Scandium, yttrium, rare earth elements (REE), titanium, zirconium, hafnium, thorium

1. Scandium, yttrium, rare earth elements (REE), titanium, zirconium, hafnium, thorium

3. Scandium (Sc) Universe: 0.005 ppm Sun: 0.04 ppm Carbonaceous meteorite: 1.4 ppm  Earth's Crust: 22 ppm  Seawater: 95 x 10-6 ppm

4. Scandium in magmatic processes Scandium is typically a trace element in most rocks andminerals. It lithophileand found in the trivalent state. Scandium differs fromother rare earth elements in being much smaller and thus tendsto substitute into early crystallizing phases with 6-fold coordination,such as pyroxenes and amphiboles. It has a similar sizeto Fe2+, for which it commonly substitutes it. Some independent Sc silicates: bazzite (Sc-analogue of beryl), jervisite (a pyroxene), thortveitite (a sorosilicate). They occur in alkaline magmatites or pegmatites. .

5. Scandium in the lithosphere It has a few phosphate, as pretulite (in metamorphic quartz-lazulite veins), and kolbeckite (in hydrothermal processes). In sedimentary environments, Sc behaves morelike other rare earth elements but differs in being more readily hydrolyzed. It concentrated than LREE by adsorption in clays, or organic matter in soils, or in Al-Fe oxides, e.g. in bauxite, laterite.

6. Yttrium (Y) Universe: 0.007 ppm Sun: 0.01 ppm Carbonaceous meteorite: 1.9 ppm  Earth's Crust: 30 ppm  Seawater: 9 x 10-6 ppm

7. Lanthanium (La) Universe: 0.002 ppm Sun: 0.002 ppm Earth's Crust: 34 ppm Europium (Eu) Universe: 0.0005 ppm Sun: 0.0005 ppm Earth's Crust: 2.1 ppm

8. Lanthanide series and yttrium The rare earth elements are perhaps the most significant group of trace elements in geochemistry. The lanthanide series develops by filling of 4f orbitalsthat are well shielded by 5s and 5p orbitals, leading to highlycoherent behavior as a group. Among other things, this resultsin the trivalent state being especially stable and the ionic radius decreases in an unusually systematic fashion. The dominant controls on the geochemicalbehavior of the REE are their size (ionic radius),volatility, redox behavior and complexing behavior.

9. REE and Y in magmatic processes The lanthanides (and Y) tend to beconcentrated in magmatic liquids and late crystallizing phases.Of the major elements in the crust and mantle, only sodiumand calcium come close in size to the REE, however substitution for these elements (especially Na) may lead to serious charge imbalance, because REE have been mainly trivalent. Of great importance to geochemistry is the fact that Eu and Ce commonly exist in other than trivalent states (Eu2+;Ce4+). Reduction of Eu occurs only at highly reducing, typically magmatic conditions.

10. REE and Y in magmatic processes An example is that Eu becomes highly concentrated in feldspars (especially in plagioclase). Plagioclase is only stable to about 40 km on Earthand anomalous Eu behavior in magmatic rocks is a sign ofrelatively shallow igneous processes. In contrast, Ce is oxidized almost exclusively underhighly oxidizing surficial conditions, notably in early marine diagenesis, to form manganese nodules and under certain weathering conditions.

11. REE and Y in magmatic processes In most rocks and minerals, REE are trace elements and in some cases minor elements; however, there are more than 70minerals in which various REE are essential structural constituents.Among the mostsignificant in geochemistry are Ianthanite, (La,Ce)2(CO3).8H2O, allanite (Ce,Ca,Y)2(AI,Fe)3(SiO4)3(OH), and the phosphates florencite CeAI3(PO4)2(OH), monazite La,Ce(PO4), xenotime Y(PO4) and fluorocarbonates (parisite, synchisite series). The name of mineral species are create the root name (monazite) plus the name of dominant REE in the structure: monazite-(Ce), monazite-(La), etc.

12. REE and Y in magmatic processes The REE substitute mainly Ca and Sr in structure (because of similar size of ions). E.g. in rock-forming silicates: amphiboles, pyroxenes, epidotes, but in apatite, fluorite. Constant REE/Y substitution is known in zircon, thorite (latter isomorphous with xenotime).

