Garnets

Narrow lines at 462, 476, 482, 501 and 590 nm in the luminescence spectrum of the Ca-variety of garnet (grossular) with a relatively short decay time are not typical for traditional trivalent REE in minerals. Evidently they may be connected to visible emission of Nd (Fig. 4.57c,d), but this has to be checked. 

Dunai and Roselieb (1996) presented the results of He solution experiments suggesting a very high closure temperature for garnet, 600°C. No data are available to assess the role of composition on He diffusion, but given the diversity of substitutions possible in garnet some compositional effects are likely. In many cases the U and Th in garnet derives from inclusions of radioactive minerals, and this is a potential pitfall for garnet thermochronometry. 

As an alternative, Farley et al. (1996) developed a quantitative model for correcting He ages for the effects of long a stopping distances based on measured grain geometry and size. Several assumptions are required  

1) Implantation from the surrounding matrix is insignificant, so only a ejection need be 

2) The distribution of U and Th in the crystal being dated must be specified. Because the distribution will in general not be known, Farley et al. (1996) presented results assuming a homogeneous distribution and discussed the error introduced by various types of zonation. 

Because U and Th have very similar decay energies, the above expression associates the He from these two parents with a single Ft value. The fraction of He derived from (a23g) can be calculated exactly from Equation (1), or approximated from the measured Th/U ratio for integration periods of less than 200 Myr as  

Due to the centrosymmetric cubic crystalline structure, its optical properties are isotropic. Owing to its high thermal stability, stable chemical properties, and unique homogenous optical properties, transparent YAG ceramics are not only an important high-temperature structural material, but also an excellent host material for 

For practical laser applications, YAG ceramics should be doped with different elements. For example, highly transparent polyciystalline Er Y3Al50i2 (Er YAG) ceramics with concentrations of Er ion from 1 to 90 % were prepared by the SSR and the vacuum sintering technique [153]. In-line transmittances of mirror-polished Er YAG ceramics were all up to 84 % at 1100 nm wavelength, which were attributed to high density of the samples. 

If the (Nd + Y) A1 molar ratio was much lower than 0.6, superfluous AI2O3 was present, while it was much higher than 0.6, superfluous Y2O3 was present which led to intermediate phases (YAP and YAM). The powders prepared within the range of pH value of 7.9-8.2 had (Nd + Y) A1 molar ratio very close to 0.6 (0.598 and 0.603). In this case, no secondary phases were observed and fully dense Nd YAG ceramics with homogeneous microstructures were obtained. The conclusion was further confirmed by the cross-sectional SEM analysis. The fracture styles of samples with (Nd + Y) A1 molar ratios of 0.576 and 0.648 are both intraciystalline, because secondary phases and pores in them enhanced strength of the grain boundaries. In contrast, the samples from the powders with ratios of 0.598 and 

603 possessed pore-free microstructure and clean grain boundaries, thus having intergranular fracture behavior. 

It has been demonstrated that freeze-diying could play an important role in the synthesis of high quahty Nd YAG nanosized powders [169]. Submicronic neodymium-doped YAG (Y3AI5O12) powder was synthesized from freeze-dried precursors. The powder calcined at 1200 °C, with small crystallite size and the lowest amount of organic residues, showed the highest sinterability. With these powder, although color centers were detected by transmission, transparent ceramics were obtained after a vacuum sintering at 1700 °C for 3 h and completed with a HIP treatment at 1700 °C, 160 MPa of Ar for 90 min. 

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Group III with electronic configuration 5s 4d . The principal ore is gadolinite (a silicate also containing lanthanides). Y2O3 containing Eu is used as a red phosphor in colour television. Yttrium iron garnets are used as microwave filters. 

Yttrium oxide also is used to produce yttrium-iron-garnets, which are very effective microwave filters. 

Gadolinium yttrium garnets are used in microwave applications and gadolinium compounds are used as phosphors in color television sets. 

Structure of (a) alizarin garnet R, and (b) its metal-ligand complex with AP+. 

Neodymium and YAG Lasers. The principle of neodymium and YAG lasers is very similar to that of the ruby laser. Neodymium ions (Nd +) are used in place of Cr + and are often distributed in glass rather than in alumina. The light from the neodymium laser has a wavelength of 1060 nm (1.06 xm) it emits in the infrared region of the electromagnetic spectrum. Yttrium (Y) ions in alumina (A) compose a form of the naturally occurring garnet (G), hence the name, YAG laser. Like the ruby laser, the Nd and YAG lasers operate from three- and four-level excited-state processes. 

Laser action can be induced in Nd ions embedded in a suitable solid matrix. Several matrices, including some special glasses, are suitable but one of the most frequently used is yttrium aluminium garnet (Y3AI5O12), which is referred to as YAG. 

Neodymium-doped yttrium aluminum garnet laser (NldiYAG)... 

Naturally occurring abrasives are still an important item of commerce, although synthetic abrasives now fill many of thek former uses. In 1987 about 156 million metric tons of natural abrasives were produced in the United States. Production was up from 1986 because of increased nonabrasive uses and increased use of garnet in sandblasting (4). 

Ca is replaced by a rare-earth element, resulting in a distorted perovskite stmcture, which is essentially orthorhombic. Orthoferrites, studied extensively in the early 1970s as potential data storage materials based on magnetic bubble domains (10), have been largely replaced by the garnet materials (see... 

Deep Bed Filters. Deep bed filtration is fundamentally different from cake filtration both in principle and appHcation. The filter medium (Fig. 4) is a deep bed with pore size much greater than the particles it is meant to remove. No cake should form on the face of the medium. Particles penetrate into the medium where they separate due to gravity settling, diffusion, and inertial forces attachment to the medium is due to molecular and electrostatic forces. Sand is the most common medium and multimedia filters also use garnet and anthracite. The filtration process is cycHc, ie, when the bed is full of sohds and the pressure drop across the bed is excessive, the flow is intermpted and solids are backwashed from the bed, sometimes aided by air scouring or wash jets. 

Synthetic gemstone materials often have multiple uses. Synthetic mby and colodess sapphire are used for watch bearings, unscratchable watch crystals, and bar-code reader windows. Synthetic quartz oscillators are used for precision time-keeping, citizen s band radio (CB) crystals, and filters. Synthetic mby, emerald, and garnets are used for masers and lasers (qv). 

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