Transcript Geochemistry Introduction

The Geochemistry of Rocks and
Natural Waters
Course no. 210301
1st part: Introduction and
Fundamentals in Geochemistry
A. Koschinsky
Geochemistry - an Introduction
What is Geochemistry?
The urge to make geology more quantitative has led to the widespread
inclusion of the so-called “basic” sciences such as physics and chemistry into
the study of geology. The term “geochemistry” was first used by the Swiss
chemist Schönbein in 1838. V.M. Goldschmidt, who is regarded as the
founder of modern geochemistry, characterized geochemistry in 1933 with
the following words:
“The major task of geochemistry is to investigate the composition of the Earth
as a whole and of its various components and to uncover the laws that
control the distribution of the various elements. To solve these problems, the
geochemist needs a comprehensive collection of analytical data of terrestrial
material, i.e. rocks, waters and atmosphere. Furthermore, he uses analyses
of meteorites, astrophysical data about the composition of other cosmic
bodies and geophysical data about the nature of the Earth’s inside. Much
useful information also came from the synthesis of minerals in the lab and
from the observation of their mode of formation and stability conditions.”
Definition and Sub-disciplines
Geochemistry uses the tools of chemistry to understand processes on
Earth.
The wide field of Geochemistry includes:
 Trace element geochemistry
 Isotope geochemistry
 Petrochemistry
 Soil geochemistry
 Sediment geochemistry
 Marine geochemistry
 Atmospheric geochemistry
 Planetary geochemistry and Cosmochemistry
 Geochemical thermodynamics and kinetics
 Aquatic chemistry
 Inorganic geochemistry
 Organic geochemistry
 Biogeochemistry
 Environmental geochemistry
…
The Periodic Table of Elements
The Periodic Table of Elements
Symbols and numbers
Isotopes
The atoms of an element can differ in
mass from each other because they
have differing numbers of neutrons.
Those with more neutrons will weigh
more and be more massive. The
atomic mass (often referred to as
atomic weight) of an element is
calculated by adding together the
number of protons and the number of
neutrons.
Examples for isotopic couples:
Stable isotopes:
H-1, H-2 (D), H-3 (T) (or 1H, 2H, 3H)
C-12, C-13, C-14 (or 12C, 13C, 14C)
O-16, O-18
Radiogenic isotopes:
Fe-54, Fe-56
U-235, U-238
The Electronic Structure of Atoms
Electrons and Orbits
The electronic structure of
an atom largely determines
the chemical properties of
the element.
Elements within the same
group of the Periodic Table
have the similar outer
electronic configuration and
behave chemically similar.
Each electron shell
corresponds to a period or
row in the Periodic Table.
The periodic nature of
chemical properties reflects
the filling of successive
shells with additional
electrons.
The Electronic Structure of Atoms
L shell
K shell
K
Electron shell representation of
carbon atom:
The inner-most (first) shell is full as it
can hold only two electrons. The second
shell can hold eight but has only four.
Protons, neutrons, electrons
N
L M
The copper atom has one lone
electron in its outer shell, which can
easily be pulled away from the atom.
The Electronic Configuration of the Elements
Chemical Properties of the Elements
Ionization potential
The First Ionization
Potential is the energy
required to remove the
least tightly bound
electron from the atom.
Example: H --> H+ + eThe second, third, …
ionization potentials are
defined
correspondingly.
Valence is the number
of electrons given up or
accepted. Transition
metals often have more
than one valence.
Example: Fe(II) and
Fe(III)
Chemical Properties of the Elements
Electron Affinity
Electron Affinity is a measure of the desire or ability of an atom to gain electrons. It is an
energy concept. The formal definition states that Electron Affinity is the amount of energy
released when an electron as added to an atom. Most atoms tend to lose energy when they
gain electrons. Some atoms do not. The elements located in the upper right corner of the
Periodic Chart have the high E.A. values (usually found as anions ) while those in the lower
left corner have the low E.A. value (usually found as cations ). A generic equation of the EA
process would be as follows.
X + e- --> X-1 + EA.
Often this is measured in electronvolts.
