Transcript Manganese in sedimentary processes

Manganese, iron, cobalt,
nickel, platinum group
elements
Manganese
Universe: 8 ppm (by weight)
Sun: 10 ppm (by weight)
Carbonaceous meteorite: 2800 ppm
Earth's Crust: 1100 ppm
Seawater: Atlantic surface: 1 x 10-4 ppm
Atlantic deep: 9.6 x 10-5 ppm
Manganese in magmatic processes
Manganese abundance is 0.112% in ultrabasic rock and
0.096% in granites, so does not shows significant
difference. Much more interesting the ratio of Mn:Mg from
the ultrabasic to acidic character (from 1:100 till 1:1) or
Mn:Fe (from 1:50 till 1:10).
Mg, Fe, Mn order the substitution of crystal lattice (ionic
radii of 2+ valence states: 0,78, 0,83, 0,91). Its independent
phase very rare, rather the Mn2+ substitute in the structure
of mafic rock-forming minerals (mainly Fe2+, Mg, Ca
substitution) in biotite, tourmaline, ilmenite, magnetite,
pyroxenes, amphiboles.
Manganese in magmatic processes
However, in post-magmatic processes it forms many
independent phases. There are silicates (spessartite),
oxides (columbite-Mn, tantalite-Mn), phosphates in
pegmatite, wolframites (e.g. hübnerite) in pneumatolithic
processes.
There are mainly carbonates with Mn-substitution (Mnbearing calcite, ankerite, siderite, magnesite) and
rhodochrosite in hydrothermal or hydrothermalmetasomatic processes. It occurs in rare sulphides such
environments, e.g. alabandine (cubic MnS).
Manganese in metamorphic
processes
There are characteristic Mn silicates in metamorphic
environments, such Mn3O4 hausmannite, rhodonite,
spessartite, tephroite. It forms mainly from the Mn-rich
black shales to metamorphic processes.
Manganese in sedimentary
processes
It has an oxidation-reduction and coordination chemistry
similar to that of iron, but exists naturally in three stable
oxidation states +2, +3, +4 in solids and only +2 state when
dissolved in natural waters. While Mn2+ is
stable at acidic to neutral pH, it is easily oxidized by O2 in
basic solutions. The six-coordinate ionic radii (nm) are 0.80
for Mn2+, 0.70 for Mn3+, and 0.60 for Mn4+. The reactivity
and cycling of manganese is primarily determined the pH
and Eh of associated water: dissolution or precipitation of
phases due to oxidation-reduction of manganese ions,
solubilization of Mn2+.
Manganese in sedimentary
processes
It is present in igneous, metamorphic and sedimentary
rocks, and it is found in most waters. Manganese can be
described by two interrelated global cycles: a terrestrial
rock-water cycle and a marine/lacustrine sediment-water
cycle. Manganese is supplied to both cycles by igneous
rock. Once exposed, igneous manganese is weathered,
primarily by oxidation-reduction, acid dissolution, and
organic chelators. From solution, manganese is
redeposited into fresh or saline waters as oxides,
carbonates, silicates and many different adsorbed phases.
Manganese movement within each cycle is quite dynamic.
Manganese also moves from the terrestrial cycle to the
marine/lacustrine cycle by land drainage into the waters.
Manganese in sedimentary
processes
Microbes are actively involved in manganese cycling by
participating in oxidation-reduction, dissolution and
precipitation of Mn. Manganese cycles are important
because manganese is an important micronutrient
to plants and animals but can be toxic at high levels of
consumption.
Manganese in sedimentary
processes
It concetrates in close association with iron in oxidized
environments, however it forms independent phases.
Strongly adsorbed some metals, such K, Ca, Ba, Ti, Co, Ni,
Cu, Zn Pb, and sometimes forms different oxides whith
various chemical compounds (cryptomelane, rancieite,
romanechite, asbolane, coronadite etc.).
It forms carbonates in reductive environments, and it has
various substitution of Fe-Mn-Mg-Ca. The Mn origins from
volcanic activities in deep-marine sediments.
So, there are oxides, oxi-hydroxides, carbonates and
sulphates in sedimentary environments.
