Smoky quartz

Smoky quartz Vitreous Transparent or translucent Pale/black Often banded... 

Smoke flavoring, 12 48 Smoke generation tests, 19 588 Smoke point, of fats and oils, 10 822 Smoke retardants, molybdenum compounds in, 17 39 Smoke suppression, by ethylene-acrylic elastomers, 10 700 Smoking, age-related macular degeneration and, 17 659 Smoky quartz color, 7 337... 

Another interesting natural example of colour centres lies in the colour of smoky quartz and amethyst. These semi-precious stones are basically crystals of silica, Si02, with some impurities present. In the case of smoky quartz, the silica contains a little aluminium impurity. The substitutes for the Si " in the lattice, and the electrical... 

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. 

The election can be trapped, for example by an interstitial H+, which is converted to an H atom. The IO 1 . is the hole color center which ahsorhs light and gives the color to smoky quartz. Bleaching is the result of thermal eneigy releasing the trapped electron, which then produces the reverse of reaction 1. The amethyst color center in quartz is exacdy like the smoky, except that Fe3+ replaces Al3+. ... 

Quartz. When colorless, quartz is also known as rock crystal irradiation of this produces smoky quartz.. The name citrine is used when quartz is colored by Fe. and irradiation of this can produce purple-colored amethyst under certain circumstances. 

Quartz is the most common mineral in the Earth s crust. It occurs in a wide variety of forms, colors, and lusters, but other properties are generally consistent for all specimens. All quartz has the same basic chemical formula, silicon dioxide (SiC ). It has a hardness of 7, and a distinctive conchoidal fracture. The color and translucency of quartz can be affected by a disruption of the molecular structure, as in smoky quartz, or by the inclusions of tiny amounts of other elements or minerals. In some classifications, quartz is listed with the oxides, but it is most often placed with the silicates. 

Smoky quartz Cairngorm Smoky topaz Morion (black) Gray, brown, black, transparent to opaque Terminated hexagonal prisms rarely massive... 

Rutile (titanium oxide) Rutilated quartz Needle stone Venus hair stone Sagenite Colorless to smoky quartz, often as crystals, with needle-shaped yellow, red, or brown crystals of rutile randomly scattered or in radiating sprays... 

Tourmaline (usually Tourma.lina.ted nuartz Clear to smoky quartz, often crystals, with black or dark green... 

Lepidochrocite (iron oxide) Strawberry auartz Clear to light smoky quartz with red platelets scattered or aligned along crystal axes... 

One of the commonest forms of hole-excess center imparts color to the minerals smoky quartz and amethyst. These minerals are forms of silica, containing aluminium as an impurity. The AP+ substitutes for 81 +, and to preserve charge neutrality equal amounts of H+ are incorporated into the crystal. The smoky purple color arises in the electron deficient [A104] group. It is formed when an electron is liberated from a neutral [A104] group by ionising radiation is trapped on one of the H+ ions present. Other hole centers have been described in a variety of crystals. 

Figure 106 An amethyst scepter (Brandberg, South Africa) (a) and an example of parallel growth in smoky quartz (Galileia, Minas Gerais, Brazil) (b).

The two forms on the left are formed by beta quartz, but are also formed, although not commonly, by alpha quartz. Quartz forms a large number of colored varieties, including rose quartz, amethyst, smoky quartz, citrine (Ught-yellow), and milky quartz. Inclusions such as rutile and hematite are fairly common. It also exists in a large number of varieties that exhibit no macroscopic crystalline structure. These include chalcedony, agate, flint, chert, and jasper. 

Smoky Quartz, Japan Law Twin (Lincoln Co., New Mexico)... 

Nassau, K. and Prescott, B. E., A reinterpretation of smoky quartz, Phys. Status. Solidi A 29, 659 (1975). Nelson, C. M. and Weeks, R. A., Vaccuum-ultraviolet absorption studies of irradiated silica and quartz, J. Appl. Phys. 32,883(1961). 

Smoky quartz and amethyst are two natural examples of color centers. 

In smoky quartz a small amount of A1 impurity atoms can substitute for Si. 

Defects produced by X-ray or neutron bombardment. Irradiation of colorless quartz crystals with X-rays will result in the formation of a dark brownish gray to black color of smoky quartz. Coloration is due to the formation of a color center, a hole color center this time. If the smoky quartz is heated to several hundred °C it will become clear again. 

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