Spinel crystals

Figure 3-3c displays the steady-state concentration profile for a spherical shell. One application of this solution is for a spinel crystal inside a magma chamber, where the spinel contains a large melt inclusion at its core. The diffusion profile in the spinel (which is a spherical shell) in equilibrium with the melt inclusion and the outside melt reservoir would follow Equation 3-3 Ig. 

Figure 1.41 The spinel crystal structure of MgAl204. Reprinted, by permission, from C. Kittel, Introduction to Solid State Physics, p. 447. Copyright 1957 by John Wiley Sons, Inc.

Nakhlites (Fig. 6.11b) are augite-rich basaltic rocks. They are cumulates formed by concentration of augite and minor olivine. They formed on or close to the surface, as indicated by the fine grain size of their groundmass, which consists of needles of feldspars, delicate spinel crystals, and glass. All the nakhlites are —1.3 billion years old, and compositional similarities indicate that they are related. 

Intercalation of cations into a framework of titanium dioxide is a process of wide interest. This is due to the electrochromic properties associated with the process (a clear blue coloration results from the intercalation) and to the system s charge storage capabilities (facilitated by the reversibility of the process) and thus the potential application in rocking-chair batteries. We have studied alkali-metal intercalation and ion diffusion in the Ti02 anatase and spinel crystals by theoretical methods ranging from condensed-phase ab initio to semiempirical computations [65, 66]. Structure relaxation, electron-density distribution, electron transfer, diffusion paths and activation energies of the ion intercalation process were modeled. 

Figure 4.18 The spinel crystal structure adopted by magnetite, Fe2+Fe3+204. Note the three-dimensional infinite chains of edge-shared [FeOe] octahedra extending along [110] directions, which accommode Fe2+ and Fe3+ ions separated by 297 pm. Fe3+ ions also occur in isolated tetrahedra linking the octahedral chains.

One of the unusual features of spinel crystal chemistry is that some transition metals form normal spinels and others inverse. The spinel-types are summarized in table 6.2. The site occupancy patterns were considered to be anomalous until they were explained by crystal field theory (McClure, 1957 Dunitz andOrgel, 1957). 

The octahedral site preference energy parameter listed in table 6.3, applied originally to spinel crystal chemistry, has had a profound influence in transition metal geochemistry following its introduction into earth science literature in 1964 (Bums and Fyfe, 1964 Curtis, 1964). The use of such site preference energies to explain distribution coefficients of transition metal ions in coexisting minerals and phenocryst/melt systems are described in 7.6, 7.8 and 8.5.3. 

Crystal chemistry of spinels. A classic example showing that transition metal ions display distinct site preferences in oxides stems from studies of spinel crystal chemistry. The spinel structure contains tetrahedral and octahedral sites normal and inverse forms exist in which divalent and trivalent ions, respectively, fill the tetrahedral sites. The type of spinel formed by a cation is related to its octahedral site preference energy (OSPE), or difference between crystal field stabilization energies in octahedral and tetrahedral coordinations in an oxide structure. Trivalent and divalent cations with large site preference energies (e.g., Cr3 and Ni2+) tend to form normal and inverse spinels, respectively. The type of spinel adopted by cations with zero CFSE (e.g., Fe3+ and Mn2+) is controlled by the preferences of the second cation in the structure. 

The views of Williams were extended by subsequent writers. Curtis (1964) considered the stability of ions in tetrahedral coordination in minerals as well as in octahedral sites. While this approach is applicable to spinels crystallizing from the magma, it does not apply to silicate minerals in which the transition metal ions occupy only six-coordinated sites. Bums and Fyfe (1964) presented crystal field spectral data indicating that cations such as Ni2+ are present in both octahedral and tetrahedral sites in silicate glasses assumed to approximate... 

