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Chromium, molybdenum, tungsten, rhenium

Chromium, molybdenum, tungsten, rhenium. 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. Chromium in magmatic processes.

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Chromium, molybdenum, tungsten, rhenium

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  1. Chromium, molybdenum, tungsten, rhenium

  2. 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

  3. Chromium in magmatic processes In natural terrestrial environments, chromium commonly exists in the hexavalent or trivalentoxidation states. The bulk of chromium in the Earth existsin the trivalent state; hexavalent chromium is restricted tonear-surface oxidizing environments. In highly reducing extraterrestrialenvironments, metal alloys with high chromiumcontents have been reported from meteorites and divalent chromiumhas 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 constituentof rock-forming minerals such as spinels, pyroxenes, and garnets.

  4. Chromium in magmatic processes 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 chromiumis commonly enriched in Fe(III)- and Al(III)-bearing minerals. The highest chromium concentrations are associated with ultramaficrocks and the lowest concentrations are found in graniticrocks. In metamorphic rocks, the granulite facies contain the highest chromium enrichments.

  5. Chromium in weathering and sedimentary processes Hexavalent chromium prefers tetrahedral coordination inminerals 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 underlyingparent rock. In sediments and soils, chromium is stronglyassociated with the clay mineral fraction. It concentrated by adsorption on clays, or as relict phases in bauxite, laterite. Hexavalent chromium into the environment via anthropogenicactivities 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.

  6. Chromium in the biosphere Cr(III) is only sparingly soluble and relatively non-toxic; whereas Cr(VI) is verysoluble, toxic and a known carcinogen material. Consequently, both homogeneous and heterogeneousredox reactions are important determinants of the solubilityand 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)-containingminerals, 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).

  7. 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

  8. Molybdenum in magmatic processes It can function as a metallic cation with a valence of +4 (as in MoS2 molybdenite), or as part of the complex molybdate anion, inwhich it has a valence of +6 (as in CaMoO4 powellite).Molybdenite (MoS2) is the most common molybdenum mineraland is the principal source of molybdenum.Powellite (CaMoO4 ) and scheelite (CaWO4 ) are end-membersof 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, anddisseminated particles. It is commonly have associatedmolybdenite-bearing aplites and pegmatites.

  9. Molybdenum in weathering and sedimentary processes 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.

  10. Molybdenum in environments 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 anioncombines with iron to form ferrimolybdite, and co-precipitates with, and adsorbs on ferric hydroxide.Thus, molybdenum is relatively immobile in acidic surficialenvironments, 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 mobilemolybdate anion is stable. Plants that grow on Mo-bearingsoils with pH 6.5 or higher tend to take up mobile molybdate ions.

  11. 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

  12. Tungsten in magmatic processes Tungsten is almost invariably hexavalent in nature, except forits 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 fractionatedto a large extent into the Earth's metallic core ( ~ 258 ppb ),leaving the bulk silicate earth (mantle+ crust) with a weightedaverage W-concentration of 16 ppb. Furthermore, under thecomparatively oxidizing conditions within the bulk silicateearth, tungsten behaves as a highly incompatible lithophileelement which, through igneous fractionation processes, concentratesin the continental crust (average concentration of 1100 ppb).

  13. Tungsten in magmatic processes 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 highlyoxidizing conditions, tungsten is an incompatible lithophileelement 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.).

  14. Tungsten in magmatic processes The primary W deposit types include quartz veins and skarns.In both deposit types, tungsten is carried as complexesin residual magmatic waters (or possibly magmatically heatedconnate waters) to the site of deposition. Reaction with Ca-richlithologies (e.g. carbonates, anorthite-rich rocks, etc.) thentypically induces precipitation of scheelite in skarn deposits. Tourmalinization and fluoride-rich greisenization are alterations frequently associated with tungsten ore deposits.

  15. Tungsten in weathering and sedimentary processes 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.

  16. 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

  17. Rhenium in magmatic processes Among them magmatics, basalts are enriched relative to peridotite and both continental and oceanic varieties typicallycontain 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 molybdenitesor magmatic Ni-Cu ores. Molybdenites typically contain up to 110 ppb Re (up to 11.5% in those from fumaroles).

  18. Rhenium in sedimentary processes 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 andis 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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