Spinels description

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Our description of meteorite mineralogy starts with the minerals characteristic of the calcium-aluminum-rich inclusions (CAIs). The mineralogy of CAIs varies systematically with their composition. The most Al-rich CAIs contain spinel, hibonite, and/or grossite. More rarely, corundum or calcium mono-aluminate is present. As the bulk composition becomes more Si-rich, the melilite solid solution becomes important. With additional Mg and Si in the bulk composition, fassaite and anorthite are present. 

One CAI was reported (Weisberg et al., 2002) in the Bencubbin-hke meteorite Gujba, which contains melihte- -pyroxene- -spinel. No photographs or detailed descriptions have been published CAIs have not (as of this writing) been reported in Bencubbin or Weatherford. 

Structure. This particle of P" -alumina contains a sheet of spinel the spinel is like a sheet of a second phase. As we have seen in Section 7.12, we can also think of the structure of the P" -Al203 as being blocks of spinel that twin on every six 111 oxygen planes. The attractive part of this description is that we can then understand the fault in the structure where the twin occurs after only four 111 oxygen planes. Remember that the twin planes are not actually spinel twin planes because the chemistry is different on the twin plane. 

It should be noted that all these descriptions concern the oxygen-deficient spinel and cannot be adopted to the oxygen stoichiometric spinel. That is, it is not overdone how strongly it is emphasized that the oxygen stoichiometric spinel does not have these three features. As mentioned above, it should be understood for the spinel compounds that the data of spinels, in which oxygen content is measured by the chemical analysis, would be reliable. 

For an example, the reader is referred to the crystallographic description of the spinel structure of MgAl204 given below under the heading Structure Types . 

Figure 2.15. Description of the spinel structure ofLiMn204 in the space group Fd-3m...

Soled [26] proposed the description of y-form, considering OH species as structural elements, and proposed formula with the stoichiometry -Al2.5[ ]o.503.5(OH)o.5. The changes of y-form to 5- and 0- by thermal treatment are than depicted as a particle growth which results from condensation of hydroxyl species and formation of water. This model accounts for a more dehydroxylated character of 8- and 0- aluminas compared to y-. However, in this formulation the ratio between trivalent and divalent cationic sites does not meet the requirement of the spinel structure that must be equal to 2. To make it correct, Burtin et al [27] introduced into the formula the new structural element, which corresponds to oxygen vacancies, noted < >. Oxygen vacancies were believed to a result from removing of the water molecules. The common features of the of y-, 8- and 0-forms of transition aluminas which were found in their structural and chemical properties allow to describe them by the general formula ... 

The above description corresponds to the normal type of spinel where the octahedral interstices are occupied in proportion of 1/2 with the more polarized cations, vdiile the tetrahedral interstices are filled only in 1/8 proportion by the cations with lower polarization ability. 

Figure 3.10 Network description of the four-component (quaternary) system CaO (q-AijOj (A)-MgO (M)-Si02 (S) in the vicinity of the giobai quaternary eutectic P, in which the phase assembly anorthite (AN)-diopside (Di)-spinel (SP)-forsterite (FO) is stabie. The nodes labeled Y, I,),. .. Q, R, U refer to quaternary compositions which are connected by phase boundary lines along which the ternary subsystems (labeled with...

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