Iron (Fe)
Universe: 1100 ppm (by weight)
Sun: 1000 ppm (by weight)
Carbonaceous meteorite: 2.2 x 105 ppm
Earth's Crust: 63000 ppm
Seawater: Atlantic surface: 1 x 10-4 ppm
Atlantic deep: 4 x 10-4 ppm
Iron in magmatic processes
Iron is a component in all mineral classes (we know about
700 Fe-bearing minerals). It occurs chiefly as reduced,
ferrous iron with magnesium in mafic silicates such as
olivines, pyroxenes, amphiboles and biotite. It is also
present in pyrite, pyrrhotite, magnetite and ilmenite. The
abundance of iron (and mafic minerals) is decrease from
ultrabasic/basic to acidic rocks. The Fe2+ substitutes
Mg2+, the Fe3+ Al3+ in minerals. In post magmatic
processes often occur as complex oxides, sulphides,
carbonates, and phoshates.
Iron in weathering and sedimentary
processes
The principal source of iron in the hydrosphere is the
weathering of iron minerals from igneous and metamorphic
rocks, including the silicates olivine, pyroxenes,
amphiboles, and biotite. Sedimentary shales can be
significant sources to the extent that they contain pyrite and
marcasite. The aqueous geochemistry of iron can be
summarized by the two rules: Rule 1: Oxidizing conditions
promote the precipitation of iron, reducing conditions
promote the solution of iron. Rule 2: Acid conditions
generally promote the solution of iron, alkaline conditions
promote the precipitation of iron.
Iron in weathering and sedimentary
processes
The average concentration in seawater is 0.01 mg/l, and it
occurs primarily in the ferric (Fe3+) state. Under reducing
conditions found near the bottom of some lakes and rivers,
where highly soluble ferrous iron is favored over oxidized
ferric iron, concentrations may reach several mg/l.
Concentrations of iron in groundwater range from 1 to 10
mg/l. Low pH water, produced by industrial waste or sulfide
oxidation (chiefly pyrite) associated with mining and natural
weathering processes, can contain hundreds to thousands
of mg/l iron.The oxidation of iron in aqueous solution can
either be driven by biologically mediated reactions involving
the genus Thiobacillus or can occur as inorganic process
with dissolved oxygen.
Iron in weathering and sedimentary
processes
The weathering of iron-bearing minerals is driven by the
dissolution of CO2 into water (either atmospheric or soil
solutions), producing carbonic acid which in turn attacks
minerals. These reactions produce cations, silica and
bicarbonate in solution. In general, weathering reactions of
iron-bearing minerals occur in the same order as they are
formed according to Bowen's reaction series. The phases
which form at the higher temperatures (e.g. olivine)
weather more rapidly than those which form later, at
lower temperatures (e.g. biotite). An interesting exception
to this order are the iron sulfides pyrite and pyrrhotite,
which can form at relatively high temperatures.
Iron in weathering and sedimentary
processes
The transport of iron in surficial exogenic environments
must be either in solution or as detrital particles. Primary
iron-bearing phases, which have not as yet chemically
weathered, are carried by wind or water. Ferrous iron
released by chemical weathering will readily oxidize to form
ferric oxides. These ferric oxides can form a discrete phase
or, very commonly, coat other solids which are present as
detritus. Some iron minerals, such as magnetite, do not
chemically weather at an appreciable rate and may be
preserved and carried significant distances. Others,
such as the sulfides pyrite and pyrrhotite, rarely escape
chemical weathering and are almost never found occurring
as detrital grains.
Iron in weathering and sedimentary
processes
Large accumulations of ferric oxides can be produced,
forming materials which can be described as banded iron
formations. Rates of precipitation reactions indicate that
phases such as amorphous ferric hydroxide (ferrihydrite or
„limonite”) form first and, with time, dehydrate (lose water)
to form the more thermodynamically favored phases such
as hematite (Fe2O3), with goethite (FeOOH) as a middle
step. Thus, the red beds (hematite) of today may have
been the yellow beds (limonite) of ages ago. Ferric oxides
in sedimentary environments in which organic matter is
abundant, are destabilized with the onset of reducing
conditions produced by bacterial degradation of the organic
matter.
