General

Chrysoberyl cat's eye

Chrysoberyl cat's eye

Chrysoberyl cat's eye

Chrysoberyl (from the Greek chrysos "gold" and beryl "quartz") is a form of chrysoberyl that contains a significant amount of the more rare yttrium-aluminum-garnet (YAG) crystal form of the mineral beryl. YAG crystals are transparent to translucent, with an orange-red colour (most often a yellowish to straw colour, but may also be orange or brown). It is formed when the YAG beryl is altered by adding a small amount of an alkaline earth metal cation.

This is done by adding an alkali to a saturated solution of beryl in sodium sulfate (saltpeter) for a few hours at. The precipitate (yttrium aluminate) will consist of a mixture of beryl, YAG, and other forms of YAG.

Properties

The light reflected from the crystal is a mixture of green and yellow, the colour of which is determined by the concentration of yttrium and the presence of some aluminum ions.

This form of chrysoberyl is most stable at a pH of about 9, which is not very stable. In general, alkaline earth metals like calcium will shift the stability of the mineral to acidic pH, whereas trivalent rare earth metals like gadolinium will shift the stability to more alkaline pH.

Synthesis

Chrysoberyl is a relatively rare form of chrysoberyl, as it can only be produced when the YAG is changed to a mixture of YAG and other phases. This is done by adding an alkali, in the form of a salt such as NaOH, to a saturated solution of the beryl.

The addition of the salt will cause the beryl to precipitate out of solution, which will then be replaced by a mixture of the beryl and other forms of YAG. This mixture is in turn mixed with an alkaline earth metal salt, such as Ca(OH)2 or Mg(OH)2. This will cause the formation of a solid solution of the beryl, the YAG, and the alkaline earth metal, which will consist of a mixture of yttrium aluminate, beryl, and alkaline earth metal halide. The solidification takes place slowly, as is common with most minerals.

The formation of a solid solution can be detected by the gradual change of the optical color from the beryl to the mixture of YAG and alkaline earth metal halide, as the alkaline earth metal ions will form an inner shell around the YAG.

The final product is then ground up, and the resulting powder sieved into fine sizes.

Formation

In nature, chrysoberyl is formed in the presence of beryl in a process known as diagenesis. Beryl is a mineral that contains a small amount of yttrium, and can also contain other rare earth elements such as samarskite and bastnäsite. In most cases, the YAG is contained in the bastnäsite and is known as bastnasite. It is possible to isolate bastnasite and to purify it to contain no other rare earth elements. The bastnasite will react with a slight excess of alkali to cause the formation of chrysoberyl, with some excess of the beryl being lost in the process. The excess is recovered in a beryl precipitate known as paulingite.

The process is as follows:

Beryl will dissolve in a solution of the salt of an alkaline earth metal, with an excess of beryl being added.

At the beryl saturation point, the beryl will start to precipitate out, causing a slight supersaturation with respect to the YAG.

Yttrium will precipitate out of the solution and be mixed with the beryl, forming the mixture of YAG and beryl.

A small amount of bastnasite (and other beryl phases) will dissolve in the supersaturated mixture.

Alkali is then added to the solution.

The alkali will react with the bastnasite, precipitating a mixture of bastnasite and alkaline earth metal halide.

The precipitated mixture will then slowly precipitate out of solution as a solid solution.

Uses

Optical

Chrysoberyl cat's eye is primarily used as an optical material. It has many of the same characteristics as zircon, and is usually used in the same applications. The main difference between these two materials is that zircon has a tendency to be very brittle, and so is more easily machined into shapes of interest. Chrysoberyl is more stable, with higher transparency, and it is more difficult to form.

Chrysoberyl cat's eye is most often seen as a component of gemstones in jewelry. These are usually in the form of a cabochon, which is polished flat on all sides.

The color is a function of the ratio of yttrium to beryl. This ratio must be in the vicinity of 1:2 to achieve the color that is typical of this mineral.

Some other uses of chrysoberyl are in the production of prisms for use in lasers. Chrysoberyl has a higher refractive index than other materials, such as zircon. The ability to refract light into different directions is extremely valuable in such uses.

Thermoelectric

Chrysoberyl cat's eye has many of the same properties as zircon, which has been used for a long time as a thermoelectric material. The temperature coefficients of electrical resistance of zircon, and chrysoberyl, are similar, being around 5 ppm/°C. This is an important property, as a thermoelectric device must have low temperature coefficients in order to work effectively. A high temperature coefficient will cause an increase in electrical resistance at low temperatures, such as at room temperature, while causing a decrease in resistance at high temperatures. Such


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