Transcript Introduction to Environmental Geochemistry

Natural Geochemical
Enrichments of Elements
GLY 4241 - Lecture 3
Fall, 2014
1
Important Low-Abundance Elements
• Elements used in steel alloys
 Vanadium, chromium, nickel, niobium
• Elements used in rechargeable batteries
 Nickel, cadmium, lithium
• Jewelery
 Gold, silver, platinum, palladium
• Nuclear fuels
 Uranium, thorium
• Miscellaneous
 Copper, used in wiring, plumbing, alloys
 Mercury, used in electrical switches, pesticides,
fluorescent bulbs, etc.
2
Need for Concentration
• Most of these elements could not be mined,
processed, and formulated into useful products at
reasonable costs if they occurred everywhere at
their average abundances in the crust
• For example, millions of tons of gold exist in
seawater, but the cost of obtaining pure gold from
seawater is many times the value of the gold
3
Enrichment
• We need to answer two questions:
 1. For any given element, how much
enrichment above natural abundance values is
needed to produce a mineable ore?
 2. What geochemical processes are responsible
for producing these natural elemental
enrichments?
4
Definition of Ore
• The naturally occurring material from
which a mineral or minerals of economic
value can be extracted at a reasonable profit
(from the Glossary of Geology, 3rd edition)
5
Factors Influencing Cost of Metals
• Exploration
• Mining Rights Acquisition
• Cost of mining,
 Includes cost of compliance with existing
environmental regulations
• Ore separation and processing
• Transportation of ore to consumer
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Frank Wigglesworth Clarke,
1847-1931
• Early career involved
teaching, including a
year at Howard
University, and 9
years at the Univ. of
Cincinnati
• Later, Chief Chemist,
USGS, 1883-1924
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Definitions
• Clarke = the average abundance of an
element in the crust of the earth
• Clarke of concentration = the concentration
of an element in a rock compared with its
average concentration in the earth's crust, or
of an element within a particular mineral
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Clarke of Concentration
• Clarke of copper is about 55 ppm, or
0.006%
• In the mineral chalcocite, Cu2S, the Cu
concentration is 79.8%
• Thus, the clarke of concentration within this
mineral is 79.8/0.006, or 13,300
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Clarke Values
Metal
Clarke
Minimum metal %
for profitable
extraction
Clarke of
Concentration
Al
8.13
30
4
Fe
5.00
20
4
Mn
0.10
35
350
Cr
0.01
30
3000
Cu
0.006
0.25
40
Ni
0.0075
1.5
200
Zn
0.007
4
600
Sn
0.0002
1
5000
Pb
0.0013
4
3000
U
0.0002
0.1
500
Ag
0.00001
0.05
5000
Au
0.0000005
0.0005
1000
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Early Earth
• The early earth was probably bombarded by
solid planetesimals, primarily chondritic
meteorites
• Chondritic meteorites may be left over from
the protoplanet stage of the solar system
• Chondritic meteorites are composed of three
different phases, or combinations of these
phases
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Chondrite Phases
• Nickel-iron metal
• Iron sulfide
• Silicates, largely olivine or pyroxene
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Exchange Reactions
• M + Fe silicate ↔ M silicate + Fe
• M + Fe sulfide ↔ M sulfide + Fe
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Goldschmidt Element Affinities
• Siderophile: Elements concentrated in the
metallic phase, along with metallic iron
• Chalcophile: Elements concentrated in the
sulfide phase
• Lithophile: Elements concentrated in the
silicate phase
• Atmophile: Elements concentrated in the
atmosphere
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Meteorite Phases
• Iron-nickel metal
• Troilite (sulfide)
• Silicate
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Geochemical Classification of
Elements
Siderophile
Chalcophile
Lithophile
Atmophile
Fe* Co* Ni*
(Cu) Ag
Li Na K Rb Cs
(H) (C) N (O)
Ru Rh Pd
Zn Cd Hg
Be Mg Ca Sr Ba
(Cl) (Br) (I)
Os Ir Pt
Ga In Tl
B Al Sc Y REE
He Ne Ar
Au Re+ Mo+
(Ge) (Sn) Pb
Si Ti Zr Hf Th
Kr Xe
Ge* Sn* W++
(As) (Sb) Bi
P V Nb Ta
C++ Cu* Ga*
S Se Te
O Cr U
(P) As+ Sb+
(Fe) Mo (Os)
H F Cl Br I
(Ru) (Rh) (Pd)
(Fe) Mn (Zn) (Ga)
*
Elements are chalcophile and lithophile in the earth's crust.
Elements are chalcophile in the earth's crust
++
Elements are lithophile in the earth's crust
() Elements show affinity for more than one group. Secondary group(s) are shown in parentheses.
After Mason and Moore (1982); Brownlow (1979)
+
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Trace Elements
• Working definition: element whose
concentration is less than 0.1%
• May form their own minerals, but typically
are too scarce to do so
• Typically, trace elements will follow a
major element into another mineral, where
they replace part of the major element
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Siderophile Characteristics
• Elements whose valence electrons are not readily
available for combination with other elements
• Positive charge on the nucleus, at least under
certain conditions, exerts a strong attraction on the
outer electrons, preventing combination
• These elements usually occur in the native state
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Chalcophile Characteristics
• Elements whose valence electrons may be shared,
but are not electropositive enough to donate
electrons or electronegative enough to accept
electrons
• Thus, the bonds formed are predominantly
covalent
• Since sulfur is much less electronegative than
oxygen, sulfur is prone to form covalent bonds
with these elements
• Generally the chalcophile elements have their
valence electrons outside a shell of 18 electrons
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Lithophile Characteristics
• Elements that are strongly electropositive or
electronegative and thus typically donate or
accept electrons, forming ionic bonds
• Most silicate minerals have oxygen ions
that can form ionic bonds to metal cations
• Generally the lithophile elements have their
valence electrons outside a shell of eight
electrons
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Atmophile Characteristics
• Elements that do not readily combine with
other elements, or which form diatomic
molecules held together in the solid or
liquid states only by very weak Van der
Waal forces
• All of the inert gases, with completed shells
or subshells, fall into this category
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Oxygen
• Secondary atmophile element would not occur in
the atmosphere of the earth if the earth were at
chemical equilibrium
• Oxygen is maintained in the atmosphere only by
the continual photosynthesis within the biosphere
• Indeed, the presence of oxygen in an atmosphere
is often regarded as an indicator of life on the
planet
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Atomic Volume vs. Atomic Number
• Vertical scale should be atomic volume
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