Chemical Formula: BeAl2O4
Familly: Oxides and hydroxides
Status: IMA-GP
Crystal System: Orthorhombic
Mineral for Display: No
Associated names (luminescent varieties, discredited names, synonymes etc.): alexandrite,UV Type | Main color | Intensity | Observation Frequency |
---|---|---|---|
Long Waves (365nm): | Red | Very weak | Mid waves (320 nm): | Red | Short Waves (254 nm): | Red | Very weak |
Other colors SW: | Greenish Yellow , |
Varété alexandrite: fluorescence rouge due au chrome (Cr2+)
Chrysobéryl classique: jaune vert, crème, verdâtre;
Most Common Activator: Cr3+
Other activators: V2+ ,
Peaks in the spectrum (nm):
Cr3+ replacing Al : 678, 680 nm V2+ : band around 720 nm (80nm half-width) (Gaft), VO6 : broad band at 645nm (FWHM: 120nm) TiO6 : 516nm (FWHM: 115nm) (Gaft), band at 540 nm (Gorobets), 689, 698, 703nm (Gaft)
Spectrum: Michael Gaft, Petah Tikva, Israel. Plot: Institute of Mineralogy, University of Vienna, Austria, with permission of the authors.
The Al3+ ions are octahedrally coordinated by the oxygen ions and occur in two not equivalent crystal field sites in the lattice. The Cr3+ ions replace substitutionally the Al3+ ions, 78 % replacing Al3+ ions in one position and the rest going into the second sites. The major features of the spectra at 300 K are the two sharp lines R1m (m for mirror, the first position for Al3+ replacement) and R2m peaking at 680 nm and a broad, structured band peaking at lower energies. The 689.5 and 695.2 nm decay time is much larger than that for the 677.9 and 679.5 nm emission (Suchoki et al. 1987). At certain excitation wavelengths, such as 488nm, the R1i (i for inversion) and R2i peaking at 690.0 and 695.0 nm lines associated with Cri ions in inversion sites (second site) appear (Powell et al. 1985) The Cr3+ luminescence properties in natural chrysoberyl minerals have been studied as a function of the Cr content as well as impurities such as Fe and V. A competition was found between Cr and V for very low Cr concentration with the vanishing of Cr3+ emission from Cr3+ ions located in inversion site. The Fe3+ ions substitute in mirror site efficiently with a strong impact on the Cr3+ lifetime of mirror site (Ollier et al. 2015). Cathodoluminescence: red;
(*)Data are not exhaustive and are limited to the most important localities for fluorescence
http://www.mindat.org/show.php?name=Chrysoberyl
http://webmineral.com/data/Chrysoberyl.shtml
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