Topaz irradiation

Gemstones may be irradiated with neutrons to improve their properties (e.g., change to a more desirable colour) in order to increase their demand and monetary value. The most common neutron irradiation being performed at research reactors is for topaz. Therefore, the following information is pertinent to topaz irradiation. 

Topaz irradiation is a time consuming process with much manpower involvement. The initial preparations will require the services of a radio-chemist and an individual trained in heat transfer calculations and techniques. Even when automated equipment is used for the post irradiation analysis, several technicians will be required for the work. 

From the other side, such a result enables us to consider the possible participation of other elements, such as Mn +, while such valence states are not stable and can appear and disappear as a result of irradiation and thermal treatment. Unfortunately, the sensitivity of Mn to the ICP-MS method is relatively low and it is confidently detected only in the red topaz. However, Mn was detected in several topaz samples, while it is not correlated with Mn, found by EPR and another valence state was supposed (Schott et al. 2003). The ionic radius of Mn of 0.67 A is practically equal to the ionic radius of... 

AP+ (0.675 A). Thus from ciystallochemical positions the presence of Mn " in topaz structure is quite possible. Strong absorption bands due to Mn " corresponding to the spin-allowed transitions to levels in the visible to near-UV region generate the yellow color of the phosphors, which corresponds to the yellow color of topaz. Thus we propose that a connection may be considered between the long-lived lines at 697 and 711 nm and Mn + centers, which may be formed as a result of natural irradiation of topaz. 

Color centers in topaz can be produced by irradiation with fast neutrons and gamma-rays (Platonov et al. 1989). It was found that luminescence bands are connected with such color centers, which may be detected by steady state and time-resolved spectroscopy (Marques et al. 2000). The detected luminescence bands have been connected with both impurities and structural changes. 

Using a combination of IR, UV/VIS reflection and Raman spectroscopies, thermally treated corundum and zircon and radioactively irradiated topaz and spodumen maybe identified (Tretyakova and Tretaykova 1996). 

Some treatments are practiced so widely that untreated material is essentially unknown in the jewelry trade. The heating of pale Fe-containing chalcedony to produce red-brown camelian is one of these. Another example involves turquoise where the treated material is far superior in color stability. Such treatments have traditionally not been disclosed. Almost all blue sapphire on the market has been heat treated, but it is not possible to distinguish whether it was near-colorless corundum containing Fe and Ti before treatment, or whether it had already been blue and was only treated in an attempt at marginal improvement. The irradiation of colorless topaz to produce a blue color more intense than any occurring naturally is, however, self-evident, and treatments used on diamond are always disclosed. 

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. 

With the exception of diamond coloring and the turning of topaz blue, the source of the irradiation is immaterial. Gamma rays are the preferred source because of uniformity of coloration and the absence of heating and induced radioactivity. The most commonly seen gemstones enhanced by irradiation are summarized in Table 4. 

Published spectra are available for various types of topazes, including Cr3+-bearing (Petrov et al., 1977) and irradiated (Aines and Rossman, 1986) sped-... 

Blue. The common blue stones are sapphire, lapis, and irradiated topaz. Less common gemstones include blue diamond, although such stones could be common if irradiated more often. 

Instead of using a reactor or other source of radiation, natural ionizing radiation can excite electrons in the crystal. Fluorite can be red, green, or purple due to natural irradiation. The same irradiation can cause topaz to be brown (see below). 

FIGURE 36.26 Topaz. Examples of irradiated rough, natural Imperial topaz and a huge natural single crystal. 

Topaz or sapphire irradiated to a yellow or brown color No No, only fact of fading Explained... 

Irradiation Topaz is sold as Swiss blue, London blue, etc. In all cases, the material has been irradiated and heated. 

Nevertheless, several research reactors worldwide are irradiating topaz commercially and performing research on irradiation of gemstones, but only three facilities have applied for the license required in the USA. 

The colour in topaz is induced by the interaction of fast neutrons. If thermal neutrons are allowed to irradiate the topaz, then significant undesirable radioactivity will be induced in the gemstone. 

The required fluence is dependent on the specific batch of topaz stones and the depth of the desired blue coloration. The fast neutron fluence is typically of the order of 10 -10 n cm. For a 2 MW research reactor, about 50 to 100 hours of irradiation is required to achieve this fluence. 

If the temperature of the gemstones approaches 300°C the damage to the topaz will anneal and de-coloration will occur. In addition, if the temperature is too high the stones will be prone to flaking during post-irradiation handling. A typical temperature during irradiation should be... 

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