Iron in weathering and sedimentary
processes
Simultaneously, sulfate sulfur (the stable form of sulfur in
oxidizing conditions) is reduced to sulfide sulfur, which can
react with ferrous iron in the low Eh environment to form
iron sulfides. However, pyrite to be the stable phase, much
as with the ferric oxides precipitates, the initial iron sulfide
precipitate is a poorly crystalline iron mono-sulfide form
(the kinetically favored phase), which, in time, alters to the
thermodynamically favored phase, pyrite (or pyrrhotite). If
sulfide sulfur is not present, siderite (FeCO3) may form if
pCO2 , pH and the ratio of Fe2+/Ca2+ are relatively high
(e.g. a fresh water swamp). If pCO2 is low, but silica is
present, iron silicates such as glauconite, berthierine may
form.
Environmental geochemistry of iron
Iron plays an important role in environmental geochemistry,
both degrading and improving the quality of natural waters
necessary for humans and many other life forms. The
sulfide minerals, particularly pyrite, are more and more a
part of the waste and ore stream of modern mining activity.
Pyrite is unstable at the Earth's surface (like most other
endogenically formed iron minerals) and weathers to form
acidic iron sulfate solutions called acid rock drainage
(ARD). These acidic waters often carry trace metals with
them (i.e. copper, cadmium, arsenic), which are also
released to solution on the oxidation of the sulfides.
Environmental geochemistry of iron
Ferrous iron which is oxidized to ferric iron readily oxidizes
in surficial environments to form amorphous precipitates
These precipitates, which may be found in aquatic as well
as terrestrial environments, exhibit a strong tendency to
scavenge trace metals. Studies of the diagenesis of
sediments have routinely showed trace metals to be
associated with ferric iron phases. The sorption of metals
from aqueous solution by ferric iron precipitates may be
approximated by a number of sorption models. The
sorption process is affected by pH and ionic strength, with
near-neutral pH and low ionic strength favoring maximal
adsorption of cations (e.g. Pb2+, Cu2+).
Cobalt (Co)
Universe: 3 ppm (by weight)
Sun: 4 ppm (by weight)
Carbonaceous meteorite: 600 ppm
Earth's Crust: 20 ppm
Seawater: Pacific surface: 6.9 x 10-6 ppm
Pacific deep: 1.1 x 10-6 ppm
Nickel (Ni)
Universe: 60 ppm (by weight)
Sun: 80 ppm (by weight)
Carbonaceous meteorite: 13000 ppm
Earth's Crust: 90 ppm
Seawater: Atlantic surface: 1 x 10-4 ppm
Atlantic deep: 4 x 10-4 ppm
Cobalt in magmatic processes
Co2+ has an ionic radius in octahedral coordination which
is intermediate between Mg2+ and Fe2+ (0.735 A); thus it
substitutes for these cations in several silicates. In basaltic
rocks, the correlation with Fe and Mg is significant, in
granitic and metamorphic rocks the correlation with Mg
persists. It shows enrichment in the high temperature post
magmatic processes, it forms arsenides, or arsenidsulphides (e.g. cobalthite – CoAsS, safflorite – CoAs2,
skutterudite – CoAs3). Easily substitutes Ni in structure of
Ni-bearing minerals, both early differentiations, or postmagmatic processes (e.g. pentlandite – (Fe,Ni)9S8 and
pyrrhotite - FeS).
Cobalt in weathering and
sedimentary processes
Co is easily solubilized during weathering and does not
form residual silicate minerals in soils (average Co
concentration is 7 ppm). The distribution of Co is mostly
determined by the fate of Fe and Mn oxides; complexation
by organic substances is of intermediate relevance.
The concentration of Co in fresh surface and groundwater
varies mostly between 0.04 and 0.35 g/l; higher
concentrations occur in contaminated areas (Netherlands,
1-11 g/l) or mineralized zones. In aquatic systems Co is
transported both adsorbed to suspended particles and
dissolved, with a large portion complexed by organic
ligands.
Cobalt in weathering and
sedimentary processes
For suspended particles, participation of Co in the Mn
redox cycle and the possible oxidation of Co2+ to Co3+
on Mn oxy-hydroxide surfaces are as important as direct
adsorption to clay particles or indirect adsorption to
different particle surfaces through organic substances.
