Uranium, vanadium, niobium, tantalum

Uranium Universe: ppm (by weight) Sun: ppm (by weight) Carbonaceous meteorite: ppm Earth's Crust: 1.8 ppm Seawater: 3.13 x 10-3 ppm

Uranium in lithosphere Uranium chiefly appears in minerals with valences 4+ and 6+. The coordination number of oxygen around U4+ is six or eight. The coordination number of U6+ around oxygen is six, seven, or eight. Crystallochemical properties of U4+ are very close to those of Th4+ (ionic radii 0.94 A and 1.05 A) and LREE3+ (ionic radii A and A, from La to Nd). The geochemistry of this element in igneous rocks is strongly coherent with that of Th and LREE. In hydrothermal and supergene processes, however, uranium is partially or totally oxidized to U6+, and does not bear any coherence with the above elements.

Uranium in magmatic processes About 110 uranium minerals are known. The most abundant is the oxide uraninite (its simplified formula is UO2). It can forms solid solution with thorianite. When uraninite is strongly oxidized it is called pitchblende. Uraninite is widespread in acidic magmatics either as minute inclusions in major rock-forming minerals or as large grains (up to several mm) in granites and pegmatites. Some common accessory minerals may contain appreciable uranium contents. The most important are thorite (~1-35 wt% UO2 ), thorianite (varies from ThO2 to UO2), xenotime and zircon (up to 5 wt% of UO2 ), monazite ( ppm U), allanite ( ppm U).

Uranium in magmatic processes Ultramafic and mafic rocks have very low concentrations (average 10 ppb). The concentration of U from the ultramafic to acidic series increases steeply with silica contents, the granites normally having about 4-5 ppm U. It occurs by Ca/REE substitution in silicates (allanite, thorite), and phosphates (monazite, xenotime, apatite). Characteristic enrichment is the pneumatholitic processes, mainly in the Co-Ni-Sn-Bi-As ore deposits. It forms various oxides with Nb-Ta-REE in pegmatites (e.g. pyrochlore, aeschynite group minerals), alkali magmatic rocks, and especially in carbonatites.

Uranium in weathering and sedimentary rocks In supergene conditions uranium is invariably oxidized to uranyl ion, which is easily mobilized. For this reason, most uranium deposits, regardless of their origin, contain an important assemblage of supergene U6+ minerals consisting of silicates (coffinite, uranophane), carbonates (andersonite, cejkaite), phosphates (torbernite, autunite), arsenates (novacekite, zeunerite), sulfates (uranopilite, zippeite, johannite), vanadates, molybdates, niobates, tantalates. The largest amounts of secondary mineral associations is found in the oxidation zone of uranium ore deposits. Characteristic phases in abandoned mines, and waste dumps, too. Most of the secondary phases rather instable, after weathering the U moves to hydrosphere.

Uranium in weathering and sedimentary rocks Distribution of U in sedimentary rocks: evaporites have very low U contents, usually < 100 ppb. Limestones contain between 0.5 and 3 ppm U. In terrigenous rocks, U generally increases as the grain size decreases. Sandstones contain about ppm U, and shales between 2 and 8 ppm. Black shales may have very high U contents, with values higher than several hundred ppm being not uncommon. Phosphate rocks, coals, bitumens can also have very high U contents, in some cases greater than 1000 ppm. The high U content correlate with the amounts of organic matter, which adsorbed not only U, but Th, V, Ge, Nb etc.

Vanadium Universe: 1 ppm (by weight) Sun: 0.4 ppm (by weight) Carbonaceous meteorite: 62 ppm Earth's Crust: 190 ppm Seawater: 1.1 x ppm

Vanadium in magmatic processes Vanadium can take on a 2+, 3+, 4+, and 5+ charge, it is typically in the 3+ valence state in primary terrestrial minerals. In the 3+ valence state it is most often octahedrally coordinated. Vanadium is a lithophile element at low-pressure, but may be siderophile at the elevated pressures suggested for core formation in the Earth. It is incompatible in most silicate minerals, although it may be moderately compatible in some pyroxenes. It has an estimated abundance of 103 ppm in the bulk Earth, 98 ppm in present-day bulk continental crust, 53 ppm in the upper crust, finally 149 ppm in the lower crust.

