Earth Materials. John O'Brien
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Название: Earth Materials
Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781119512219
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Because the silicate groups constitute the most significant rock‐forming minerals in Earth's crust and upper mantle they are discussed more fully in Chapter 5. In Chapter 3, we will further investigate significant aspects of mineral chemistry, including substitution solid solution and the uses of isotopes and phase stability diagrams in understanding Earth materials.
CONTENT ASSESSMENT
1 Explain the difference between:An isotope and an ion.A cation and an anion and the reason the latter two types of ions exist.A stable isotope and an unstable isotope and how the latter evolve through time.
2 Detail the major factors that determine the effective radii of different atoms and ions.
3 What is the diagonal rule and how does it help to predict the electron configuration of most major elements whose atomic number is known? Use the diagonal rule to write the “ground state” electron configurations for the following elements.(a)Figure 2.21 Major silicate structures: (a) nesosilicate, (b) sorosilicate, (c) cyclosilicate, (d) single‐chain inosilicate, (e) double chain inosilicate, (f) phyllosilicate, (g) tectosilicate.Source: Wenk and Bulakh (2004). © Cambridge University Press.helium (H)carbon (C)oxygen (O)aluminum (Al)argon (Ar)iron (Fe).
4 Referring to the periodic table of the elements (Table 2.3), which of the reactive (non‐Noble) elements is the most metallic (electropositive) and which of these is the most nonmetallic (electronegative). Use their electron configurations and position on the periodic table to explain why.
5 Explain the differences between ionic, covalent, metallic, and transitional (hybrid) bonds and the properties that commonly characterize materials that possess that type of bond.
6 Using the periodic table of the elements (Table 2.3), calculate the electronegativity difference and then predict the bond type and properties for each of the following minerals:Native platinum (Pt)Periclase (MgO)Sylvite (KCl)Pyrite (FeS2)Sphalerite (ZnS).
7 Explain the concepts of radius ratio and electrostatic valency and use them to explain why silica tetrahedra tend to link by sharing oxygen ions. Then describe and explain the basic differences between nesosilicate, sorosilicate, cyclosilicate, nesosilicate, phyllosilicate and tectosilicate structures.
REFERENCES
1 Klein, C. and Dutrow, B. (2007). Manual of Mineral Science (Manual of Mineralogy), 23e. New York: Wiley 704 pp.
2 Pauling, L. (1929). The principles determining the structure of complex ionic structures. Journal American Chemical Society 51: 1010–1026.
3 Railsback, L.B. (2003). An earth scientist's periodic table of the elements and their ions. Geology 31: 737–740.
4 Wenk, H.R. and Bulakh, A. (2004). Minerals: Their Constitution and Origin, 3e. Cambridge, UK: Cambridge University Press 646 pp.
5 Wenk, H.R. and Bulakh, A. (2016). Minerals: Their Constitution and Origin, 3e. Cambridge, UK: Cambridge University Press 621 pp.
Chapter 3 Atomic substitution, phase diagrams, and isotopes
1 3.1 Atomic (ionic) substitutio
2 3.2 Phase stability (equilibrium) diagrams
3.1 ATOMIC (IONIC) SUBSTITUTION
Minerals are composed of atoms or ions that occupy structural sites in a crystal structure (Chapter 2). Different ions can occupy the same structural site if (1) they have similar size, (2) have similar charge, and (3) are available in the environment in which the mineral is forming. This process of one ion replacing another ion is called ionic substitution. In mineral formulas, ions that commonly substitute for one another are generally placed within a single set of parentheses. In the olivine group, iron and magnesium can freely substitute for one another in the sixfold, octahedral site. As a result, the formula for olivine is commonly written as (Mg,Fe)2SiO4.
Substitution is favored for ions of similar ionic radius. In general, cation substitution at surface temperatures and pressures is limited when the larger cation radii exceed the smaller by 10–15% and becomes negligible for differences greater than 30%. Such ions are “too big” or “too small” to easily substitute for one another (Figure 3.1a), while ions of similar size are “just right.” Substitution of ions of significantly different radii distorts coordination polyhedra and decreases the stability of crystals. However, at higher temperatures, where the crystal structure is expanded, ions with larger differences in radius may more easily substitute for one another.
Substitution is favored for ions of similar charge. Where substitutions occur in only one coordination site, substitution is largely limited to ions with the same charge (Figure 3.1b). This enables the mineral to remain electrically neutral, which increases its stability. However, where substitution can occur in multiple coordination sites, ions of different charge may substitute for one another in one site so long as this charge difference is balanced by a second substitution of ions of different charge in a second coordination site.
Figure 3.1 Criteria for substitution are (a) similar size, (b) similar charge and (not shown) availability.
Substitution is favored for ions that are widely available in the environment in which the mineral is growing (Figure 3.1). As minerals grow, coordination sites will preferentially select ions with the appropriate radii and charge that are available in the vicinity of the growing crystal. The ions that occupy a coordination site in a mineral provide vital clues to the chemical composition of the system and environmental conditions under which crystallization occurred.
3.1.1 Simple СКАЧАТЬ