Topaz structure

A chain like structure of connected irregular octahedrons controls topaz structure. These octahedrons have A1 in the middle surrounded by four O atoms. Above and below the A1 are the (OH) or F ions. The chains of octahedrons are held together by individual Si tetrahedrons. The crystal system is orthorhombic (21 m21 m21 m). 

AP+ (0.675 A). Thus from ciystallochemical positions the presence of Mn " in topaz structure is quite possible. Strong absorption bands due to Mn " corresponding to the spin-allowed transitions to levels in the visible to near-UV region generate the yellow color of the phosphors, which corresponds to the yellow color of topaz. Thus we propose that a connection may be considered between the long-lived lines at 697 and 711 nm and Mn + centers, which may be formed as a result of natural irradiation of topaz. 

Steady-state luminescence of topaz from Ouro Preto, Brazil has been studied in (Tarashchan 1978 Tarashchan et al. 2005 Taran et al. 2003). It was established that all photoluminescence characteristics of variously colored topazes are due to three structurally non-equivalent Cr " centers isomorphically substituting Al " in the topaz structure and forming [Cr04F2] , [Cr040H,Fn , and... 

J. Ladell, Refinement of the topaz structure, Norelco Reporter 12, 34-39 (1965). 

The elucidation of the factors determining the relative stability of alternative crystalline structures of a substance would be of the greatest significance in the development of the theory of the solid state. Why, for example, do some of the alkali halides crystallize with the sodium chloride structure and some with the cesium chloride structure Why does titanium dioxide under different conditions assume the different structures of rutile, brookite and anatase Why does aluminum fluosilicate, AljSiCV F2, crystallize with the structure of topaz and not with some other structure These questions are answered formally by the statement that in each case the structure with the minimum free energy is stable. This answer, however, is not satisfying what is desired in our atomistic and quantum theoretical era is the explanation of this minimum free energy in terms of atoms or ions and their properties. 

The method was then applied in predicting the structure of the orthorhombic crystal topaz, AljSiC>4F. l It was assumed that each aluminum ion is surrounded by four oxygen ions and two fluorine ions at the corners 10 Linus Pauling, Proc. Nat. Acad. Sd., 14, 603 (1928). 

The Principles Determining the Structure of Complex Ionic Crystals.—The success of the coordination method in predicting structures for brookite and topaz has led to the proposal of a set of principles governing the structure of a rather extensive class of complex ionic crystals. 

Fig. 4.—The structure of topaz. The layers are to be superposed in the order abed, with d uppermost. The crosses are the traces of the corners of the unit of structure in the plane of the paper. Large circles represent oxygen, large double circles fluorine, small open circles aluminum, and small solid circles silicon ions.

By making use of the coordination theory of ionic crystals, I have predicted a structure for topaz which agrees satisfactorily with the experimental data. This structure is described in the following paragraphs. 

The further discussion of the structure of topaz will be published in the Zeitschriftfur Kristallographie. 

Bragg and West report the dimensions of the unit of structure of topaz from their measurements to be [Pg.504]

A variety of minerals are prized for their exquisite beauty, rarity, and exceptional durability. These extraordinary materials are classified as gemstones. One such mineral, silica, with a chemical composition of SK>2 (silicon dioxide), exhibits several crystal structures. Several gemstones are crystalline forms of silica, including amethyst, aquamarine, emerald, garnet, peridot, topaz, tourmaline, and zircon J l... 

Color centers in topaz can be produced by irradiation with fast neutrons and gamma-rays (Platonov et al. 1989). It was found that luminescence bands are connected with such color centers, which may be detected by steady state and time-resolved spectroscopy (Marques et al. 2000). The detected luminescence bands have been connected with both impurities and structural changes. 

Fluoride and hydroxide ions are saturated by bondB of total strength 1. This is achieved by two aluminum octahedral bonds, as in hydrar-gillite (Al(OH)t), with the structure shown in Figure 13-17, topaz (AljSiChF ), jmnyite, described below, and many other crystals, and also by three magnesium octahedra in brucite, Mg(OH)t, and other crystals. 

SiC>4 tetrahedra around the divalent cation produces an octahedral configuration (see Section 10.3.1). Forsterite has oxide ions hep (P layers), one-eighth of the T sites occupied by Si, and half of the O sites occupied by Mg. Topaz, [A1(F, OH)]2SiC>4 (Section 10.3.2) contains SiC>4 tetrahedra and A1(0, F)6 octahedra. Orthosilicates (SiO ) are the most compact of the silicate structures. 

Access to information was not enough by itself, however. The gathered facts also had to be put together in the right way to form an answer. In 1928, Pauling applied himself to solving the complicated structures of an important family of minerals called the silicates, which include topaz, talc, and mica. He knew it was likely that these minerals... 

Hampar, M. S. (1971). Thermal and electron beam induced breakdown of topaz. In Electron Microscopy and Structure of Materials, edited by Gareth Thomas, pp. 1256-66. Berkeley and Los Angeles Univ. of California Press. 

The vAl-O-Al IR band was used to study the effects of the caustic ratio on the structure of sodium aluminate solutions.118 Variable-temperature Raman spectra of the spinel MgAl204 showed a band at 727 cm" 1 for samples annealed above 800°C. This was ascribed to cation disorder in these samples.119 The angular force constant method was used to analyse vibrational modes for MA1204, where M = Mg or Zn.120 Ab initio calculations have been made of vibrational wavenumbers involving the OH group in topaz, Al2Si04(0H)2.121... 

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