Silicate absorption band

Calculation was based on the absorbance of the 1969cm-1 band of the 3-aminotriazolium cation using montmorillonite saturated with the cation as the standard. Film weights were standardized either by weight or from the absorbance of a silicate absorption band at 800cm-1. 

The third oxide used for physical decolorizing is neodymium oxide. Its absorption curve closely compliments an average mixture of ferrous and ferric oxides especially with the strong absorption band at 589 nm. Neodymium oxide is also stable against any state of oxidation change in the furnace. Neodymium is exceptionally good as a decolorizer for potassium silicate and lead glasses. If the redox balance is not quite correct for the... 

The activity data confirm that an IR absorption band at 960 cm" is a necessary condition for titanium silicates to be active for the selective oxidation of hydrocarbons with aqueous H2O2 as suggested by Huybrechts et al. (9). However, this band is not a sufficient condition for predicting the activity of the TS-1 catalyst. Although TS-l(B) and TS-l(C) show intensities for the 960 cm- band similar to TS-1 (A), their activities are different First of all, the reaction data reveal that TS-1 (A) is much more active than TS-l(B) for phenol hydroxylation, while both samples show similar activity for n-octane oxidation and 1-hexene epoxidation. Therefore, the presence of the IR band at 960 cm-i in TS-1 catalysts may correlate with the activities for the oxidation of n-octane and the epoxidation of 1-hexene but not for phenol hydroxylation. However, note that the amorphous Ti02-Si02 also has an IR absorption band at 960 cm- and it does not activate either substrate. 

The D- and P-class asteroids dominate the outer main belt and Trojan asteroids located in Jupiter s orbit. With only a few exceptions, the spectra of these asteroids show no 3 pm absorption bands (Jones et al., 1990). The D and P asteroids are thought to contain ice that has never been melted. However, it is also possible that D and P asteroids could contain hydrated silicates, and that the 3 pm feature is masked by an increasing abundance of elemental carbon with heliocentric distance. The unique carbonaceous chondrite Tagish Lake has a reflectance spectrum quite similar to D-class asteroids, and it has been hypothesized to be a sample of this class. However, Tagish Lake shows a significant 3 pm absorption. 

Fig. 4 shows the IR spectra of various Ti-mesoporous materials obtained by the solvent evaporation method. These samples were synthesized using a C22TMaC1 surfactant and methanol solvent. The IR spectra of Ti containing MCM-41 exhibited an absorption band near 970 cm"1, which was also found for the purely siliceous MCM-41 samples in the Fig. 4. 

For pure Si-MCM-41. this band has been assigned to the Si-O stretching vibrations and the presence of this band in the pure siliceous is due to the great amount of silanol groups present. A characteristic absorption band at about 970 cm 1 has been observed in all the framework IR spectra of titanium-silicalites. It was also reported that the intensity of 970 cm 1 band increased as a function of titanium in the lattice[17] and this absorption band is attributed to an asymmetric stretching mode of tetrahetral Si-O-Ti linkages [18] in the zeolitic framework. The increase in intensity of this peak with the Ti content has been taken as a proof of incorporation of titanium into the framework. 

The formation of a peroxo complex between H202 and a titanium silicates has been demonstrated in several ways, the most convincing being the appearance of an absorption band in the UV-visible spectra at 26,000 cm 1 when H202 is added to a titanium silicate. A band at the same frequency is present in the UV-visible spectra of the peroxo complex [TiF5(02)]3, and the absorption has been attributed to a charge-transfer process 02 - Ti4+ (Geobaldo et al., 1992). The stability of these complexes is limited to a temperature of 333 K they decompose rapidly at 373 K (Huybrechts et al., 1991). The thermal stability of the peroxo complex formed on TS-1 is markedly increased in the presence of... 

Figure 12. Molecular orbital diagram for an FegOjg cluster used to understand the orbitals involved in Fe Fe3 charge transfer. The absorption band observed near 13,000 cm 1 in the spectra of mixed-valence silicates is due to the transition from the Fe2 (t2g)- Fe3+(t2g) orbitals. A transition state calculation for that energy in the cluster presented here gives 10,570 cm"1 in fair agreement with experiment.

Chlorites have been studied spectroscopically mainly on account of Fe2+- Fe3+ IVCT bands near 14,300 cm-1 that contribute to their optical spectra (e.g., White and Keester, 1966 Faye, 1968b Smith and Strens, 1976 Smith, 1977). Two other bands centred near 11,500 cm-1 and 9,500 cm-1 provide estimates for the crystal field parameters of Fe2+ ions in chlorite of A0 = 11,200 cm-1 and CFSE = 4,300 cm-1. Crystal spectra of Cr3+-bearing chlorite, kammererite, yield absorption bands at 18,450 cm-1 and 25,000 cm-1, giving A0 = 18,450 cm-1 and a CFSE of 22,140 cm-1 for octahedrally coordinated Cr3+ ions surrounded by OH- ions in the brucite sheets. The spectra of other Cr3+-bearing clay silicates have been described (Calas et al., 1984), including clinochlore and stichtite. 

Chloritoid. Optical spectra of chloritoids, again studied mainly on account of the Fe2+ —> Fe3+ IVCT band at 16,300 cm-1 (Faye et al., 1968 H lenius et al., 1981), also contain features assignable to CF transitions in Fe2+ ions. These cations are located in the M1B positions in brucite layers which are surrounded by four OH" ions and two trans- non-bridging oxygens belonging to isolated [Si04] tetrahedra in silicate layers in the chloritoid structure. The two absorption bands at 10,900 cm-1 and 8,000 cm-1 yield approximate values of A0 = 9,000 cm-1 and CFSE = 4,050 cm-1, respectively, for the Fe2+ ions. 

The electronic spectra of a variety of transition metal-bearing oxide and silicate minerals have been measured at high pressures and/or elevated temperatures. Trends for absorption bands originating from crystal field (CF), metal-metal intervalence charge transfer (IVCT) and oxygen —> metal charge transfer (OMCT) transitions are summarized in tables 9.2,9.3 and 9.4, respectively. 

In earlier chapters, allusions were made to die effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites ( 3.7.1) patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations ( 6.8.4) and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations ( 8.3.2). 

Covalency of ligands clearly influences the positions and intensities of absorption bands in crystal field spectra of oxides and silicates, so that it is pertinent to discuss the types of covalent bonds that exist when transition elements are present in mineral structures. In this section, the more qualitative aspects of molecular orbital theory are described. 

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