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The Mineral Topaz
by Aubrey Weese
How Topaz Got its Name
Topaz is a word that has been in use since ancient times. Before the Middle Ages, it was used by Greeks and Romans to describe a green gemstone the color of 'fresh oil' or 'the juice of leeks'. These gemstones were probably actually the mineral peridot or olivine. The gems were found on a legendary island called Topazios, which is probably present day Zabargad in the Red Sea. The island was far away from land and often so covered in fog that sailors had difficulty finding it. This supports the speculation that the name topaz may have come from topazin, the troglodyte (cave dweller) word meaning to search for something, or from topazein, meaning to conjecture. It could also have come from the Greek words topos (meaning land or country) and azos (meaning tree branch). The words signify a desert island when put together.
In the Middle Ages the name topaz changed from describing a green mineral to describing one that was yellow or gold. It is uncertain why this change came about but it might have been due to a typographical or translation error of the ancient documents. The study of minerals continued under this new classification and in 1747 Henckel is attributed with first assigning the name to the mineral we know as topaz today, without including in other minerals in the term.
Real topaz actually is not found on the on the island of Zabargad today, and it's very unlikely it would have been there in ancient times either. Zabargad is made up of mafic and ultramafic rocks, and topaz is found only in granitic rocks. Ancient people would have seen the mineral while they were mining for tin, however, because it is usually found along with tin ore. It is most common in its colorless form and often was confused with quartz. The kind of granitic rocks that contain tin ore and topaz are called "A type" granite. "A" is short for anorgenic and means the granite is not related to mountain building processes such as subduction. It is formed under high temperatures and is high in Fluorine and low in water. Topaz would be found in this granite along with minerals such cassiterite (tin oxide), muscovite and quartz.
Bibliography
Hoover, D. B. Topaz. Butterworth-Heinemann Gem Books: Jordan Hill, Oxford. 1992.
Pabian, Roger K. "November Birthstone - Topaz." Nebraska State Geological Survey. http://csd.unl.edu/csd/birthstones/topaz/topaz.html. August 4, 2000.
"The Mineral Topaz." Amethyst Galleries, Inc. http://www.galleries.com/minerals/silicate/topaz/topaz.htm. 1996.
Importance of Topaz
The most important use of Topaz is as a gemstone. It is one of the most popular and widely used gemstones in jewelry, and has been used that way for centuries. The mineral is almost always found in the form of large, well developed crystals. There are specimens of topaz that have been found weighing up to several hundred pounds. Gems have been cut from huge specimens like this that are several thousands of carats, such as a 144,000 carat golden brown topaz shown at the 1974 National Gem and Mineral show in Lincoln, Nebraska; and a 36,853 carat champagne topaz carved in 1989. Most of these large topaz deposits are found in Brazil.
Topaz is one the hardest minerals in existence, and is the hardest silicate mineral, with a hardness of eight. But this feature is undermined by the fact that is has perfect basal cleavage in one direction. This makes it difficult to cut and polish, and carvings made out of topaz are very rare because of it.
It is still valuable as a gemstone, however, because of it's high luster, good crystal form with many facets, and good color. It comes in many colors ranging from colorless to red, pink, yellow, orange, brown, and pale blue. Its colorless variety is fairly common, and can be cut to look like diamond, or heat-irradiated to turn it into blue topaz, which has become a very popular, less expensive substitute for aquamarine. The orange-yellow variety most characteristic of the mineral is called "imperial topaz," and is the most valuable form. Prices for imperial topaz range from $50-$400 per carat. Prices for blue topaz are around $0.75-$10.00 per carat.
Bibliography
Friedman, Hershel. (1999) The Gemstone Topaz. The Mineral and Gemstone Kingdom, http://www.minerals.net/gemstone/gemstone/topaz/topaz.htm.
Helper, Mark. (1998) Geo 347k: Gems & Gem Minerals - Topaz. Department of Geological Studies - The University of Texas at Austin, http://www.geo.utexas.edu/courses/347k/redesign/gem_notes/Topaz/topaz_three_frames.htm.
Hoover, D.B. (1992) Topaz. Butterworth-Heinemann Ltd. Jordan Hill, Oxford.
Pabian, Roger K. (2000) November Birthstone - Topaz. Conservation and Survey Division - Nebraska State Geological Survey, http://csd.unl.edu/csd/birthstones/topaz/topaz.html.
