Yellow sapphire

The destruction of color centers (1,3) by heating can result in bleaching or fading. Examples are brown or blue topaz, red tourmaline, smoky quartz, and some yellow sapphire. In other instances there may be a color change as when amethyst turns into yellow citrine, or when the heating of a brown topaz reveals the presence of a previously hidden Cr-derived color in a pinked topaz. These changes can usually be reversed by an irradiation treatment. 

Figure 3.16 Energy level diagram for ferric iron matched to spin-forbidden crystal field transitions within Fe3+ ions, which are portrayed by the polarized absorption spectra of yellow sapphire (adapted from Ferguson Fielding, 1972 Sherman, 1985a). Note that the unassigned band at -17,600 cm-1 represents a paired transition within magnetically coupled Fe3+ ions located in adjacent face-shared octahedra in the corundum structure.

The crystal structures of transition metal compounds and minerals have either cubic or lower symmetries. The cations may occur in regular octahedral (or tetrahedral) sites or be present in distorted coordination polyhedra in the crystal structures. When cations are located in low-symmetry coordination environments in non-cubic minerals, different absorption spectrum profiles may result when linearly polarized light is transmitted through single crystals of the anisotropic phases. Such polarization dependence of absorption bands is illustrated by the spectra ofFe2+ in gillespite (fig. 3.3) and of Fe3+in yellow sapphire (fig. 3.16). 

Figure 4.16 Polarized absorption spectra of natural and synthetic sapphires (from Bums and Bums, 1984a). (a) Natural yellow sapphire (b) natural dark blue sapphire (c) synthetic Ti-doped A1203 (d) synthetic Fe-Ti-doped A1203. —Ellc spectra -----EJx...

Representative optical spectra of various sapphires are illustrated in fig. 4.16. It is apparent from the spectra of the natural blue sapphire (fig. 4.16b) that absorption minima in the violet-indigo and blue-green regions, which are located between sharp peaks at 25,680 cm-1 and 22,220 cm-1 and broad bands spanning 17,800 to 14,200 cm-1, are responsible for the blue coloration. Absorption at 17,800 to 14,200 cm-1 is less intense in spectra of natural yellow sapphire (fig. 4.16a see also fig. 3.21) containing negligible Ti. The spectra of synthetic Ti3+-doped A1203 (fig. 4.16c) show absorption maxima at... 

Yellow. Yellow stones are less common but include citrine, yellow sapphire, and yellow diamond. 

Nickel Chrysoprase, synthetic green and yellow sapphires... 

Natural green sapphire Glue layer (colorless) Vemeuil yellow sapphire... 

Corundum [1344-28-1] with the chemical formula Aip, and the relative molar mass of 101.96127 is named after the Hindi, kurund, or the Tamil, kurundam. Corundum with a trigonal space lattice is the second hardest mineral on the Mohs scale just after diamond. The well-known gem varieties are the blood-red ruby while all other colors are named sapphire (e.g., pink sapphire, yellow sapphire) despite it is commonly used to denote the blue... 

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