Earth Materials. John O'Brien

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Earth Materials - John  O'Brien


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href="#u2c228299-ce38-5dd7-b3f1-bbc8b5e82b34">Chapter 16).

      Point defects can occur on still smaller scales. In some cases electrons are missing from a quantum level, which produces an electron hole in the crystal structure. In others, an electron substitutes for an anion in the crystal structure. As with other point defects, the existence of electron holes plays an important role in the properties of the crystalline materials in which they occur. In most minerals, as temperature increases, the number of omission defects tends to increase. This allows minerals to deform more readily in a plastic manner at higher temperatures.

Schematic illustration of (a) Frenkel defect, with a vacancy due to an ion displaced to the interstitial site. (b) Shottky defect in pyrrhotite (Fe1–xS) where a vacancy (absent Fe plus 2) is balanced by the substitution of Fe plus 3 for Fe plus 2 in two lattice sites.

      4.8.1 Point defects

      Point defects involve individual atoms and therefore do not have longer range extent; they are considered to be zero‐dimensional defects. Many types of point defects exist, and they are important in explaining the properties of minerals as well as other materials that include steel, cement, glass products, semiconductors, and superconductors. These include the following:

      1 Substitution defects (Figure 4.32b) form when anomalous ions of inappropriate size and/or charge substitute for ions of appropriate size and/or charge in a structural site. These anomalous ions tend to distort the crystal lattice locally and to be somewhat randomly distributed within the crystal lattice.

      2 Interstitial defects (Figure 4.32c) occur when anomalous ions occupy the spaces between structural sites. Such “extra” ions are trapped in the interstices between the “normal” locations of ions in the crystal lattice.

      3 Omission defects (Figure 4.32d) form when structural sites that should contain ions are unoccupied. In such cases, ions that should occur within the ideal crystal structure are omitted from the crystal lattice leaving a “hole” in the ideal crystal structure.

      4.8.2 Line defects

Schematic illustration of (a) Perfect crystal lattice; (b) substitution defect; (c) interstitial defect; (d) omission defect. Image described by caption.

      Source: Klein and Hurlbut (1985). © John Wiley & Sons.