Earth Materials

Kevin Hefferan, John O'Brien

• Publisher: Wiley-Blackwell
• Year: 2010
• ISBN:
  9781444391213
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Geologia e scienze della terra

Minerals and rocks form the foundation of geologic studies. This new textbook has been written to address the needs of students at the increasing number of universities that have compressed separate mineralogy and petrology courses into a one- or two-semester Earth materials course. Key features of this book include:

• equal coverage of mineralogy, sedimentary petrology, igneous petrology and metamorphic petrology;
• copious field examples and regional relationships with graphics that illustrate the concepts discussed;
• numerous case studies to show the uses of earth materials as resources and their fundamental role in our lives and the global economy, and their relation to natural and human-induced hazards;
• the integration of earth materials into a cohesive process-based earth systems framework;
• two color thoughout with 48 pages of four color.

Readership: students taking an earth materials, or combined mineralogy and petrology course in an earth science degree program. It will also be useful for environmental scientists, engineering geologists, and physical geographers who need to learn about minerals, rocks, soil and water in a comprehensive framework.

A companion website for this book is available at: www.wiley.com/go/hefferan/earthmaterials.

Chapter 1
Earth materials and the geosphere

• 1.1 Earth materials 1
• 1.2 The geosphere 2
• 1.3 Detailed model of the geosphere 3
• 1.4 Global tectonics 7
• 1.5 Hotspots and mantle convection 17

1.1 EARTH MATERIALS

This book concerns the nature, origin, evolution and significance of Earth materials. Earth is composed of a variety of naturally occurring and synthetic materials whose composition can be expressed in many ways. Solid Earth materials are described by their chemical, mineral and rock composition. Atoms combine to form minerals and minerals combine to form rocks. Discussion of the relationships between atoms, minerals and rocks is fundamental to an understanding of Earth materials and their behavior.

The term mineral is used in a number of ways. For example, elements on your typical breakfast cereal box are listed as minerals. Oil and gas are considered mineral resources. All these are loose interpretations of the term mineral. In the narrowest sense, minerals are defined by the following five properties:

1. 1 Minerals are solid, so they do not include liquids and gases. Minerals are solid because all the atoms in them are held together in fixed positions by forces called chemical bonds (Chapter 2).
2. 2 Minerals are naturally occurring. This definition excludes synthetic solids produced through technology. Many solid Earth materials are produced by both natural and synthetic processes. Natural and synthetic diamonds are a good example. Another example is the solid materials synthesized in high temperature and high pressure laboratory experiments that are thought to be analogous to real minerals that occur only in the deep interior of Earth.
3. 3 Minerals usually form by inorganic processes. Some solid Earth materials form by both inorganic and organic processes. For example, the mineral calcite (CaCO₃) forms by inorganic processes (stalactites and other cavestones) and is also precipitated as shell material by organisms such as clams, snails and corals.
4. 4 Each mineral species has a specific chemical composition which can be expressed by a chemical formula. An example is common table salt or halite which is composed of sodium and chlorine atoms in a 1 : 1 ratio (NaCl). Chemical compositions may vary within well-defined limits because minerals incorporate impurities, have atoms missing, or otherwise vary from their ideal compositions. In addition some types of atoms may substitute freely for one another when a mineral forms, generating a well-defined range of chemical compositions. For example, magnesium (Mg) and iron (Fe) may substitute freely for one another in the mineral olivine whose composition is expressed as (Mg,Fe)₂SiO_4. The parentheses are used to indicate the variable amounts of Mg and Fe that may substitute for each other in olivine group minerals (Chapter 3).
5. 5 Every mineral species possesses a long-range, geometric arrangement of constituent atoms or ions. This implies that the atoms in minerals are not randomly arranged. Instead minerals crystallize in geometric patterns so that the same pattern is repeated throughout the mineral. In this sense, minerals are like three-dimensional wall paper. A basic pattern of atoms, a motif, is repeated systematically to produce the entire geometric design. This long-range pattern of atoms characteristic of each mineral species is called its crystal structure. All materials that possess geometric crystal structures are crystalline materials. Solid materials that lack a long-range crystal structure are amorphous materials, where amorphous means without form; without a long-range geometric order.