Electronegativity
The concept of Electronegativity refers to the ability of a bonded atom to pull electrons
towards itself.
It is defined as the relative ability of an atom in a molecule to attract electrons towards itself.
As atoms bond, electrons are shared or transferred. The atom with the higher
electronegativity will dominate the electrons.
In order to be able to determine electronegativity values it is important to observe the
behavior of atoms in a bonded situation. Consequently, the Noble Gases do not usually
appear with listed electronegativity values.
Chemical Properties of the Elements
Pauling Scale
The Pauling Scale is the most commonly used scale of electronegativity values. The
calculations used to arrive at the numbers in the scale are complex. It is most common to
simply know the results of those calculations. The scale is based on Fluorine having the
largest electronegativity with a value of 4.0. The Francium atom is assigned the lowest
electronegativity value at 0.7. All other values are located between these extremes.
Examples: Li--1.0 Be--1.5 B--2.0
C--2.5
N--3.0
O--3.5
F--4.0.
(Pauling scale)
Chemical Properties of the Elements
Chemical Properties of the Elements
R.S. Mulliken (1934) proposed an electronegativity scale in which the electronegativity, M is
related to the electron affinity EAv (a measure of the tendency of an atom to form a negative
species) and the ionization potential IEv (a measure of the tendency of an atom to form a
positive species) by the equation:
M = (IEv + EAv)/2
The subscript v denotes a specific valence state.
The Mulliken electronegativities are expressed directly in energy units, usually electron volts.
Chemical Properties of the Elements
Ionic radius
Cations have smaller radii than anions. Ionic radius decreases with increasing charge.
Ionic radius is important for geochemical reactions such as substitution in crystal lattices,
solubility, and diffusion rates.
Comparison of some atomic and respective ionic
radii (in nanometers)
Chemical Bonding
Ionic Bond: total transfer of electrons from one atom to another
Covalent Bond: the outer electrons of
the bound atoms are in hybrid orbits that
encompass both atoms.
Due to different electronegativity, covalent
bonds are often polar --> dipole
interactions (Van der Waals interactions)
Chemical Bonding
Metallic Bond: valence electrons are not
associated with any single atom, but are
mobile (“electron sea”).
This bond type is less important in
geochemistry than the other bonds.
Chemical Properties of the Elements - Summary
Hydrogen
Alkali Metals
Alkaline Earth
Metals
Transition
Metals
Other Metals
Hydrogen is unique as it is the simplest possible atom c onsisting of
just one proton and one electron
These are very reactive metals that do not occur f reely i n nature .
These metals have only one electron in th eir out er shell, therefore
they are ready t o lose that one electron in ionic bonding with other
elements. The alkali metals are softer than most othe r metals.
Cesium and francium are the m ost reactive elements in this group.
The alkaline earth elements are metallic. All alkaline earth elements
have an oxidation number of +2, making them very reactive.
Because of the ir reactivity, the alkaline metals are not fo und free in
nature.
The transition elements are both du ctile and malleable, and conduct
electricity and heat. The interesting thing about t ransition metals is
that their valence electrons, or the electrons they use to combine
with ot her elements, are present in more than one shell. This is the
reason why t hey often exhibit several co mmon oxidation s tates.
The 7 elements class ified as other metals, unlike the transition
elements, do not exhibit variable oxidation states, and their valence
electrons are only p resent in the ir outer shell. All of these elements
are solid. They have oxidation nu mbers of +3, +4, -4, and -3.
Chemical Properties of the Elements - Summary
Metalloids
Non-Metals
Rare Earth
Metals
Halogens
Noble Gases
Metalloids are the elements found along the stair-step line that
distinguishes metals from no n-metals. This line is drawn from
between Boron and Aluminum t o the b order between Polonium and
Astatine. Metalloids have properties of both metals and non-metals.
Some of the metalloids, s uch as silic on and germanium, are semiconductors.
Non-metals a re not able to conduct electricity or heat very w ell. As
opposed to metals, non-metallic elements are very brittle. Th e nonmetals exist in two of the thre e states of matter at room temperature:
gases (such as oxygen) and solids (such as c arbon). Th ey have
oxidation nu mbers of +4, -4, -3, and -2.
