 ppm



Tungsten is almost invariably hexavalent in nature, except for its 4+ valence in a few rare sulfide minerals. Tungsten can, however, also take on 2+, 3+, or 5+ valences. Tungsten is moderately siderophile under highly reducing conditions, and has consequently fractionated to a large extent into the Earth's metallic core ( ~ 258 ppb ), leaving the bulk silicate earth (mantle+ crust) with a weighted average W-concentration of 16 ppb. Furthermore, under the comparatively oxidizing conditions within the bulk silicate earth, tungsten behaves as a highly incompatible lithophile element which, through igneous fractionation processes, concentrates in the continental crust (average concentration of 1100 ppb).

  • Tungsten is almost invariably hexavalent in nature, except for its 4+ valence in a few rare sulfide minerals. Tungsten can, however, also take on 2+, 3+, or 5+ valences. Tungsten is moderately siderophile under highly reducing conditions, and has consequently fractionated to a large extent into the Earth's metallic core ( ~ 258 ppb ), leaving the bulk silicate earth (mantle+ crust) with a weighted average W-concentration of 16 ppb. Furthermore, under the comparatively oxidizing conditions within the bulk silicate earth, tungsten behaves as a highly incompatible lithophile element which, through igneous fractionation processes, concentrates in the continental crust (average concentration of 1100 ppb).



Igneous tungsten concentrations increase from ultramafic

  • Igneous tungsten concentrations increase from ultramafic

  • (typically 0.1-0.7 ppm) to mafic (typically 0.1-1.3 ppm) to

  • intermediate/felsic rocks. These data confirm that under slightly to highly oxidizing conditions, tungsten is an incompatible lithophile element that partitions into residual, fluid-rich magmatic segregations enriched in elements such as Si, AI, Na, Li, F, Be, B, Sn, Nb, Ta, U, Th, Zr, REE.

  • The most common and economically significant W minerals are scheelite and wolframite series minerals (FeWO4 ferberite and MnWO4 hübnerite). It occurs in pegmatitic, pneumatholitic processes (together with topaz, lepidolite, fluorite, cassiterite, tourmaline, albite etc.).



The primary W deposit types include quartz veins and skarns. In both deposit types, tungsten is carried as complexes in residual magmatic waters (or possibly magmatically heated connate waters) to the site of deposition. Reaction with Ca-rich lithologies (e.g. carbonates, anorthite-rich rocks, etc.) then typically induces precipitation of scheelite in skarn deposits. Tourmalinization and fluoride-rich greisenization are

  • The primary W deposit types include quartz veins and skarns. In both deposit types, tungsten is carried as complexes in residual magmatic waters (or possibly magmatically heated connate waters) to the site of deposition. Reaction with Ca-rich lithologies (e.g. carbonates, anorthite-rich rocks, etc.) then typically induces precipitation of scheelite in skarn deposits. Tourmalinization and fluoride-rich greisenization are

  • alterations frequently associated with tungsten ore deposits.



The wolframates are more stable than molybdates. It forms mainly oxides, hydrated oxides (tungstite, russellite). They move long way, so they does not concentrate in the oxidation zone of W ore deposit.

  • The wolframates are more stable than molybdates. It forms mainly oxides, hydrated oxides (tungstite, russellite). They move long way, so they does not concentrate in the oxidation zone of W ore deposit.

  • The scheelite and wolframite stable minerals, so they rather concentrate in clastic sediments, than cassiterite.



Rhenium

  • Rhenium

  • Universe: 0.0002 ppm (by weight) 

  • Sun: 0.0001 ppm (by weight) 

  • Carbonaceous meteorite: 0.05 ppm 

  • Earth's Crust: 0.0004 ppm 

  • Seawater: 4 x 10-6 ppm



Among them magmatics, basalts are enriched relative

  • Among them magmatics, basalts are enriched relative

  • to peridotite and both continental and oceanic varieties typically contain 0.5 to 1.5 ppb Re. In other upper crustal rocks, the following abundances have been reported (in ppb Re): komatiites, 0.5-3; diabase, 0.42; andesite, 0.34; granite 0.22-0.56.

  • Rhenium may form its own sulfide in volcanic fumaroles (ReS2 rheniite – the only known Re mineral), but economic occurrences are chiefly molybdenites or magmatic Ni-Cu ores. Molybdenites typically contain up to 110 ppb Re (up to 11.5% in those from fumaroles).



Abundances are very low in oxidized sediments (ppb Re); shale, 0.009-0.051; quartz-rich sandstone, 0.021-0.034. In the euxinic regime, however, Re is reduced and is found highly enriched in sedimentary rocks (ppb Re), 56-285 in black shales. One of most important concentrations is knows in sedimentary copper shales together with Mo.

  • Abundances are very low in oxidized sediments (ppb Re); shale, 0.009-0.051; quartz-rich sandstone, 0.021-0.034. In the euxinic regime, however, Re is reduced and is found highly enriched in sedimentary rocks (ppb Re), 56-285 in black shales. One of most important concentrations is knows in sedimentary copper shales together with Mo.



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