Earth Materials. John O'Brien
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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)2SiO4. 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).
4 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. They are minerals, in the narrow sense, if they are naturally occurring, inorganic solids with a well‐defined chemical composition. Solid materials that lack a long‐range crystal structure are amorphous materials, where amorphous means without form and without a long‐range geometric order.
Many would add a fifth property that requires minerals to sometimes be formed by inorganic processes. It is certainly true that the vast majority of minerals conform to this property and that the vast majority of organically formed crystalline solids are not considered to be minerals. However, many solid Earth materials that form by both inorganic and organic processes are considered minerals, especially if they are important constituents of naturally formed rocks. For example, the mineral calcite is also precipitated as shell material by organisms such as clams, snails, and corals and is the major constituent of the rock limestone (Chapter 14).
Over 5500 minerals have been discovered to date (www.mindat.com) and each is distinguished by a unique combination of chemical composition and crystal structure. 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 in which the atoms lack long‐range order and amber or ivory which are formed only by organic processes.
1.2.1 Rocks
Earth is largely composed of various types of rock. A rock is an aggregate of mineral crystals and/or mineraloids. Scarce monominerallic rocks consist of multiple crystals of a single mineral. Examples include the sedimentary rock quartz sandstone which may consist entirely of quartz grains held together by quartz cement and the igneous rock dunite which can consist entirely of olivine crystals. The vast majority of rocks are polyminerallic; they are composed of many types of mineral crystals. For example, granite commonly contains quartz, potassium feldspar, plagioclase, hornblende and/or biotite, and various other minerals in small amounts.
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 minerals and rocks is the major emphasis of this text.
1.3 THE GEOSPHERE
Earth materials can occur anywhere on or within the geosphere, the portion the Earth from its surface to its center, whose radius is approximately 6370 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.
Figure 1.1 Standard cross‐section model of the geosphere. Major compositional layers are shown on the left: core (red shade), mantle (brown shade), and continental and oceanic crust (blue shades). Major mechanical layers are shown on the right: inner core and outer core (red shades), lithosphere (light brown), mesosphere (dark brown), and asthenosphere (burnt orange).
1.3.1 Compositional layers
The layers within Earth that are defined largely on the basis of chemical composition (Figure 1.1; left side) include the: (1) crust, which is subdivided into continental and oceanic crust, (2) mantle, and (3) core. Each of these layers has a distinctive combination of chemical, mineral, and rock compositions that distinguishes it from the others, as described in the next section. The thin crust typically ranges from 5 to 85 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 ~3470 km and comprises ~16% of Earth's volume.
1.3.2 Mechanical layers
The layers within Earth defined principally on the basis of mechanical properties (Figure 1.1; right side) include: (1) a relatively strong lithosphere of variable thickness to an average depth of ~100 km that includes all of the crust and the upper part of the mantle, (2) a weaker asthenosphere that extends to depths between ~100 and 660 km and includes a transition zone from ~400 to 660 km, and (3) a mesosphere or lower mantle from ~660 to 2900 km. The underlying core is divided into a liquid outer core (~2900–5150 km) and a solid inner core below ~5150 km to the center of Earth. Each of these layers is distinguished from the layers above and below by its unique mechanical properties. The major features of each of these layers are summarized in the next section.
1.4 DETAILED MODEL OF THE GEOSPHERE
1.4.1 Earth's crust
The outermost layer of the geosphere, Earth's crust, is extremely thin; in some ways it is analogous to the very thin skin on an apple. The crust is separated from the underlying mantle by a boundary called the Mohorovičić (Moho) discontinuity. Two major types of crust occur.
Oceanic crust
Oceanic crust is composed largely of dark colored, basic (45–52% SiO2) rocks (Chapter 7) enriched in oxides of magnesium, iron, and calcium (MgO, FeO, and CaO) relative to average crust. The elevated iron (Fe) content is responsible for the both the dark color and elevated density of oceanic crust. Oceanic crust is thin; the depth to the Moho averages 5–7 km. Under some oceanic islands, its thickness reaches 18 km. The elevated