The Geochemistry of Rocks and Natural Waters Course no. 210301 1st part: Introduction and Fundamentals in Geochemistry A. Koschinsky

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.

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 wide field of Geochemistry includes:

The Periodic Table of Elements

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-18Radiogenic isotopes:Fe-54, Fe-56U-235, U-238

The Periodic Table of Elements

Symbols and numbers

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

The Electronic Structure of Atoms

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

K shell

L shell

The copper atom has one lone electron in its outer shell, which can easily be pulled away from the atom.

K

N

ML

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+ + e-

The 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)


Electron AffinityElectron 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.

ElectronegativityThe 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.


Pauling ScaleThe 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)



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.


Ionic radiusCations 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 Hydrogen is unique as it is the simplest possible atom consisting ofjust one proton and one electron

Alkali Metals These are very reactive metals that do not occur freely in nature.These metals have only one electron in their outer shell, thereforethey are ready to lose that one electron in ionic bonding with otherelements. The alkali metals are softer than most other metals.Cesium and francium are the most reactive elements in this group.

Alkaline EarthMetals

The alkaline earth elements are metallic. All alkaline earth elementshave an oxidation number of +2, making them very reactive.Because of their reactivity, the alkaline metals are not found free innature.

TransitionMetals

The transition elements are both ductile and malleable, and conductelectricity and heat. The interesting thing about transition metals isthat their valence electrons, or the electrons they use to combinewith other elements, are present in more than one shell. This is thereason why they often exhibit several common oxidation states.

Other Metals The 7 elements classified as other metals, unlike the transitionelements, do not exhibit variable oxidation states, and their valenceelectrons are only present in their outer shell. All of these elementsare solid. They have oxidation numbers of +3, +4, -4, and -3.

Chemical Properties of the Elements - Summary

Metalloids Metalloids are the elements found along the stair-step line thatdistinguishes metals from non-metals. This line is drawn frombetween Boron and Aluminum to the border between Polonium andAstatine. Metalloids have properties of both metals and non-metals.Some of the metalloids, such as silicon and germanium, are semi-conductors.

Non-Metals Non-metals are not able to conduct electricity or heat very well. Asopposed to metals, non-metallic elements are very brittle. The non-metals exist in two of the three states of matter at room temperature:gases (such as oxygen) and solids (such as carbon). They haveoxidation numbers of +4, -4, -3, and -2.

Rare EarthMetals

The thirty rare earth elements are composed of the lanthanide andactinide series. They are transition metals. One element of thelanthanide series and most of the elements in the actinide series arecalled trans-uranic, and are synthetic or man-made

Halogens The term “halogen” means “salt-former” and compounds containinghalogens are called “salts”. All halogens have 7 electrons in theirouter shell, giving them an oxidation number of -1. The halogens arenon-metallic and exist, at room temperature, in all three states ofmatter

Noble Gases All noble gases have the maximum number of electrons possible intheir outer shell (2 for Helium, 8 for all others), making them stableand preventing them from forming compounds 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 .


From the figure, we see four patterns:

An overwhelming abundance of light elements A strong preference for even-numbered 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.


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.


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.


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.


c. In pyroxenes, the tetrahedral each share two oxygen and are bound together into chains. Metal ions are located between the chains.

d. In sheet silicates, such as talc, mica, and clays, the tetrahedra each share 3 oxygens and are bound together into sheets.

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The Geochemistry of Rocks and Natural Waters Course no. 210301 1st part: Introduction and Fundamentals in Geochemistry A. Koschinsky