Chromia Chromite Cr2Fe04 is the most commonly used chromium-containing mineral for ceramic formulations. This mineral has a spinel crystal structure, where the iron may be replaced by magnesium and aluminum. Chromite is used in ceramics largely as a refractory in the form of burned and chemically bonded bricks. For this purpose, a low-silica material is desired. When low silica is desired, chromic oxide is extracted from chromite by dissolution in add, removal of the iron impiu-ity by liquid—liquid extraction, and precipitation of the hydroxide, which is subsequently calcined to the oxide. Chromic oxide is used as a color additive to azes and enamels and in ferrite production to give magnetic materials. 

Figures 3 and 13 show two spinel-pyroxene-rich CAIs from Mighei (CM) that are typical of the kinds of seen in many chondrite types, including CO, CR, ordinary, and enstatite chondrites. Besides being much smaller than their CV3 kindred, these CM examples typically are deficient in melilite and anorthite. Figure 3 shows what MacPherson et al. (1983) called a nodular spinel-pyroxene inclusion, in which the central spinel region consists of numerous spinel crystals in a dense overgrowth. In contrast, the CAl in Figure 13 is a chain-like inclusion (MacPherson and Davis, 1994) that consists of numerous small linear chains of spinel and even some individual spinel crystals. Each has its own individual pyroxene rim. Although clearly the ensemble is part of a single object, its individual components are separated from one another by up to 50 p,m of intervening meteorite matrix.

The cubic spinel crystal structure (Fd3m) is a close-packed array of oxygen ions, which has the general form AB204. A is a divalent cation and B trivalent [60, 71]. 

In high radiation fields, the spinel crystal structure has been shown to change. The structure, while still cubic, becomes disordered with a reduction in lattice parameter. The disordered rock-salt structure has a smaller unit cell reflecting the more random occupation of the octahedral sites by both trivalent and divalent ions. Increased radiation damage results in the formation of completely amorphous spinels. Radial distribution functions (g(r)) of these amorphous phases have Al-0 and Mg-O radial distances that are different from equivalent crystalline phases. The Al-0 distance in the amorphous form is reduced from Al-O of 0.194nm in the crystalline phase to 0.18nm in the amorphous phase, while the Mg-O distance is increased (0.19nm in the crystal to 0.21 nm in the amorphous phase). Differences between the Al-O distances of crystalline and amorphous phases are a characteristic of both calcium and rare earth aluminates. 

Alumina is a material that has been studied extensively [6,19,20]. Its y-form is characterized as a tetragonally distorted defect spinel lattice, with a unit cell composed of 32 oxygen atoms and 21 A aluminum atoms. There are 2-/i vacant cation positions per unit cell. Among the 2116 aluminum atoms in the unit cell are a significant number of octahedral, as well as tetrahedral, sites. For the y-form, one can consider an idealized surface to be composed of two low-index defect-spinel crystal planes, specifically the (110) and (100) planes [19]. The presence of these planes implies that there is a mixture of octahedral and tetrahedral aluminum sites exposed on the surface. Recent solid-state NMR experiments have observed these sites indirectly [21] and suggest that the surface can also be described by (110) and (111) planes. In general, however, it is best to consider the surface of y-alumina as being composed of the (110), (100), and (111) planes. 

The optimal material for magnetic recording media was, for many years, y-FejOs (maghemite). This form of FejOj, which has a defect-spinel crystal structure, can be prepared as small, elongated particles. The coercive field is produced within the required range by shape anisotropy (see Section 4.5.1). The process used to prepare small particles of... 

Raman studies have been made on natural and a number of synthetic MgAl204 spinel crystals. Deviations in Raman selection rules in the synthetic crystals can be ascribed to disordering in the Mg-Al sites, and can be useful in characterizing such crystals. 

In addition, it is found from the spectra that the magnesium indium spinel crystal system is not likely to reveal new compounds. Further, the channeling measurement indicates that the amount of ion irradiation does not cause amorphization of the lattice surface layers and that the range of investigated doses for the defects of the interstitial hydrogen and lithium type is predominant. 

The presence of a hydrated film (FeOOH) was found to depend on the concentration of iron cations at the passive layer-solution interface [64]. In situ surface X-ray diffraction studies indicated that the film consists of a spinel crystal structure [65]. 

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