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Geochemistry - an 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?

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Geochemistry - an Introduction

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  1. The Geochemistry of Rocks and Natural WatersCourse no. 2103011st part: Introduction and Fundamentals in GeochemistryA. Koschinsky

  2. 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.”

  3. 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 • …

  4. The Periodic Table of Elements

  5. 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

  6. 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.

  7. L shell K shell N L M 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 The copper atom has one lone electron in its outer shell, which can easily be pulled away from the atom. The Electronic Structure of Atoms

  8. The Electronic Configuration of the Elements

  9. 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)

  10. 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.

  11. (Pauling scale) 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.

  12. Chemical Properties of the Elements

  13. Chemical Properties of the Elements R.S. Mulliken (1934) proposed an electronegativity scale in which the electronegativity, Mis 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.

  14. Comparison of some atomic and respective ionic radii (in nanometers) 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.

  15. 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)

  16. 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.

  17. Chemical Properties of the Elements - Summary

  18. Chemical Properties of the Elements - Summary

  19. 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.

  20. What is the Solar System made of? • 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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? The silica tetrahedron and the structure of silicate minerals

  25. 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. What is the Earth made of?

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