Physical and Optical Properties of Topaz
Topaz is an orthorhombic mineral. This means that within its crystal lattice, each unit cell is shaped like a rectangle. In crystallography the sides of the unit cell are named a, b and c, with a being the side that comes out towards you, b being the side that goes to right, and c being the side that goes up. In a cube a = b = c, but in an orthorhombic mineral a does not equal b does not equal c. However, all the edges comes to together at 90° like they do in a cube. In the topaz unit cell, a = 4.65 Angstroms, b = 8.39 Angstroms and c = 8.80 Angstroms.
Because the length, width and height of each unit cell is different, topaz has three refractive indices. A refractive index is a number that represents how much light is slowed down as it passes through a substance. The higher the number, the more the light is slowed down. The refractive index for air is 1, and for water is 1.53. If a substance has more than one refractive index, it means that light will travel through it at different velocities depending on which direction it is traveling in. The refractive indices for topaz are na = 1.606-1.634, nb = 1.609-1.637, and ng = 1.616-1.644. na refers to light traveling along a, nb refers to light traveling along b and ng refers to light traveling along c. As you can see, the refractive index for light traveling along c is highest, because c is the longest edge. The light has to travel through more material going in that direction, so it gets slowed down more.
These refractive indices are given in a range, instead of just as one number, because they vary depending on how much OH is in the topaz. The chemical formula for topaz is Al2[SiO4](OH,F)2. The comma between OH and F means that hydroxyl can be interchanged with fluorine in the lattice and the mineral will still be topaz. The more hydroxyl there is, the closer to the higher end of the range the refractive indices will be.
The birefringence of topaz is 0.008-0.011. Birefringence is simply the difference between the highest refractive index and the lowest refractive index. The birefringence for topaz is pretty low compared to that of other minerals.
Topaz is a biaxial mineral. This means it has two optic axes. An optic axis is a special line that when you cut the mineral exactly perpendicular to it (so that you are looking straight down it when you look at a thin section of the mineral) the two sides of the unit cell in the plane directly below you will be equal, and light will be slowed down the same amount in every direction (all refractive indices will be the same). The angle between these two optic axes varies with different biaxial minerals, and is called 2V. For topaz 2V = 48-68°. The angle is smaller the more OH there is in the mineral.
Topaz is also designated as being biaxial "positive," instead of biaxial negative. Being biaxial positive means that the acute part of the 2V angle is bisected by the Z axis (which is the axis pointing upward, or in the c direction). A biaxial negative mineral would have the acute angle between its optic axes bisected by the X axis (the axis pointing to the right, or in the b direction).
Topaz has a specific gravity of 3.49-3.57. (It is lighter when it has more OH in it.) This range is about average for a mineral. The specific gravity of water is 1. The specific gravity of quartz (an average mineral) is 2.65. The specific gravity of gold (a very heavy mineral) is 15.6-19.3.
It has a hardness of 8, which makes it one of the hardest minerals that exists. Hardness for minerals is reported on a scale of 1 to 10 with the hardest mineral (diamond) being 10 and the softest (Talc) being 1. Glass has a hardness of 5.5. A mineral can only scratch something less hard than it, and can only be scratched by something more hard than it.
Topaz has perfect cleavage in the 001 plane (which is the plane that is pierced through by c but not by a or b - in other words, the plane perpendicular to c). This means that its bonds have a natural weakness in that plane, and it breaks easily and smoothly there. This kind of cleavage is also called basal cleavage.
Topaz is an orthosilicate, which means it is composed of isolated silica tetrahedra. A silica tetrahedron is a SiO4 molecule that has a silicon atom in the middle surrounded by four oxygen atoms sticking out at 119° to each other, making up the four corners of a pyramid. Silica tetrahedra are the building blocks of all the silicate minerals. In topaz, these tetrahedra don't touch each other directly, but are surrounded by Al atoms which bond to the O atoms in the silica tetrahedra as well as to (OH) or F ions.
Some of the distinguishing features of topaz when compared with other minerals are its hardness, cleavage, low birefringence, positive optic sign and moderate 2V angle. In addition, it shows high relief, which means its edges stand out distinctly from the surroundings when it is viewed under a microscope. It can be pleochroic, which means it changes color when it is rotated under the microscope.
The mineral is also distinguished by the fact that its cleavage trace is parallel to the fast ray. Under the microscope, minerals will have dark straight lines on them going in the direction of their cleavage. These lines are called the cleavage trace. When light enters any mineral, it is split into two rays, and one travels faster than another. The direction of the fast ray is an optical property of a mineral. In topaz, this ray travels parallel to the cleavage trace.
These properties allow topaz to be distinguished from similar minerals like andalusite, melilite, vesuvianite, quartz and feldspars.