The Bruce Dice Mineralogical Museum contains many diﬀerent specimens of beautiful gemstones. This article highlights a specimen of the mineral topaz that is in the museum (ﬁg. 1). This article will explore the location of the mine that this specimen was found in along with the geological setting of the mine, general information about the topaz mineral, and how it has been used throughout history.
Location of Mine
This sample of topaz is from the Xanda Mine in Minas Gerais, Brazil (ﬁg. 2). This mine is considered a prime location to ﬁnd topaz minerals of excellent quality. Other minerals, such as elbaite, lepidolite, and cleavelandite, along with many others, are also mined in this location (Sauer, 1982). Virgem Da Lapa is a part of the Araçuaí Pegmatite District which is located in the Eastern Brazilian Pegmatite Province (EBP). The pegmatites in this region produce a variety of gemstones, including beryls, topazes, and tourmalines, of excellent quality and over 1000 pegmatites in this area have been mined since the 1940’s (Pedrosa-Soares et al., 2009). In 1974, work on the pegmatite that was found in Xanda Mine was started. Some of the tunnels within the mine reached up to 150 meters in length. These tunnels have no apparent direction and many sharp turns. However, it was inside of these sharp turns that gem pockets, which contained large and beautifully formed blue topaz crystals, were found. Although the mine was closed from 1979 to 1982, it reopened in 1983 and remains open to this day (Proctor, 1985).
Topaz is formed in a large variety of geologic settings as a late-stage crystalliza-tion mineral. It can be found in metamorphic rocks and hydrothermal veins as well as magmatic, late-magmatic, and post-magmatic rocks (T. Gauzzi et al., 2019). The Araçuaí District is made up of the geotectonic unit called the Araçuaí orogen (ﬁg. 3). The Araçuaí orogen contains about 90% of the EBP area and stretches from the Sáo Francisco craton’s eastern edge causing it to occupy 400,000 km of land in southeast-ern Brazil. One third of this area is covered by EBP pegmatites along with Cambro-Or-dovician and late Neoproterozoic granitic rocks. These granitic structures include batholiths, stocks, and plutons, all containing gem-bearing pegmatites which were found to crystallize from 630 Ma to 490 Ma. The pegmatites that are found in this re-gion can be classiﬁed as anatectic and residual. Anatectic pegmatites are formed from the partial melting of rocks. Residual pegmatites result from the fractional crystalliza-tion of their parent granites in a ﬂuid-rich silicate melt. These pegmatites are responsi-ble for major mineral deposits of many diﬀerent types of gemstones such as tourma-line, beryl, and topaz. Most of the pegmatites that were found in the Araçuaí District were residual, which explains the abundance of the topaz mineral that is mined there (Pedrosa-Soares et al., 2009).
Topaz, which is a silicate mineral, has the formula Al2SiO4(F,OH)2. It is a rare but extremely well-known mineral that is used as a beautiful gemstone. This silicate mineral has a high hardness of 8 on the Moh’s hardness scale, which is one of its most characterizing physical features and makes it the hardest silicate mineral. Topaz can come in a variety of diﬀerent colors, the most common being colorless, pale yellow, and brown. Some of the rarer colors include pink, orange, red, purple, and blue. It has an orthorhombic crystal system, a prismatic crystal form, and a glassy appearance. It is normally translucent or transparent and has a speciﬁc gravity of 3.4 to 3.8 (King,
Mineral Uses as a Treasure
Topaz has been most commonly used as a gemstone, although it has had vari-ous other uses throughout history. Topaz’s name is derived from an island in the Red Sea called Topazios. Although the island didn’t actually produce topaz, it did produce the gemstone peridot, which can easily be mistaken as topaz. In ancient Greece, topaz was believed to give people strength and to make the wearer invisible. During the Re-naissance period in Europe, it was thought that magic spells could be broken and anger could be eliminated by using topaz. It is a strong belief in India that topaz inﬂu-ences intelligence, beauty, and long life if it is worn above the heart. Imperial topaz, which is rare and valuable because of its unique pink color, was extremely popular in Russia and only the royal family of the Russian czar were allowed to wear it (“Topaz History and Lore”). Ancient Egyptians put topaz in their amulets to prevent injuries. The “Braganza Diamond”, which was mistaken to be a diamond, is the most famous stone of topaz weighing 1680 carats and is currently set in the Portuguese Crown Jewels (“Topaz Facts, Information and Description”). Today, blue topaz is the most popular gemstone to buy because of its low price and beautiful color (ﬁg. 4) (King, n.d.).
King, M. R. Topaz: A Gemstone That Occurs in a Wide Range of Natural and Treated Colors. Geoscience News and Information. Retrieved from https://geology.com/minerals/topaz.shtml
Pedrosa-Soares, Antonio Carlos & Chaves, M & Scholz, Ricardo. (2009). Eastern Brazilian Pegmatite Province. 10.13140/2.1.4499.9688.
Proctor, K. (1985). Gem Pegmatites of Minas Gerais, Brazil: The Tourmalines of the Araçuaí Districts. Gemological Institute of America, Gems & Gemology. Retrieved from https://www.gia.edu/doc/Gem-Pegmatites-of-Minas-Gerais-Brazil.pdf
Sauer, J.R. (1982). Brazil, Paradise of Gemstones. Gemological Institute of America,135 pp. (p. 62). Rocks & Minerals (xxxx) 63 (pp. 45, 389).
Topaz Facts, Information and Description. Bernardine Fine Art Jewelry. http://www.bernardine.com/gemstones/topaz.htm
Topaz History and Lore. Gemological Institute Of America. https://www.gia.edu/topaz-history-lore