Weathering

An Introduction to the Scientific Principles

Will J Bland, David Rolls, Will Bland

• Publisher: Routledge
• Year: 2016
• Print ISBN:
  9780340677452
• eBook ISBN:
  9781317836292
• Topic: Physical Sciences
• Subtopic: Environmental Science

Our landscape is constantly changing, but before the dramatic effects of erosion and mass movement take place, more subtle forces work on the rocks, minerals and soils around us. Weathering is the initial process which exposes the top few layers of the Earth to the potential for change.

This book provides an introduction to the scientific principles behind mechanical, chemical and biological weathering. Starting with a consideration of the chemical and physical properties of rocks and water, the authors proceed to an accessible explanation of the weathering processes themselves, concluding with a review of weathering rates and intensities, and a survey of the effects of weathering on the landscape. Assuming little background knowledge, the authors develop ideas from first principles to provide a straightforward introduction to weathering for students of geography, geology and earth and environmental science.

1

GENERAL PRINCIPLES OF BONDING AND STRUCTURE IN EARTH MATERIALS

Simple observation tells you that earth materials differ greatly in their physical properties. If you dig up a spadeful of soil you will see earth which has a crumbly structure mixed up with stones which are hard. In dry environments you will find rock salt crystals which are hard and brittle, or you could find sparkling micas which are hard but easily sheared. Water, also found in soil, is a liquid.

The physical differences in these materials reflect both the ways in which the sub-microscopic constituents are organised and bound together and the larger-scale physical characteristics.

The first three chapters are concerned with the following items:

• basic principles;

• application of principles;

• bulk physical properties.

In this chapter the basic ideas about atoms are developed to explain the different types of bonding which occur. The two main types are ionic bonding and covalent bonding. An important third type of bonding, hydrogen bonding, which only occurs under particular conditions, is also reviewed.

1.1 ATOMS, IONS AND THE CLASSIFICATION OF THE ELEMENTS

All matter, rocks, minerals, water, etc., is made up of atoms. An element is an example of matter which contains only one type of atom; for example, diamond contains only carbon atoms, iron contains only iron atoms and gold contains only gold atoms.

A compound is an example of matter where two or more elements are chemically combined in some definite fixed ratio; for example, sodium chloride (NaCl) has one sodium for every one chlorine atom. Rocks and minerals are made up of compounds and only the most inert of elements occur pure (in native form) in nature, e.g. gold is found as nuggets.

Atoms are made up of three particles: protons, neutrons and electrons. Atoms have a small dense nucleus made up of protons and neutrons and is surrounded by the electrons which are very spread out. Most of the mass of an atom is in its nucleus and in addition the nucleus is positively charged. This can be understood by looking at the properties of the particles:

The hydrogen atom is the simplest of all atoms and contains one proton and one electron. The atom can be drawn as a nucleus with the electron orbiting round it:

The helium atom has two protons and two neutrons in its nucleus and two electrons orbiting round it:

Notice that in neutral atoms the number of positively charged protons is equal to the number of negatively charged electrons. The number of protons is called the atomic number. It is 1 for hydrogen and 2 for helium.

The lithium atom has three protons, four neutrons and three electrons:

Notice that the third electron is contained in a second shell of electrons outside the first one.

1.1.1 Ions

These are charged. Sodium (Na) has an atomic number of 11 so a neutral atom of sodium contains 11 protons and 11 electrons. If a sodium atom loses one of its electrons it will still contain 11 positively charged protons, but only 10 negatively charged electrons, and so the overall charge will be +1. This is the sodium ion and is written Na⁺.

Chlorine (Cl) has a tendency to gain an electron which will give it one extra negative charge. The chlorine ion is written Cl⁻.

Ions can come together to form ionic solids.

Ionic bonding

In the formation of the ionic solid, sodium chloride, one electron is transferred from a sodium atom to a chlorine atom, so changing the charge of both and resulting in mutual attraction. Sodium chloride can be written Na⁺Cl⁻ to show the charges on the ions which are present, though usually ionic solids are written without the charges being shown in this way. Calcium chloride (CaCl₂) is another example of an ionic solid. It contains calcium ions (Ca²⁺) and two chloride ions (Cl⁻) for every calcium ion.

Sodium forms a singly charged cation (cation is another way of describing a positive ion) while calcium forms a doubly charged cation. In order to explain this it is necessary to know that electrons in atoms are arranged in shells and sodium has one more electron than it needs to achieve a stable outer shell of eight electrons, while calcium has two more electrons than it needs to achieve a stable outer shell of eight electrons.

Periodic table of the elements and ionic bonding

This is a method of classifying the elements which was first developed in its modern form by Mendeleev (see Box 1.1). The formation of ionic bonds and the nature of ions can usefully be discussed in general terms by use of the periodic table. If you look at the periodic table of the elements in Appendix D you will see that group 1 (the first vertical group on the left) contains the elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and caesium (Cs). Each of these elements has one more electron than it needs to achieve a stable outer shell of eight electrons, so they form singly charged cations, e.g. Li⁺ and chlorides of formula LiCl, NaCl, etc. Similarly for group 2 elements they will form doubly charged cations, e.g. magnesium forms Mg²⁺, and the chloride has the formula MgCl₂.

Negatively charged ions (also called anions) occur where the atoms have just less than a stable outer shell of eight electrons, so that they will tend to gain one or more extra electrons in order to have a stable outer shell. Examples are the elements in group 17 (also called group VII) called the halogens, e.g. fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). They need one more electron and form singly charged anions, e.g. fluorine forms the fluoride ion (F⁻). (Note that for anions the name is formed by removing the end of the name from the element and adding the ending ‘ide’.)

Box 1.1 The periodic table of the elements

The modern form of the periodic table came originally from the Russian scientist, Dimitri Mendeleev. Mendeleev observed that if the elements known at that time were written down in order of increasing atomic weight (now called relative atomic mass), every so often the properties of the elements seemed to repeat in nature; for example, highly reactive light-coloured metals recurred. This periodic variation in properties was the basis of what Mendeleev called his periodic law. In 1869 Mendeleev proposed that the elements could be written down in rows in order of increasing atomic weight, returning to the start of the next row when properties were repeated. This led to columns of elements (called groups) with similar properties and rows of elements across which trends in properties were observed.

The modern form of th...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Chapter 1 General principles of bonding and structure in earth materials
• Chapter 2 Application of principles: structures of silicates and other earth materials
• Chapter 3 Bulk physical properties of rocks
• Chapter 4 Physical properties of water
• Chapter 5 Chemical properties of water
• Chapter 6 Mechanical weathering processes
• Chapter 7 Chemical weathering
• Chapter 8 Biological weathering
• Chapter 9 Intensity and rate of weathering
• Chapter 10 Products of weathering
• Chapter 11 Weathering-controlled landforms
• Appendix A Named compounds and minerals
• Appendix B Glossary
• Appendix C Units, constants and conversion factors
• Appendix D Periodic table of the elements
• Index