Chrysoberyl

Beryllium is found in some 30 mineral species, the most important of which are bertrandite, beryl, chrysoberyl, and phenacite. Aquamarine and emerald are precious forms of beryl. Beryl and bertrandite are the most important commercial sources of the element and its compounds. Most of the metal is now prepared by reducing beryllium fluoride with magnesium metal. Beryllium metal did not become readily available to industry until 1957. 

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. 

The principal minerals of Be arc listed in Table 1, the most abundant being beryl, the only one of commercial significance. Phcnacitc, chrysoberyl, bertrandite and barylitc arc constituents of recently discovered Be-containing deposits future extraction of Be from these ores is currently being considered. The other minerals are not found in sufficient quantities to constitute possible commercial sources of Be. The majority of the ores, including beryl, are complex silicate materials from which it is difficult to extract the metal consequently. Be extractive metallurgy is both complex and expensive. 

Chrysoberyl Metal Oxides Berylium aluminum oxide ... 

The number of oxide type minerals is quite large. Rostov (1956) has identified 160 specific minerals, grouped them into classes (chrysoberyl, spinel, corundum, periclase, etc.), and proposed a classification system. Only a few examples will be discussed here. 

The host crystal of chrysoberyl has a hexagonal-close-packed structure. The space group is orthorhombic Pnma with four molecules per unit cell. The AP ions are octahedrally coordinated by the oxygen ions and occur in two not equivalent crystal field sites in the lattice. The AP" sites lying in the mirror-... 

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. 

Luminescence similar to those in zoisite has been found in the laser-induced time-resolved spectrum of chrysoberyl (Fig. 4.54d). A relatively broad band accompanied by narrow lines at 698,703 and 717 nm with a decay time of 150 ps, which are not connected with Cr emission, may be preUminary ascribed to V luminescence. 

On April 18, 1821, he was elected to membership in the Swedish Academy of Sciences. In the same year he published some analyses of cyanite from St. Gotthard and Roras and nepheline and sodalite from Vesuvius (13). In 1822 he published analyses of cinnamon stone, chrysoberyl, and boracite (14). He found the cinnamon stone which Berzelius had brought back from Vermland to be a calcium aluminum iron silicate and regarded it as a true garnet like the one from Ceylon which Klaproth had analyzed. 

Arfwedson s analysis of Brazilian chrysoberyl was severely criticized by Thomas Thomson, who said that by some inadvertence, he has taken a compound of glucina and alumina for silica (15). Glucina, or beryllia, had been discovered by N.-L. Vauquelin 24 years before (16). 

Arfwedson fused the chrysoberyl three times with caustic potash in a silver crucible. Since a portion of the melt corresponding to about 18 per cent of the mineral failed to dissolve in hydrochloric acid, he reported this residue as silica. It is now known that beryllium hydroxide, when freshly precipitated, dissolves readily in hydrochloric acid, but becomes after a time almost completely insoluble in it (17). Therefore, it is probable that Arfwedson s silica was really the beryllium hydroxide. He then precipitated the alumina by adding ammonium hydroxide to the acid filtrate. To satisfy himself of the purity of his alumina, he saturated the alkaline solution with hydrochloric acid until the precipitate dissolved, and added a large excess of ammonium carbonate. Had any glucina [beryllia] or yttria existed in the matter, said Arfwedson, it would have been dissolved by this excess of carbonate of ammonia, and would have fallen when the filtered liquid was boiled till the excess of ammonia was driven off but the liquid stood this test without any precipitate appearing. Arfwedson was evidently unable to detect beryllia here because he had already filtered it off and reported it as silica. When American chemist Henry Seybert analyzed the same mineral in 1824 he found it to contain 15 to 16 per cent of beryllia (22). 

Figure 7.1. Various growth twins, (a) Contact twin (albite) (b) inclined twin (quartz) (c) elbow twin (rutile) (d) cyclic twin (chrysoberyl) (e) lamellar twin (albite) ...

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. 

When the term liger s eye is used, this applies only to chrysoberyl. Other gemstones that exhibit this phenomenon include sillimanite. scapolite, cordierite, onhoclase, albite. and beryl. See also Chrysoberyl and Cmcidolite (Bine Asbestos). 

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. 

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