Point defects in minerals

Hobbs, B.E. (1985) in Point Defects in Minerals (Ed. R.N. Schock), Am. Geophys. Union, Washington... 

Point defects are an important part of the work in this paper. There are many reasons for the formation of point defects in minerals and their presence can exert important perturbations on the properties of the material (4). Point defects are formed because of the thermally driven intrinsic disorder in a lattice, the addition of aliovalent impurities or dopants, the presence of metal-nonmetal nonstoichiometry, and the creation of nonideal cation ratios. The first three source of defects are well-known from binary compounds but the last is unique to ternary compounds. Ternary compounds are much more complex than the binary compounds but they also have gained a great deal of attention because of the variety of important behavior they exhibit including now the presence of superconductivity at high temperatures. The point defects can be measured by introducing probe ions into the lattice. 

The hydrolytic weakening effect in quartz. In Point Defects in Minerals, Geophysical Monograph 31, edited by R. N. Schock, pp. 151-70. Washington, DC American Geophysical Union. 

Condit RH, Weed HC, Piwinskii AJ (1985) A technique for observing diffusion along grain boundary regions in synthetic forsterite. In Schock RN (ed) Point Defects in Minerals, Geophys Monogr 31, Mineral Phys 1 97-105... 

Nakamura A. and Schmalzried H. (1983). On the nonstoichiometry and point defects in olivine. Phys. Chem. Minerals, 10 27-37. 

Water in the mantle is fonnd in different states as a fluid especially near sub-duction zones, as a hydrous phase and as a hydroxyl point defect in nominally anhydrous minerals. 8D-values between -90 and -110%c have been obtained by Bell and Ihinger (2000) analyzing nominally anhydrous mantle minerals (garnet, pyroxene) containing trace quantities of OH. Nominally anhydrous minerals from mantle xenoliths are the most D-depleted of all mantle materials with 5D-values 50%c lower than MORE (O Leary et al. 2005). This difference may either imply that these minerals represent an isotopically distinct mantle reservoir or that the samples analyzed have exchanged hydrogen dnring or after their ascent from the mantle (meteoric/water interaction ). 

Brodholt J (1997) Ab initio calculations on point defects in forsterite (Mg2Si04) and implications for diffusion and creep. Am Mineral 82 1049-1053... 

In the previous sections composition variation has been attributed more or less to point defects and extensions of the point defect concept. In this section structures that can be considered to be built from slabs of one or more parent structures are described. They are frequently found in mineral specimens, and the piecemeal way in which early examples were discovered has led to a number of more or less synonymic terms for their description, including intergrowth phases, composite structures, polysynthetic twinned phases, polysomatic phases, and tropochemical cell-twinned phases. In general, they are all considered to be modular structures. 

With decreasing temperature, as we have seen, the intrinsic defect population decreases exponentially and, at low T, extrinsic disorder becomes dominant. Moreover, extrinsic disorder for oxygen-based minerals (such as silicates and oxides) is significantly alfected by the partial pressure of oxygen in the system (see section 4.4) and, in the region of intrinsic pressure, by the concentration of point impurities. In this new region, term Qj does not embody the enthalpy of defect formation, but simply the enthalpy of migration of the defect—i.e.,... 

Vitamin D deficiency leads to defects in the structure of bones, a disease known as rickets , common in miners who lived and worked underground during the daytime. They did not have the normal share of sunlight, but it took some time to realize that the lack of vitamin D was related to a photochemical reaction. This reaction is shown in Figure 5.23 in a simplified form, in order to point out the important electrocyclic process. 

The rare earth elements are different from other elements because the optical transitions between levels of the fn configuration are inherently very sharp-lined and have well-resolved structure characteristic of the local crystal fields around the ion. In minerals, this characteristic provides an excellent probe of the local structure at the atomic level. Examples will be shown from our work of how site selective laser spectroscopy can be used to determine the thermal history of a sample, the point defect equilibria that are important, the presence of coupled ion substitution, the determination of multiple phases, and stoichiometry of the phase. The paper will also emphasize the fact that the usefulness and the interpretation of the rare earth luminescence is complicated by the presence of quenching and disorder in mineral samples. One in fact needs to know a great deal about a sample before the wealth of information contained in the site selective luminescence spectrum can be understood. 

For the weathering of trace minerals from the solid matrix, the dissolution occurs selectively on spots where the mineral is exposed to the surface. These mineral surfaces are usually not smooth, but show dislocations (screw, jump, step dislocations) and point defects (vacant sites, interstitial sites) (Fig. 23 left). Dissolved ions are immediately transported from the surface into solution, so that no gradient can develop. Since the total concentrations of trace minerals in the solution are low, no equilibrium can be reached. In the following the dissolution of trace minerals is called surface-controlled. 

Virtually all minerals contain defects. In addition to point defects (e.g., vacancies that exist in a thermodynamically determined equilibrium number, impurities etc ), macroscopic minerals contain line defects (dislocations), and planar defects such as stacking foults, antiphase boundaries and twins. Intergrown layers of different structure or composition, and polytypic disorder also may be present. 

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