Download Scandium in magmatic processes

Scandium, yttrium, rare earth
elements (REE), titanium,
zirconium, hafnium, thorium
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
Scandium in magmatic processes
Scandium is typically a trace element in most rocks and
minerals. It lithophile and found in the trivalent state.
Scandium differs from other rare earth elements in being
much smaller and thus tends to substitute into early
crystallizing phases with 6-fold coordination, such as
pyroxenes and amphiboles. It has a similar size to 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.
.
Scandium in the lithosphere
It has a few phosphate, as pretulite (in metamorphic quartzlazulite veins), and kolbeckite (in hydrothermal processes).
In sedimentary environments, Sc behaves more like 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.
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
Lanthanium (La)
Europium (Eu)
Universe: 0.002 ppm
Sun: 0.002 ppm
Earth's Crust: 34 ppm
Universe: 0.0005 ppm
Sun: 0.0005 ppm
Earth's Crust: 2.1 ppm
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 orbitals that are well shielded
by 5s and 5p orbitals, leading to highly coherent behavior
as a group. Among other things, this results in the trivalent
state being especially stable and the ionic radius decreases
in an unusually systematic fashion. The dominant controls
on the geochemical behavior of the REE are their size
(ionic radius), volatility, redox behavior and complexing
behavior.
REE and Y in magmatic processes
The lanthanides (and Y) tend to be concentrated in
magmatic liquids and late crystallizing phases. Of the major
elements in the crust and mantle, only sodium and 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.
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 Earth and anomalous Eu behavior
in magmatic rocks is a sign of relatively shallow igneous
processes.
In contrast, Ce is oxidized almost exclusively under highly
oxidizing surficial conditions, notably in early marine
diagenesis, to form manganese nodules and under certain
weathering conditions.
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
70 minerals in which various REE are essential structural
constituents. Among the most significant 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.
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).
Some tendencies of
enrichment of LREE
and HREE in minerals
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 Feoxides/hydroxydes).
REE and Y in weathering and
sedimentary rocks
Under aqueous conditions, the rare earth elements exist
mostly as a variety of complexes. Carbonates and
bicarbonates dominate in seawater. For a number of rare
earth complexes, such as fluorides and carbonates, the
heavier (smaller) REE show a marked increase in stability.
Titanium (Ti)
Universe: 3 ppm
Sun: 4 ppm
Carbonaceous meteorite: 550 ppm
Earth's Crust: 6600 ppm
Seawater: 4.8 x 10-4 ppm
Titanium in magmatic processes
Titanium (4+) coordination is usually 6 (octahedral), but can
be 4 coordinated (in some Al-deficient Ti-amphiboles and
pyroxenes). Common 6 coordinated Ti phases are 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 Tioxides/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+ rich periclase (MgO)
phases are also known.
Titanium in magmatic processes
Silicate glasses and melts (such as basalts) show a
contrasted coordination chemistry for Ti: they contain
essentially 5 coordinated Ti, as titanyl units or (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, Ticontaining 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).
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 Ticontents 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.
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
very sensitive to external agents such as chemical
weathering. They can be common in detritic sediments and
metamorphic rocks. Such Ti minerals are useful tracers for
valuable placers of gold, diamond, bauxites (etc.). The low
solubility of Ti in water 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).
Titanium in weathering and sedimentary
processes
The leucoxene is mixture of different oxides, mainly rutilebrookite-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 Ticontents of bauxite consist of not only relict phases, but
diagenetic origin minerals, too.
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
Zirconium in magmatic processes
Zr is a lithophile element, present essentially as Zr4+ in
silicates but occurs sometimes in oxides. In Earth
materials, Zr4+ coordination may be 6, 7 or 8. Sixcoordinated Zr is observed in a wide number of rare
minerals: as a major element in zirconosilicates and as a
minor element in rock-forming silicates. For instance, Zr
substitutes to Ti in a number of rock-forming silicates such
as garnets, alkali-rich pyroxenes and amphiboles (like
aegirine and arfvedsonite). 7-coordinated Zr is rare in
minerals (as in baddeleyite and zirconolite, but also in
metamict zircon, a naturally radiation damaged
zircon). Finally, 8-coordinated Zr is known mainly in
crystalline zircon.
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).
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
sensitive to weathering but radiation effects can partially
destroy their atomic structure (when actinides, mainly U-Th
are substituted to Zr). Zircon can be common in detritic
sediments and metamorphic rocks 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.
Hafnium (Hf)
Universe: 0.0004 ppm
Sun: 0.0003 ppm
Carbonaceous meteorite: 0.04 ppm
Earth's Crust: 12 ppm
Seawater: 9.2 ppm
Hafnium in the lithosphere
Chemically it shows close analog of zirconium, and is
almost always enriched or depleted to the same degree.
The most important mineral host by far in the Earth's crust
is zircon (Zr,Hf)SiO4 , where Hf averages 1%,
corresponding to the terrestrial Zr/Hf ratio of ca. 37.
We know only two independent Hf minerals: hafnon,
(Hf,Zr)SiO4, the Hf-dominant analogue of zircon.
Thorium (Th)
Universe: 0.0004 ppm
Sun: 0.0003 ppm
Carbonaceous meteorite: 0.04 ppm
Earth's Crust: 12 ppm
Seawater: 9.2 ppm
Thorium in magmatic processes
Thorium occurs as a trace element in common rocks and
rock-forming minerals, with concentrations in the range of a
few ppb to tens of ppm. Th4+ has an ionic radius of~ 1 A
(similar to that of U4+). Th along with the other incompatible
elements (e.g. U, K, Rb and REE) accumulates in the
residual magma and is incorporated into the late
crystallizing silicate phases. Th and U are more abundant
in granites and associated accessory minerals than in
mafic and ultramafic rocks. There are many minerals in
which Th is a major constituent. There are relatively
common and they generally occur as accessory minerals.
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 is monazite, which is a rare earth
phosphate in which Th almost all substitutes for REE.
Thorium in weathering and sediments
During weathering of rocks and minerals, Th is by and
large retained in the regolith. This results from the
association of Th with resistant accessory minerals (e.g.
monazite, xenotime) and its chemically reactive nature in
solution. Any Th which is solubilized from the host-rocks
during weathering is rapidly adsorbed from dissolved phase
to the surface of particles (on clay minerals, Fe-Mn
oxides/hydroxydes).
The concentration of dissolved Th in natural waters is quite
low. Some studies in soil profiles indicate that Th is
mobilized by organic matter in top soil, but is precipitated in
regions of low organic content.