Kyanite structure

Those lines have been previously ascribed to Eg luminescence of Cr " " in an intermediate crystal field site (Tolstoy and Shinfue 1960 Wojtowicz 1991). Nevertheless, several contradictions prevent us from accepting such an interpretation. Excitation spectra of two types of enussion lines are very similar to those pubhshed earlier and determined as a and b environments (Platonov et al. 1998). Crystal field parameters calculated based on their excitation and polarized absorption spectra gave Dq = 1720 cm and B = 730 cm for a and Dq= 1600 cm and B = 570 cm for b environments. As was already mentioned, it was concluded that these great differences in the crystal field parameters cannot be explained by a distribution of Cr between two or more of the four crystallographicaUy different octahedral sites in the kyanite structure. The presence of a corundum precursor in kyanite was confirmed by our experiments. Nevertheless, those lines have long decay times typical for Cr in strong crystal field. 

Kossel model, 34 208, 223 low-energy Kossel structure, 34 223 Kyanite, 33 255... 

If we now recall the phase rule, it is evident that, at the P-T conditions represented by point D in figure 2.5, slight variations in the P or T values will not induce any change in the structural state of the phase (there are one phase and one component the variance is 2). At point A in the same figure, any change in one of the two intensive variables will induce a phase transition. To maintain the coexistence of kyanite and andalusite, a dP increment consistent with the slope of the univariant curve (there are two phases and one component the variance... 

Sillimanite is polymorph of kyanite. It has an orthorhombic symmetry with a space group Pbnm. The structure contains edge-sharing, strictly centrosym-metric octahedra (Al sites) arranged in straight chains parallel to [001] and... 

In aluminosilicates, too, including epidote and the Al2SiOs polymorphs andalusite, kyanite and sillimanite, Al3+ ions occupy several coordination sites. Again, trivalent cations such as Cr3+, V3+, Mn3+ and Fe3+ may show cation ordering and be relatively enriched in one specific Al3+ site in a crystal structure. 

Beran A. and Gdtzinger M. A. (1987) The quantitative IR spectroscopic determination of structural OH groups in kyanites. Mineral. Petrol. 36, 41-49. 

Fig. 1. Geological map of the Isua Greenstone Belt, showing the five structural domains identified in this study (I-V) and the location of the samples examined in this paper. Also shown are localities where kyanite has been recorded and the sites where biogenic graphic is reported (M, Mojzsis et al. 1996 R, Rosing 1999). The inset map shows the location of Isua in West Greenland.

Wilkins and Sabine (1973) used IR spectroscopy to determine water contents of kyanite, andalusite, sillimanite, grossular, andradite, pyrope, diopside, olivine, and feldspars. They found low water contents 0.008 wt.% in olivine, 0.02 wt.% in diopside, and 0.009 wt.% in pyrope. Zemann, Beran, and co-workers published a series of papers on IR spectroscopy of both hydrous minerals and NAMs (e.g., Tillmanns and Zemann, 1965 Beran and Zemann, 1971, 1986 Beran, 1971, 1986, 1987 Beran and Gotzinger, 1987 Beran et al., 1993). For the most part, these contributions were focused on the substitutional mechanisms by which hydrogen entered the crystal structure, rather than on the absolute amount of hydrogen in the crystal structure. 

The three sillimanite minerals are structurally similar and have structures that are related to that of mullite. It is not surprising that they all form mullite upon decomposition. Kyanite crystallizes in the triclinic system, while sillimanite, andalusite, as well as mullite have orthorhombic crystal structures. In these structures, all the Si4+ cations are in fourfold coordination with 02 anions, but the Al3+ cations exist in four-, five-, and sixfold coordination with 02 anions, and therein lie the structural differences. The fivefold coordination of some Al3+ cations within A105 polyhedra is rather unusual, perhaps the result of formation at high pressures. The other structural differences among the three minerals are quite small. They are associated with the double chain structures of these three minerals and the linkages of the chains to one another by different alumina and silica polyhedra. Those concepts are readily extended to mullite. 

The structures and properties of the three sillimanite minerals (andalusite, kyanite, and sillimanite) have been discussed. The uniqueness of the three, perhaps the result of their high-temperature, high-pressure genesis is described. Their decompositions at 1 atm pressure are also described. The availability and the properties of these minerals have served them well in industrial applications in the past. It appears that their worldwide source locations provide them with numerous opportunities for future uses in a growing market. 

Figure 6 static and MAS (11 kHz) NMR spectra of kyanite, black and green (dark gray in the print version) traces, respectively, and their crystal structure. The bottom spectrum (blue (black in the print version) trace) is a triple-quantum sky-line projection (isotropic dimension) of the Al 3Q/MAS NMR spectrum of kyanite. All spectra were measured at 11.7 T. 

By simulating the extracted quadrupolar profiles, four nonequivalent structure units of kyanite, 241— 44, were characterized by the isotropic chemical shifts 5qs, = 13.0, 4.0, 5.7, and 5.9 ppm, the quadrupolar coupling constant Q c = 10.1, 3.8, 6.4, and 9.2 MHz, and the asymmetry parameter rj = 0.27, 0.85, 0.70, and 0.38, respectively [76]. Kyanite thus represents a system with considerable distribution of quadrupolar coupling constants while the distribution of isotropic chemical shifts is less significant. This situation can lead to a counterintuitive decrease in the dispersion of Al 3Q NMR signals in the indirect 3Q dimension Fj with increasing intensity of the static magnetic field (Fig. 18). 

Fig. 4.106 Crystal structures of kyanite, sillimanite and andalusite showing the various A1 and Si sites...

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