13. Some tendencies of enrichment of LREE and HREE in minerals

14. REE and Y in weathering and sedimentary rocks Often enriched in chemical weathering in clays and carbonates (with common substitutions of Ca), or by adsorption on surface of Mn-Fe oxides/hydroxides. Hydrated REE minerals can form other sediments or soils (e.g. lanthanite). There are high REE concentration in bauxite, than relict phases (monazite, xenotime), secondary REE minerals (e.g. bastnasite), other rock-forming minerals with substitutions, finally as independent cations which can adsorbed on solid or gel-like phases (mainly on Fe-oxides/hydroxydes).

15. REE and Y in weathering and sedimentary rocks Under aqueous conditions, the rare earth elements existmostly as a variety of complexes. Carbonates and bicarbonatesdominate in seawater. For a number of rare earth complexes,such as fluorides and carbonates, the heavier (smaller) REEshow a marked increase in stability.

16. Titanium (Ti) Universe: 3 ppm  Sun: 4 ppm Carbonaceous meteorite: 550 ppm Earth's Crust: 6600 ppm Seawater: 4.8 x 10-4 ppm

17. Titanium in magmatic processes Titanium (4+) coordination is usually 6 (octahedral), but canbe 4 coordinated (in some Al-deficient Ti-amphiboles and pyroxenes).Common 6 coordinated Tiphasesare the rutile modification for TiO2 (rutile, brookite, anatase, there are polymorphs) and ilmenite(FeTiO3). 6 coordinated Ti is also known in kimzeyite and schorlomite garnets, titanite, various inosilicates such as titanaugite and in complex Ti-oxides/fluorides such as pyrochlore group minerals, and zirkelite, betafite, brannerite. In the Earth's mantle, the perovskite(CaTiO3 ) may be the most common Ti-phase, but Ti3+richpericlase (MgO) phases are also known.

18. Titanium in magmatic processes Silicate glasses and melts (such as basalts) show a contrasted coordinationchemistry for Ti: they contain essentially 5 coordinated Ti, as titanyl unitsor (Ti=O)O4. Highly polymerized magmas and glasses also show significant amounts of tetrahedrally coordinated Ti. There are common Ti-containing minerals, as oxides in plutonic rock, in contrary as silicates in volcanics. The most important Ti-oxides are rutile, ilmenite, perovskite, Ti-containing magnetite. Titanite often shows Ca- REE/Nb substitutions. It forms complex oxides and silicates in pegmatites with REE, Nb, Ta and Ca (e.g. pyrochlore minerals).

19. Titanium in magmatic processes Ti3+ often occurs in mafic silicates, than pyroxenes, amphiboles (e.g. titanaugite). It can substitutes Fe3+, Al3+ and rare Mg2+. It concentrates high amounts in early basic magmatites (e.g. gabbros, norites) as Ti-magnetite, or ilmenite. High Ti-contents is known in alkali magmatites (e.g. phonolites, nepheline syenites), and their pegmatites. However, not only the simple oxides can be occur in this rocks, but complex silicates, than astrophyllite, Ti-garnets.

20. Titanium in weathering and sedimentary processes Near to the Earth's surface, Ti-oxides (rutile and ilmenite)are the most abundant Ti phases, because they are not verysensitive to external agents such as chemical weathering.They can be common in detritic sediments and metamorphicrocks. Such Ti minerals are useful tracers for valuable placers ofgold, diamond, bauxites (etc.). The low solubility of Ti inwater makes it unaggressive to the environment; however,reactive bio-inorganic molecules may be chemisorbed onto TiO2 surfaces. In contrary, the titanite (a rock-forming CaTiSiO5 mineral) often weathered and Ti moves in the hydrosphere, and later it forms secondary mixture of oxides, so-called leucoxene).

21. Titanium in weathering and sedimentary processes The leucoxene is mixture of different oxides, mainly rutile-brookite-anatase. The Ti-containing mafic rock-forming minerals relatively easy weathered and move to the hydrosphere. The Ti forms secondary phases (rutile, anatase, brookite) by diagenetic processes in sediments, such laterites, bauxites, clays or soils. The high Ti-contents of bauxite consist of not only relict phases, but diagenetic origin minerals, too.

