Download Chromium in magmatic processes

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
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.
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 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.
Chromium in weathering and
sedimentary processes
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.
Chromium in the biosphere
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
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
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, 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.
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.
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 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
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 in magmatic processes
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 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 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.).
Tungsten in magmatic processes
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.
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.
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
Rhenium in magmatic processes
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).
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 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.