Chrysoberyl alexandrite

Farrell, E. F. Newnham, R. E. (1965) Crystal-field spectra of chrysoberyl, alexandrite, peridot and sinhalite. Amer. Mineral., 50, 1972-81. 

Alexandrite, like ruby, contains Cr ions but they are substituted in the lattice of chrysoberyl, BeAl204. The chromium ions occupy two symmetrically non-equivalent positions which would otherwise be occupied by aluminium ions. In this environment the 2 ground state of Cr is broadened, compared with that in ruby, by coupling to vibrations of the crystal lattice. 

Two types of Cr + luminescence centers have been found in steady-state natural alexandrite, characterized by J -lines at approximately 680 and 692 nm, accompanied by very many M-lines of Cr-Cr pairs (Tarashchan 1978). Those centers have been identified as connected with substitutions of AP" in different structural sites. It was found that natural alexandrites with very rare exceptions are characterized by very low CL intensities (Ponahlo 2000). Pulse CL study revealed that the spectrum consists of a relatively broad red band peaking at 685-695 nm, accompanied by narrow lines with the strongest one at 679 nm and the weaker ones at 650,655,664,700,707 and 716 nm. All lines and bands have been ascribed to several Cr + centers (Solomonov et al. 2002). The natural chrysoberyl and alexandrite in our study consisted of six samples. The laser-induced time-resolved technique enables us to detect two different Cr and possibly Mn + and V emission centers (Figs. 4.54-4.55). 

Fig. 4.54. Laser-induced steady-state luminescence spectra of alexandrite (a-c) and chrysoberyl (d) demonstrating different Cr and possibly V centers. Vertical polarisation - straight line, horizontal polarization - dash line...

Alexandrite, the common name for Cr-doped chrysoberyl, is a laser material capable of continuously tunable laser output in the 700-800 nm region. It was established that alexandrite is an intermediate crystal field matrix, thus the non-phonon emitting state is coupled to the 72 relaxed state and behaves as a storage level for the latter. The laser-emitted light is strongly polarized due to its biaxial structure and is characterized by a decay time of 260 ps (Fabeni et al. 1991 Schepler 1984 Suchoki et al. 2002). Two pairs of sharp i -lines are detected connected with Cr " in two different structural positions the first near 680 nm with a decay time of approximately 330 ps is connected with mirror site fluorescence and the second at 690 nm with a much longer decay of approximately 44 ms is connected with inversion symmetry sites (Powell et al. 1985). The group of narrow lines between 640 and 660 nm was connected with an anti-Stokes vibronic sideband of the mirror site fluorescence. 

ALEXANDRITE. A variety of chrysoberyl, originally found in the schists of the Ural Mountains. It absorbs yellow and blue light rays to such an extent dial it appears emerald green by daylight but columbine-red by artificial light. It is used as a gem, and was named in honor of Czar Alexander II of Russia See also Chrysoberyl. 

CHRYSOBERYL. The mineral chrysoberyl. an aluminaie of hery Ilium corresponds to the formula BeALO. , crystallizes in the orthorhombic system with both contact and penetration twins common, often repeated resulting in ro.setted structures. Hardness. 8.5 specific gravity. 3.75 luster vitreous color various shades of green sometimes yellow. A variety which is red by transmitted light is known as alexandrite. Streak colorless transparent to translucent, occasionally opalescent. Chrysoberyl also is known as cymopbane and golden beryl. 

Alexandrite. Alexandrite, which is a colorless chrysoberyl, BeALOj, when pure, has a color change derived from Cr. 

June alexandrite (chrysoberyl) Al2Be04 red/green Cr3+ Crystal field transitions in Cr+ concentrated in non-centrosymmetric distorted six-coordinated site. 

Chrysoberyl, AyJeO which is iso-structural with olivine, exists as the gems alexandrite, (Al,Cr)2Be04, and sinhalite, (Al,Fe)2Be04. Structure refinements of these gem minerals (Farrell et al., 1963), as well as EPR measurements... 

Schmetzer, K., Bank, H. GUbelin, E. (1980) The alexandrite effect in minerals chrysoberyl, garnet, corundum and fluorite. Neues Jahrb. Miner. Abh., 138,147-64. 

Alexandrite is a term that should be applied only to naturally dichroic specimens of the mineral chrysoberyl. This stone appears red in one type of light and green in another. The name is in honor of Crown Prince Alexander... 

A gem closely related to ruby and emerald is alexandrite, named after Alexander II of Russia. This gem is based on the mineral chrysoberyl, a beryllium aluminate with the empirical formula BeO AI2O3 in which approximately 1% of the Al3+ ions are replaced by Cr3+ ions. In the chrysoberyl environment Cr3+ absorbs strongly in the yellow region of the spectrum. Alexandrite has the interesting property of changing colors depend-... 

Alexandrite is a variety of chrysoberyl. It is special because absorption is so different in two directions. When viewed along the different crystallographic axes, its color changes from red to orange-yellow to emerald-green. In... 

Beryllium aluminum silicate, just discussed, is not the only beryllium mineral to occur as precious stones. Chrysoberyl BeAl O with a small amount of iron is also used as a gem. It is usually yeUow, pale green or brown. A transparent variety of chrysoberyl is alexandrite, named after Alexander II, tsar of Russia. It has a green color in daylight but is red in artificial light. 

Alexandrite is the common name for Cr-doped chrysoberyl, a crystal whose unit cell contains four formula tmits BeAl204, forming an orthorhombic structure of... 

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