Structure beryl

Emerald, Cr " doped beryl, has a beryl structure with the Cr " impurity ions in highly distorted octahedron sites. The discovery of lasing action in emerald stimulated investigation of its luminescence properties. It was established that its tuning range is approximately 730-810 nm, while luminescence consists of a narrow line at 684 nm and a band peaking at 715 nm with similar decay times of 62 ps. The relative intensities of those line and band are different in a- and 7T-polarized spectra (Fabeni et al. 1991). 

A plan of the beryl structure is shown in fig. 11.05. F°r clarity the Si04 groups are here shown as solid tetrahedra, and only the beryllium and aluminium ions are separately indicated. The Si6018 rings are clearly revealed and it will be seen that these rings are bound together... 

Vorma, A., Th. G. Sahama, and I. Haapala, 1965. Alkali position in the beryl structure. Bull. Comm. Geol. Finlande 218 119. 

On silicon carbide, it is easier to see and measure step heights than in crystals like beryl, because SiC has polytypes, first discovered by the German crystallog-rapher Baumhauer (1912). The crystal structure is built up of a succession of close-packed layers of identical structure, but stacked on top of each other in alternative ways (Figure 3.24). The simplest kind of SiC simply repeats steps ABCABC, etc., and the step height corresponds to three layers only. Many other stacking sequences... 

The discovery of beryllium in 1798 followed an unusual train of events. The mineralogist R.-J. Haiiy had observed the remarkable similarity in external crystalline structure, hardness and density of a beryl from Limoges and an emerald from Peru, and suggested to L.-N. Vauquelin that he should analyse them to see if they were chemically identical. As a result, Vauquelin showed... 

Beryl. 385 Beryllium atomic size, 379 boiling point, 374 bonding capacity, 285 chemistry of, 382 electron configuration. 378 heat of vaporization, 374 ionization energies, 379 occurrence, 384 preparation, 385 properties, 381 structure, 381... 

In Table XVIII are given values of the radius ratio for the salts of beryllium, magnesium and calcium (those of barium and strontium, with the sodium chloride structure, also obviously satisfy the radius ratio criterion). It is seen that all of the sodium chloride type crystals containing eight-shell cations have radius ratios greater than the limit 0.33, and the beryl-... 

The structures of aluminum silicates of divalent metals which are simplest from the coordination standpoint are shown to correspond to the formulas R3 + +Al2Si30i2 and R3++Al2Si60i8, which include the most important minerals of this class, the garnets and beryl. 

Photo 14 Linus Pauling in 1950, showing his still evident enthusiasm for the structures of complex minerals (Chapters 5, 6), in this case possibly beryl. In a typical pose, he holds a specimen of the mineral and stands beside an atomic model. The enthusiasm for minerals continued even though Pauling had by this time largely moved on to studies of biological macromolecules (Part III). 

Photos of four silicate minerals, red beryl, emerald, mica, and asbestos. Silicates take many forms, depending on the detailed structure of the Si—O bonding network. 

A colorless mineral known as corundum (composed of aluminum oxide) is colorless. A red variety of corundum known as ruby, a precious stone, owes its color to impurities of chromium within the crystal structure of corundum. Blue and violet varieties of corundum are classified as sapphires, the blue being the result of iron and titanium impurities, and the violet of vanadium impurities within the corundum crystal structure. Another colorless mineral is beryl (composed of beryllium aluminum silicate) but blue aquamarine, green emerald, and pink morganite, are precious varieties of beryl including different impurities aquamarine includes iron, emerald chromium and vanadium, and morganite manganese. 

Mineral gemstones that have the same basic chemical composition, that is, are composed of the same major elements and differ only in color, are considered as variations of the same mineral species. As gemstones, however, minerals that have the same composition and crystalline structure but exhibit different colors are classified as different gemstones. Beryl, for example, a mineral (composed of beryllium aluminum silicate), includes a pink variety, known by the gemstone name of morganite, and also a well-known green variety, emerald. Table 18 lists and classifies, by composition and color, gemstones that have been appreciated since antiquity. 

From the days of the Egyptians, when emeralds were a particular favorite of kings, beryl has also been a favored gemstone. It was not until the late eighteenth century that Abbe Rene Just Haiiy (1743—1822), the father of crystallography, studied the crystalline structures and densities of emeralds and beryl and determined that they were the same mineral. At about the same time, in 1798, Louis-Nicolas Vauquehn (1763—1829) discovered that both emeralds and beryl were composed of a new element with four protons in its nucleus. The element was named glucina because of its sweet taste. It was not until the nineteenth century that the metal berylhum was extracted from berylhum chloride (BeCy by chemical reactions. Late in the nineteenth century, E Lebeau (dates unknown) separated the metal by the electrolytic process. 

The correct composition of beryl and the emerald was not known until the close of the eighteenth century, when the Abb6 R.-J. Hauy pointed out the remarkable similarity in crystalline structure, hardness, and density of a beryl from Limoges and an emerald from Peru, and N.-L. Vauquelin discovered that they both contain as an essential constituent glucinum, or beryllium, and that the emerald, except for the presence in it of a little chromium, has the same composition as the beryl (25,27,91). 

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. 

The mineral beryls, cordierite, and tourmaline, collectively designated as cyclosilicates, have large open channels in their crystal structures. These channels are large enough to accommodate atoms and molecules extraneous to the structure, and these minerals are well known as hosts of such extraneous elements as noble gases. Beryl and tourmaline are characteristic minerals of pegmatites, and cordierite is a common metamorphic mineral. 

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