Nesosilicate – Topaz Gemstone
Previous articles discussed the polymorph gemstones andalusite, sillimanite, and kyanite of the Al2SiO5 group of the nesosilicates. Topaz and staurolite are the two remaining minerals of this group used as gems. In the nesosilicate structure of orthorhombic topaz, independent.
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Previous articles discussed the polymorph gemstones andalusite, sillimanite, and kyanite of the Al2SiO5 group of the nesosilicate. Topaz and Staurolite are the two remaining minerals of this group used as gems. In the nesosilicate structure of orthorhombic topaz, independent SiO4 tetrahedra cross-link chains of AlO4Fe2 octahedra parallel to the c axis.
The perfect basal cleavage of the commonly stubby prismatic crystals breaks only the AlO and the AlF bonds, leaving the SiO4 bonds intact. A close packing arrangement of fluorine and oxygen atoms causes its rather high density. Of the well known gems, topaz is the only one with a refractive index range of 1.61 to 1.63 to exhibit a specific gravity range above 3.32.
Topaz occurs most frequently in non-gem granular and columnar forms that bear a resemblance to fat. These non-transparent forms derive their name "pycnite" from the Greek word "puknos" meaning fat. Either the Sanskrit word "tapas" meaning fire or the Red Sea Island known as Topazion in ancient times is the source of the name for crystalline topaz. The usual upward growth habit of the crystalline form of topaz often causes the pyramidal terminations of the frequently vertically striated prisms to be visible only at one end. The crystals develop in a pneumatolytic environment from fluorine-bearing vapors in igneous rocks that contain abundant free silica. Topaz is found in contact zones, in cavities in granite and rhyolite lava rocks, pegmatites, high temperature quartz veins, and as worn pebbles in alluvial deposits. The alluvial deposits in northeast Brazil yield colorless pebbles called "pingos d'agoa" (drops of water).
Gemmy topaz crystals can weigh in the hundreds of pounds. The American Natural Science Museum in New York has a magnificent 300 kilogram translucent specimen from Minas Gerais, Brazil on display. The Mining College Museum in St. Petersburg displays a giant blue specimen recovered from Murzinka in the Urals. The legendary 1,640 metric carat colorless Braganza stone is reputed to have been found in Minas Gerais in 1740. The King of Portugal, believing the crystal was a huge diamond, claimed it for his own. It disappeared after having been worn as a rough suspended gem by King John VI from 1816-1826. In The Illustrated Encyclopedia of Minerals and Rocks, Dr. J. Kourimsky tells us it is the cut and polished 1680 carat topaz now set in the Portuguese crown. The Smithsonian Institution's collection of gemstones contains three magnificent faceted topazes from Brazil: the very large American Golden of 22,892.5 carats, a blue of 3,273 carats, and a 1,469 carat yellow-green gem. The 21,327 carat faceted light blue, treated, emerald-cut Brazilian Princess gem resides in a private collection. Natural pink topaz crystals seldom occur in sizes above a few carats. However, an exception is the 150 kilogram translucent specimen found in Minas Gerais now displayed in the Mineralogical Institute in Florence, Italy.
Before the advent of chemical, mineralogical, and crystallographic techniques were applied to identify Earth's minerals, the name "topaz" was used by our ancestors to designate many golden-hued gem minerals. The olivine found on the Red Sea island of Topazos (St. John's Island, now known as Zebirget) is an example. Today, some jewelers still refer to the yellow andradite garnet as "topazolite". We can be even further confused by the terms "topaz citrine" and "smoky topaz" that have been applied to yellow and brown quartz. Yellow sapphire was frequently called "oriental topaz" in the past, but the term is seldom used now. The name topaz began to be applied to the aluminum silicate containing fluorine and hydroxyl in the early part of the eighteenth century. The wine-yellow crystals from the Saxony region of Germany were the first to be scientifically identified as topaz.
