Spinel phase relationships

Ric] Richards, R.G., White, J., Phase Relationships of Iron-Oxide-Containing Spinels. Part II. 

Yonemura M et al (2004) Synthesis, strueUrre, and phase relationship in lithium manganese oxide spinel. J Mater Chem 14 1948-1958... 

AIME., 173-178 (1953) (Phase Relations, Phase Diagram, Experimental, Thermodyn., 12) [1954Ric] Richards, R.G., White, J., Phase Relationships of Iron-Oxide-Containing Spinels. Part I. 

Nickel K. G. (1986) Phase equilibria in the system Si02-Mg0-Al203-Ca0-Cr203 (SMACCR) and their bearing on the spinel/garnet Iherzolite relationships. Neus. Jahrb. Mineral. Abh. 155, 259-287. 

To trace the relationship between the valence transition and the evolution of crystal structure, electron diffraction patterns were recorded in situ at different temperatures, as shown in Figure 4.5. The crystal structure is Mn02 (with rutile structure), and no visible change in crystallography is observed up to 400 C. From 400 to 450°C, the crystal structure experiences a rapid change from rutile to spinel, and the final phase at 500°C is identified to be dominated by Mn304, with... 

It provides a sensitive check for pyroxene entry as a crystallizing phase. V-Cr and V-Al substitutional trends provide a good control for the positions of both pyroxene and plagioclase in the crystallization sequence. The positive V-Al sympathetic trend and the negative V-Cr relationship of early spinels (Fig. 5a b) demonstrate the unique chemistry of those early chromites and indicate crystallization before pyroxene and plagioclase. The trends for chromites with the second chemical zoning trend and late ulvbspinel are antipathetic both for V-Cr and V-Al (Fig. 

The chemistry of alumina is more complex. There are several modifications of the defect spinel structure adopted. Al iminium ions reside in both octahedral and tetrahedral sites, and these are in different relationships in these various phases [1,3,7]. This provides a substantial range of potential surface sites, and indeed five hydroxyl types have been classified according to their 0-H stretching frequencies. Some hydroxyl content is present after treatment at temperatures below 1200 C, and the strength and quantity of their acidity is highly dependent upon the pretreatment temperature. Surface oxide or hydroxyl groups can act as electron transfer reagents to an acceptor such as TONE [8]. However, the reactivity of dehydrated aluminas is not primarily due to Br nsted acidity, but to the Lewis acid sites at exposed aluminium ions. 

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