The thirty rare earth e lements are composed of the lantha nide and
actinide series. The y are transition metals. One e lement of the
lanthanide series and most of the elements i n the actinide series are
called trans-uranic, and are synthet ic or man-made
The term ТhalogenУmeans Тsalt-formerУand compounds c ontaining
halogens are called ТsaltsУ.All halogens have 7 electrons in their
outer shell, giving the m an oxidation number of -1. The ha logens are
non-metallic and exist, at room temperature, in all three states of
matter
All noble gases have the maximum num ber of electrons possible in
their outer shell ( 2 for Helium, 8 for all o thers), making the m s table
and preventing them from forming compound s readily.
What is the Solar System made of?
What is the relative abundances of the various elements throughout the Universe?
This turns out to be a difficult task for one obvious reason. Spectroscopic measurements
of elements from the distant stars are strongly biased towards only those elements in
excited states at or near the stellar surface. Those elements principally in the interior do
not contribute to surface radiation in the same proportions as actually exist in a star.
The situation is better for the Sun. When element distributions are stated as Cosmic
Abundances, they actually are rough estimates made from Solar Abundances .
What is the Solar
System made of?
From the figure, we see four
patterns:
 An overwhelming abundance
of light elements
 A strong preference for evennumbered elements
 A peak in abundance at iron,
followed by a steady decrease.
 Elements 3-5, Lithium,
Beryllium and Boron, are very
low in abundance.
These patterns have to do with
nucleosynthesis (element
formation) in the stars.
What is the Solar System made of?
If the Sun and Solar
System formed from the
same material, we would
expect the raw material of
the planets to match the
composition of the Sun,
minus those elements that
would remain as gases.
We find such a
composition in a class of
meteorites called
chondrites, which are
thought to be the most
primitive remaining solar
system material.
Chondrites are considered
the raw material of the
inner Solar System and
probably reflect the bulk
composition of the Earth.
What is the
Earth made of?
Relative abundance by weight
of elements in the whole
Earth and in the Earth’s crust.
Differentiation has created a
light crust depleted in iron
and enriched in oxygen,
silicon, aluminum, calcium,
potassium, and sodium.
What is the Earth made of?
Crustal Element Distribution
The abundance of elements in the Earth's crust is much different from the abundance of
elements that are to be found on the other planets and our Sun. The continental crust of
the Earth also differs radically from the overall composition of the Earth.
Our Earth as a whole and its crust, in particular, have extraordinary concentrations of
elements, all associated with silicate minerals like olivine, pyroxene, amphibole,
plagioclase, the micas, and quartz. Although there are a vast number of silicate minerals,
most silicate minerals are made from just eight elements.
The two most common elements in the Earth's crust, oxygen and silicon, combine to form
the "backbone" of the silicate minerals, along with, occasionally, aluminum and
iron. These four elements alone account for about 87% of the Earth's crust. This silicate
or alumina-silicate "backbone" carries excess negative charge, however. Positive charge
in the form of cations has to be brought in to balance this negative charge. The four most
important elements that fit in the mineralogical structures of the silicates are calcium,
sodium, potassium and magnesium. Taken all together, constituting nearly 99% of crustal
elements, leaves little room for all of the other elements.
As a consequence, all other elements are either nearly absent from the Earth's crust or
are found primarily in non-silicate rocks.
What is the Earth made of?
The silica tetrahedron and the structure of silicate minerals
a. The silica tetrahedron
consists of a central
silicon atom bound to 4
oxygens.
b. In orthosilicates such as olivine, the tetrahedra are
separate and each oxygen is also bound to other
metal ions that occupy interstitial sites between the
tetrahedra.
What is the Earth made of?
d. In sheet silicates,
such as talc, mica, and
clays, the tetrahedra
each share 3 oxygens
and are bound together
into sheets.
c. In pyroxenes, the tetrahedral each
share two oxygen and are bound
together into chains. Metal ions are
located between the chains.