23. Zirconium (Zr) Universe: 0.05 ppm Sun: 0.04 ppm Carbonaceous meteorite: 6.7 ppm Earth's Crust: 130 ppm  Seawater: 9 x 10-6 ppm

24. Zirconium in magmatic processes Zr is a lithophile element, present essentially as Zr4+ in silicatesbut occurs sometimes in oxides. In Earth materials, Zr4+coordinationmay be 6, 7 or 8. Six-coordinated Zr is observedin a wide number of rare minerals: as a major element inzirconosilicates and as a minor element in rock-forming silicates.For instance, Zr substitutes to Ti in a number of rock-formingsilicates such as garnets, alkali-richpyroxenes and amphiboles (like aegirine and arfvedsonite). 7-coordinated Zr is rare in minerals (as in baddeleyite andzirconolite, but also in metamict zircon, a naturally radiationdamaged zircon). Finally, 8-coordinated Zr is known mainly in crystalline zircon.

25. Zirconium in magmatic processes Simple, but most important and highly stable zirconium silicate is the zircon. Zr is concentrates in larger amounts in the acidic (e.g. granites) or in alkali magmatites (e.g. phonolites, nepheline syenites). However, the most highest Zr-content can be found in carbonatites. There are many complex Zr-silicates in alkali magmatites (e.g. eudialyte group minerals, catapleiite). Zr is one of the most used trace elements in geochemistry because this highly charged cation usually shows a clearly incompatible behavior for most rock-forming minerals (olivines, pyroxenes, amphiboles, felspars). However, in peralkaline melts Zr can partition efficiently towards garnets (kimzeyite), inosilicates (aegirine, arfvedsonite, aenigmatite).

26. Zirconium in weathering and sedimentary processes Zircon is the most abundant Zr phase close to the Earth's surface but baddeleyite has also been mined from alkaline rocks (syenites, carbonatites). These phases are not very sensitiveto weathering but radiation effectscan partially destroytheir atomic structure (when actinides, mainly U-Th are substituted to Zr).Zircon can be common in detritic sediments and metamorphicrocks and constitutes a useful tracer for gold, diamond, bauxites. Zr is not particularly aggressive to the environment,except in some nuclear waste sites (because of radiogenic isotopes of Zr). Because of its very stable minerals, small part of Zr move to the hydrosphere, later it adsorbed in the surface of clay minerals or Fe-Mn oxides/hydroxydes.

27. Hafnium (Hf) Universe: 0.0004 ppm Sun: 0.0003 ppm Carbonaceousmeteorite: 0.04 ppm Earth's Crust: 12 ppm Seawater: 9.2 ppm

28. Hafnium in the lithosphere Chemically it shows close analog of zirconium, and is almost always enrichedor depleted to the same degree. The most importantmineral host by far in the Earth's crust is zircon(Zr,Hf)SiO4 , where Hf averages 1%, corresponding to theterrestrial Zr/Hf ratio of ca. 37. We know only two independent Hf minerals: hafnon, (Hf,Zr)SiO4, the Hf-dominant analogue of zircon.

29. Thorium (Th) Universe: 0.0004 ppm Sun: 0.0003 ppm Carbonaceous meteorite: 0.04 ppm Earth's Crust: 12 ppm  Seawater: 9.2 ppm

30. Thorium in magmatic processes Thorium occurs as a trace element in common rocks and rock-formingminerals, with concentrations in the range of a fewppb totens of ppm. Th4+ has an ionic radiusof~ 1 A (similar to that of U4+). Th along with the other incompatible elements (e.g. U, K, Rb andREE) accumulates in the residual magma and is incorporatedinto the late crystallizing silicate phases. Th and U are moreabundant in granites and associated accessory minerals thanin mafic and ultramafic rocks. There are many minerals in which Th is a major constituent. There arerelatively common and they generally occur as accessoryminerals.

31. Thorium in magmatic processes Common Th minerals are thorianite (ThO2) and thorite (ThSiO4), latter is isomorphic with zircon (because of similar size of cations – 1.10 and 0.87). It concentrates in large amounts in pegmatites of granitoids. Often substitutes Zr or forms compounds with Zr in magmatic processes. However, the most important commercial mineral of Th ismonazite, which is a rare earth phosphate in which Th almost all substitutes for REE.

32. Thorium in weathering and sediments During weathering of rocks and minerals, Th is by and largeretained in the regolith. This results from the association ofTh with resistant accessory minerals (e.g. monazite, xenotime) and itschemically reactivenature in solution. Any Th which is solubilized from the host-rocks during weathering is rapidly adsorbed from dissolvedphase to the surface of particles (on clay minerals, Fe-Mn oxides/hydroxydes). The concentrationof dissolved Th in natural waters is quite low.Some studies in soil profiles indicate that Th is mobilized by organicmatter in top soil, but is precipitated in regions of low organic content.