In seawater Co concentration shows the behavior of
scavenged elements, decreasing with depth from about
0.05 to 0.01 ~ g/1. High Co concentrations are found in
deep-sea clays (74 ppm) and Mn-nodules (up to 2%) with
maxima occurring in the vicinity of mid-ocean ridges. There
are many arsenates/sulphates of cobalt in the oxidation
zone of Co-bearing ore deposits (e.g. erytrine, a Coarsenate).
Nickel in magmatic processes
Nickel and iron are the most important components of the
Earth's core. In the Earth's mantle, the concentration is
quite low (2000 ppm) and even lower in the Earth's crust
(105 ppm). Nickel occurs in trace amounts in most rockforming minerals, especially in olivines. In minerals, Ni2+ is
mostly 6-fold coordinated (giving a green or yellow
coloration) due to a high crystal field stabilization energy in
this site. 4- and 5-coordinated Ni2+ found in silicate glasses
and melts. It concentrates to early differenciates as
sulphides: pentlandite-pyrrhotite association – (Fe,Ni)9S8
and FeS. Because of similar ionic radii of Ni and Mg (0.78)
it substitutes mainly to Mg in phyllosilicates (e.g. chlorites,
serpentine minerals).
Nickel in magmatic processes
It shows enrichment in high temperature post-magmatic
processes, where it forms mainly arsenides, and arsenidessulphides: nickeline – NiAs, millerite – NiS, gersdorffite –
NiAsS, ullmannite – NiSbS. Its largest accumulations are in
the five-elements ore deposits (Ni-Co-As-Bi-Ag).
Nickel in weathering and
sedimentary processes
Nickel is easily mobilized during weathering and it is often
co-precipitated with iron and manganese oxides. In tropical
rain belt areas the ultramafic rocks are weathered, giving
nickel-rich silicate ores, such as garnierite, a mixture of
hydrous trioctahedral phyllosilicates. There are many
arsenates/phosphates/sulphates of nickel in the oxidation
zone of Ni-bearing ore deposits. Among them the most
important the annabergite (hydrated Ni-arsenate).
In seawater, Ni concentration increases with depth and
follows the distribution of silicate and phosphate.
Platinum group elements
Platinum, ruthenium, osmium,
palladium, iridium, rhodium
Platinum (Pt)
Universe: 0.005 ppm (by weight)
Sun: 0.009 ppm (by weight)
Carbonaceous meteorite: 0.1 ppm
Earth's Crust: 0.0037 ppm
Seawater: Pacific surface: 1.1 x 10-7 ppm
Pacific deep: 2.7 x 10-7 ppm
Platinum group elements in
magmatic processes
Platinum is both a siderophile, e.g. native platinum;
platinidiridium (Pt,Ir), some Pt-alloys, and a chalcophile,
e.g. braggite (Pt,Ni,PdS ), sperrylite (PtAs2) element.
Platinum is typically more abundant in ultramafic and mafic
rocks than in sedimentary or felsic igneous rocks. Highest
concentrations of Pt in ultramafic rocks range from 1 to
30 ppm; the concentrations of Pt in igneous rocks range
from 1 to 75 ppb. The PGE form together with chromite and
pentlandite-pyrrhotite association in the early differenciated
rocks. They miss in the intermedate to acidic magmatics,
however show relative enrichment a few special Co-Nisulphide associations.
Platinum group elements in
magmatic processes
Anomalous concentrations of Pt (low ppb) along with Ir are
present in approximate chondrite-like proportions in a clay
layer marking the Cretaceous/Tertiary boundary, and this is
one of the key pieces of evidence for a massive meteorite
impact at the Cretaceous/Tertiary boundary.
The main elements of the PGE are platinum and palladium,
while rhodium, iridium, rhutenium and osmium comprise
about 10% of the total platinum metals.
Platina group elements in
sedimentary processes
Most of PGE minerals (except sulphides-arsenides) are
very stable in weathering, so they move as relicts to the
clastic sediments.
Platinum anomalies ranging from 48 to 150 ppb have been
found in Mississippian black shales. Impact-derived phases
were not found in these shales and Pt is believed to have
been derived from detritus weathered from nearby
ultramafic rocks.