Vanadium in magmatic processes It concentrates in spinel minerals in early magmatic differentiation (e.g in Ti-magnetite, in V-containing spinel coulsonite). It substitutes Fe, Al, Ti in some rock-forming silicates (e.g. pyroxenes, micas). However, it forms independent phases, mainly sulphides in the post- magmatic processes, as patronite, VS 4, and sulvanite – Cu 3 VS 4.

Vanadium in magmatic rocks

Vanadium in weathering and sedimentary processes It has many secondary minerals in sedimentary environments. Various vanadates found in the oxidation zone of V-bearing ore deposits. The most important are: vanadinite [Pb5(VO4)3Cl], carnotite [K2(UO2)(VO4)2 3H2O), descloizite PbZn(VO4)(OH). The vanadates show some similarities to arsenates and phosphates. There are some substituting possibilities between P-As-V in these compounds. Characteristic Al – V substitution is known in some clay minerals, e.g. in illite (so-called vanadium-illite)..

Vanadium in sedimentary rocks

Vanadium in sedimentary processes It enriched by adsorption in organic-matter-rich sediments in reductive condition: coals, lignites, black shales, bitumens, sandstones. There are some independent V- bearing minerals in these rocks: oxides, sulphates, etc. In bauxites and clays the V 4+, V 5+ cations concentrated by adsorption on the surface of Fe-Mn-Al-oxides/hydroxides or clay minerals, too. Glauconite- and chlorite-bearing sediments can also show some V enrichments.

Niobium Universe: ppm Sun: ppm Carbonaceous meteorite: 0.19 ppm Earth's Crust: 17 ppm Seawater: 9 x ppm Tantalum Universe: 8 x ppm Carbonaceous meteorite: 0.02 ppm Earth's Crust: 2 ppm Seawater: 2 x ppm

Niobium and tantalum in magmatic processes They are chemically very similar and often occur together in some pegmatites, alkaline rocks and carbonatites. The most important mineral of niobium is pyrochlore, NaCaNb2O6 (OH,F), and columbite (Fe,Mn)(Nb,Ta)2O6, while Ta is tantalite (Fe,Mn)(Ta,Nb)2O6. Nb and Ta commonly substitute for Ti in rutile (Nb-rich rutile so-called ilmenorutile, Ta-rich rutile so-called strüverite), titanite, perovskite and ilmenite, and for Zr, W and Sn in other minerals (e.g. eudialyte, astrophyllite). Alkaline rock complexes (e.g. syenites, nepheline syenites and alkaline ultrabasites) have the highest Nb content of all magmatic rocks; niobium is, therefore, mainly recovered from carbonatites and associated alkaline rocks.

Niobium and tantalum in magmatic processes High enrichment in Nb and Ta in carbonatite-alkalic rock complexes results in differentiation of partial melts of the asthenosphere (carbonatite) and the metasomatized mantle (alkalic rocks). The high enrichment in Nb and Ta in pegmatites is the result of extreme fractionation of granitic magma. Specialized granites (alkali granites, biotite and/or muscovite granite, lepidolite-albite granites) often present associated niobium and tantalum mineralization, but the strong niobium enrichment is characteristic of alkali granites. The most important Nb/Ta enrichment is known in pegmatites (with many complex Nb-Ta oxides in the pyrochlore, aeschynite, columbite, tantalite groups).

Tantalum in magmatic processes Ta-dominant minerals (e.g. tantalite-columbite series, wodginite, microlite, tapiolite) are mainly found in the highly fractionated rare element granitic pegrnatites. The most important economic sources of tantalum are alkali granites, greisenized granites, rare element granitic pegmatites and tantalum-bearing cassiterite (SnO2 ) deposits. From the most primitive to the highly fractionated rare element granitic pegmatites there is an increase in the Ta/Nb ratio of the (Nb,Ta) mineral species. TheTa enrichment trend is continued in most of the alteration products that replace these pre-existing mineral species in highly fractionated pegmatites.

Niobium and tantalum in weathering and sedimentary processes Weathering of carbonatites enriches the alluvial sediments in Nb/Ta minerals, because of the most abundant Nb/Ta minerals are rather stable compounds. A few portion of Nb/Ta phases, after chemical weathering connect by adsorption to clay minerals, or Fe-Al-Mn oxides. Relative enrichment of Nb/Ta is known in deep-marine Mn- nodules.