Over 3500 minerals have been discovered to date (Wenk and Bulakh, 2004) and each is distinguished by a unique combination of crystal structure and chemical composition. Strictly speaking, naturally-occurring, solid materials that lack one of the properties described above are commonly referred to as mineraloids. Common examples include amorphous materials such as volcanic glass and organic crystalline materials such as those in organic sedimentary rocks such as coal.

Most of the solid Earth is composed of various types of rock. A rock is an aggregate of mineral crystals and/or mineraloids. A monominerallic rock consists of multiple crystals of a single mineral. Examples include the sedimentary rock quartz sandstone, which may consist of nothing but grains of quartz held together by quartz cement, and the igneous rock dunite, which can consist entirely of olivine crystals. Most rocks are polyminerallic; they are composed of many types of mineral crystals. For example, granite commonly contains quartz, potassium feldspar, plagioclase, hornblende and biotite and may include other mineral species.

Mineral composition is one of the major defining characteristics of rocks. Rock textures and structures are also important defining characteristics. It is not surprising that the number of rock types is very large indeed, given the large number of different minerals that occur in nature, the different conditions under which they form, and the different proportions in which they can combine to form aggregates with various textures and structures. Helping students to understand the properties, classification, origin and significance of rocks is the major emphasis of this text.

1.2 THE GEOSPHERE

Earth materials can occur anywhere within the geosphere, whose radius is approximately 6380 km (Figure 1.1). In static standard models of the geosphere, Earth is depicted with a number of roughly concentric layers. Some of these layers are distinguished primarily on the basis of differences in composition and others by differences in their state or mechanical properties. These two characteristics by which the internal layers of Earth are distinguished are not totally independent, because differences in chemical, mineralogical and/or rock composition influence mechanical properties and state.

Figure 1.1 Standard cross-section model of the geosphere showing the major compositional layers on the left and the major mechanical layers on the right.

1.2.1 Compositional layers

The layers within Earth that are defined largely on the basis of chemical composition (Figure 1.1; left side) include: (1) the crust, which is subdivided into continental and oceanic crust, (2) the mantle, and (3) the core. Each of these layers has a distinct combination of chemical, mineral and rock compositions that distinguishes it from the others as described in the next section. The thin crust ranges from 5 to 80 km thick and occupies <1% of Earth’s volume. The much thicker mantle has an average radius of ∼2885 km and occupies ∼83% of Earth’s volume. The core has a radius of ∼3480 km and comprises ∼16% of Earth’s volume.

1.2.2 Mechanical layers

The layers within Earth defined principally on the basis of mechanical properties (Figure 1.1; right side) include: (1) a strong lithosphere to an average depth of ∼100 km that includes all of the crust and the upper part of the mantle; (2) a weaker asthenosphere extending to depths ranging from 100 to 660 km and including a transition zone from ∼400 to 660 km; and (3) a mesosphere or lower mantle from ∼660 to 2900 km. The core is divided into a liquid outer core (∼2900–5150 km) and a solid inner core, below ∼5150 km to the ...

Table of contents

• Cover
• Table of Contents
• Dedication
• Title
• Copyright
• Preface
• Acknowledgments
• Chapter 1: Earth materials and the geosphere
• Chapter 2: Atoms, elements, bonds and coordination polyhedra
• Chapter 3: Atomic substitution, phase diagrams and isotopes
• Chapter 4: Crystallography
• Chapter 5: Mineral properties and rock-forming minerals
• Chapter 6: Optical identification of minerals
• Chapter 7: Classification of igneous rocks
• Chapter 8: Magma and intrusive structures
• Chapter 9: Volcanic features and landforms
• Chapter 10: Igneous rock associations
• Chapter 11: The sedimentary cycle: erosion, transportation, deposition and sedimentary structures
• Chapter 12: Weathering, sediment production and soils
• Chapter 13: Detrital sediments and sedimentary rocks
• Chapter 14: Biochemical sedimentary rocks
• Chapter 15: Metamorphism
• Chapter 16: Metamorphism: stress, deformation and structures
• Chapter 17: Texture and classification of metamorphic rocks
• Chapter 18: Metamorphic zones, facies and facies series
• Chapter 19: Mineral resources and hazards
• References
• Index
• Periodic table of the elements
• Table of chemical elements
• End User License Agreement