Читать книгу Earth Materials - John O'Brien - Страница 120

Frenkel defects (Figure B4.1a) are formed when the ions in question move to an interstitial site, leaving unoccupied structural sites or holes behind. Frenkel defects combine omission and interstitial defects. Because the ion has simply moved to another location, the overall charge balance of the crystal is maintained, but local lattice distortions occur in the vicinities of both the holes and the extra ions. Schottky defects (Figure B4.1b) are omissions formed when the ions migrate out of the crystal structure or were never there. Schottky defects create a charge imbalance in the crystal lattice. Such charge imbalances may be balanced by the creation of a second hole in the crystal structure; for example, an anion omission may be created to balance a cation omission. They may also be balanced by the substitution of ions of appropriate charge difference elsewhere in the structure. The highly magnetic mineral pyrrhotite (Fe_1–XS) provides a good example. When a ferrous iron (Fe⁺²) ion is omitted from a cation site in the crystal structure, leaving a charge deficit of 2, two ferric iron (Fe⁺³) ions can substitute for ferrous iron (Fe⁺²) ions to increase the charge by 2 and produce an electrically neutral lattice (Figure B4.1b). The formula for pyrrhotite (Fe_1‐xS) reflects the fact that there are fewer iron (total Fe⁺² and Fe⁺³) ions than sulfur (S⁻²) ions in the crystal structure due to the existence of a substantial number of such Schottky omission defects.

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.

Figure B4.1 (a) Frenkel defect, with a vacancy due to an ion displaced to the interstitial site. (b) Shottky defect in pyrrhotite (Fe_1–xS) where a vacancy (absent Fe⁺²) is balanced by the substitution of Fe⁺³ for Fe⁺² in two lattice sites.