Danburite

Average of reported values for zircon, andalusite, silli-manite, staurolite, topaz, titanite, thortveitite, muscovite, apophyllite, hardystonite, analcite, carnegieite, sodalite, danburite, scapolite, and cristobalite mean deviation 0.02 A. 6 Average for BP04 (1.54 A.) and KHjPO, (1.56 A.). For KjSOi and other sulfates. i For Mg(C104)-6H,0. 

Covalent contribution to the bonding is confirmed by analysis of the components of V2p. The average values of 2, and X2, representing the contraction of the density in the directions perpendicular to the bond (chapter 6), are found to be — 9eA"s, compared with —4eA 5 for the independent-atom model (IAM) density and — 6eA"5 for the ionic model. In addition, the density at the bond critical point ph is l.leA"3, higher than 0.85 eA"3 and 0.68 eA"3 calculated for the IAM and ionic models, respectively. Results for the borosilicate danburite CaB2Si208, are similar, with ph(SiO) 0.95 eA-3, and large positive values of V2p for the Si—O bonds (Downs and Swope 1992). 

Other Zircon ZrSi04, datolite CaBSi04(0Fl), danburite... 

Danburite is calcium borosilicate with an orthorhombic structure (2/m2/ mljm) and space group Pnam. The natural danburite in our study consisted of five samples from a variety of geologic environments. Concentrations of potential luminescence impurities in one sample are presented in Table 4.8. 

Table 4.8. Concentrations of rare-earth elements in danburite and datolite (ppm)...

The luminescence centers Eu ", Yb, Ce, Dy, and Sm + characterize the steady-state spectra of danburite (Gaft et al. 1979 Gaft 1989). By using laser-induced time-resolved spectroscopy we were able to detect the following emission centers Ce, Eu, Eu, Sm ", Dy (Fig. 4.15)... 

Fig. 4.15. a-d Laser-induced time-resolved luminescence spectra of danburite demonstrating Ce, Eu and Eu ... 

The narrow band at 437 nm with a decay time of 650 ns in the danburite luminescence spectrum belongs to Eu " luminescence (Fig. 4.15a), which was also detected by steady-state spectroscopy (Gaft et al. 1979). Besides that, under 308 nm excitation narrow lines appear at 580, 592, 611, 618, 655 and 692 nm (Eig. 4.15c) with a long decay time, which confidently may be ascribed to Eu. Under 266 nm excitation another group of hnes appear with the main line at 575 nm connected with a different type of Eu " (Fig. 4.15d). 

Fig. 5.22. Luminescence and excitation spectra of in danburite and datolite (Gaft and Gorobets 1979)...

CFsli) anhydrite, calcite, feldspars, danburite, datolite, esperite, apophyllite, charoite, fluorite, hardystonite... 

The rule reqiiires that in silicates the silicon tetrahedra share no elements with each other if the oxygen-silicon ratio is equal to or greater than four. This is found t o be vrue in general (topaz, zircon, olivine, other orthosilicates), Most of the few exceptional substances now known contain extra oxygen atoms in the form of hydroxide ions. These include the clay minerals, micas, and chlorites, discussed above, and also the mineral hemimorphite, Zn[Pg.561]

Beran A. (1987) OH groups in nominally anhydrous framework structures an infrared spectroscopic investigation of danburite and labradorite. Phys. Chem. Mineral. 14, 441-445. 

In the following crystals we may regard the Si207 groups as part of more extensive tetrahedral systems in which the larger cations are 8-coordinated (melilite, danburite) ... 

Infrared Spectroscopy. Infrared spectroscopy has been used to study borosilicate molecular sieves (22.25.33-361. Vibrational bands associated with trigonal framework boron occur near 900 cm"1 and 1400 cm 1 (22.25.331. The presence of the Si-0-B asymmetric stretching vibration, indicative of tetrahedral framework boron incorporation, cannot be observed directly because it is masked by the strong Si-0-Si band near 1100 cm-1 in the pentasil structures. The tetrahedral Si-O-B vibration has been observed for the borosilicate mineral danburite (.33). Shifts of bands in the framework vibrational region at 550 cm"1 and 560 cm"1 to higher frequencies as a function of boron content has been used to study boron incorporation in the framework of AMS-1B borosilicate (36). 

A large number of phases (barium silicates and borates, and borosilicates) have been identified from the hydrothermal crystallization of the Ba0-B203-Si02-H2O system at 250 and 415°C. ° Investigations of the Ca0-B203-Si02 H20 system at the same two temperatures showed that cristobalite, quartz, five identified silicates, six identified borates, and the borosilicates danburite and datolite were formed. Besides these at least 10 unidentified, and possibly new, borates were prepared. 

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