We know now that topaz also occurs in blue, pale green, pink, and colorless crystals. The various tones of pure yellow material are often referred to as honey and golden topaz. The term "precious topaz" is slowly disappearing. "Sherry topaz" is the rich, brownish golden yellow variety. Both the natural and heat-treated brownish red-orange stones are sometimes called "burnt" topaz. A vast range of color gradations exists from these through the pure pinks to those of red and almost violet tones of the darker colored gems. The deep pinkish-orange and reddish-orange toned materials are the prized "imperial topaz." Some Brazilian crystal tips yield the extremely rare red topaz known locally as "Brazilian ruby."
Natural pink topaz is very rare. Katlang, Pakistan is the source of fine rich pink crystals. Most pink topaz is obtained by carefully heating brownish red-yellow chrome-bearing crystals found at Ouro Preto in the Minas Gerais region. Although the color of some natural crystals fades when exposed to sunlight, this heat-induced color-change is permanent. Irradiation and heat-treatment of colorless and greenish-brown crystals can produce smoky grey, cinnamon-brown, yellow-orange, and blue materials. The yellow-orange, which develops in a matter of minutes upon exposure to the process, can closely resemble "imperial topaz". Its color fades rapidly. A longer period of exposure is needed to produce the more slowly fading brown. Greenish-brown crystals exposed to such treatment yield the popular permanent blue colors not found in natural topaz. These are known by such terms as "London blue", "Swiss blue", and "sky blue." In his Color Encyclopedia of Gemstones, Dr. Joel Arem states "no detection test exists for the irradiation treatment". Dr. Kurt Nassau confirms this in his book, Gems Made by Man.
The processes of linear acceleration use neutron bombardment and gamma radiation to effect the color changes in topaz. Since this is a well known practice, the National Regulatory Commission requires that all imported topaz gems and material undergo examination and meet strict safety standards. A strictly monitored facility in Missouri processes irradiated topaz gems produced in the United States. A consultation with Mr. Ray Zajicek of Equatorian Imports in Dallas, Texas and Mr. Moghadam of MP Gem Corporation in Los Angeles, California shed more light on the processes. They agreed that undetectable gamma cobalt 60 radiation is the most commonly used process. Linear acceleration processing may leave faint residual radio-activity, which dissipates within a few days. This method can be detected only with the use of very sophisticated equipment. They also said that the less-used neutron bombardment is the most apt to be discovered.
In spite of its perfect plane of cleavage, topaz can be an excellent choice as a gem for almost all types of jewelry. A superb cut will enhance its dispersion. With the attribute of its hardness of 8 on the Mohs scale and a variety of colors in a wide range of sizes, one can make very desirable additions to a jewelry wardrobe at a very reasonable cost.
Topaz Gemstone Properties
|Composition:||Al 2 SiO 4 (F,OH) 2 + Cr Hydrous aluminum fluorosilicate|
|Group||Al 2 SiO 5|
|Colors:||colorless, yellow, orange, red- brown, pale blue, pale green, pink, and red|
|Habit:||prismatic, granular, massive|
|Cleavage:||indistinct and poor|
|Specific Gravity||3.53 to 3.57|
|Refractive Index||a=1.607 - 1.629; b=1.610 - 1.632; y=1.618 - 1.649|
|Birefringence:||0.008 - 0.010|
|Optic Character||biaxial positive|
|Pleochroism||Distinct Yellow = greenish-yellow/honey-yellow/pale yellow; Brown = yellow-brown/yellow-brown/pale yellow-brown; Red Brown = yellow/reddish/reddish; Red = red/yellow/rose-red; Pink = pale violet/violet/yellow; Pink (treated) = rose/rose/colorless; Pale Blue - distinct blue/pale pink/colorless; Green = colorless/blue-green/distinct green|
|Luminescence||Blue and Colorless = LW/weak yellow-green, SW/weaker; Brown and Pink = LW/orange-yellow, SW/weaker or greenish white|
|Absorption Spectrum||strong line at 6828|
|Aqua Filter||Blue = blue-grey|
|Chelsea Filter||Blue = green; Bright Blue = brownish pink|
|Solubility||not resistant to sulfuric acid|
|Treatments||heat treatment and irradiation|
|Inclusions||planes of small liquid inclusions occupied by gas bubbles; three-phase inclusions are not uncommon|
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