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Topaz

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Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO4 (F,OH) 2. Topaz crystallizes in the orthorhombic system, and its crystals are mostly prismatic terminated by pyramidal and other faces. It is one of the hardest naturally occurring minerals (Mohs hardness of 8) and is the hardest of any silicate mineral. This hardness combined with its usual transparency and variety of colors means that acquired wide use in jewellery as a cut gemstone as well as for intaglios and other gemstone carvings.

Name: From the Greek topazion, meaning to seek, apparently in allusion to the Island of Zabargad (Zabirget or St. Johns), in the Red Sea, Egypt; the location of which was long hidden, known for olivine (\peridot” and \chrysolite”), referred to since antiquity as topaz.

Association: Tourmaline, beryl, microcline, albite, °uorite, cassiterite, zinnwaldite, quartz

Crystallography: Orthorhombic; dipyramidal. In prismatic crystals terminated by pyramids, domes, and basal plane. Often highly modified. Prism faces frequently vertically striated. Usually in crystals but also in crystalline masses; granular, coarse or fine.

Composition: An aluminum fluosilicate, Al2Si0 4 (F,0 H )2

Diagnostic Features: Recognized chiefly by its crystals, its basal cleavage, its hardness (8), and its high specific gravity.

Chemical Properties

Chemical Classification Nesosilicate minerals
Formula Al2SiO4(F,OH)2

Physical Properties

Crystal habit Prismatic crystal
Color Natural colors include: colorless, yellow, orange, brown, red, pink, blue, green. Occurs in a wide range of treated colors, most often blue.x”
Streak Colorless – harder than the streak plate.
Luster Vitreous
Cleavage Perfect basal cleavage.
Diaphaneity Translucent to transparent.
Mohs Hardness 8
Crystal System Orthorhombic
Tenacity Brittle
Density 3.4 to 3.6
Fracture Irregular/Uneven, Sub-Conchoidal

Optical Properties

Color / Pleochroism Weak X= yellow Y= yellow, violet, reddish Z= violet, bluish, yellow, pink
2V: Measured: 48° to 68°, Calculated: 58° to 68°
RI values: nα = 1.606 – 1.629 nβ = 1.609 – 1.631 nγ = 1.616 – 1.638
Optic Sign Biaxial (+)
Birefringence δ = 0.010
Relief Moderate
Dispersion: noticable r > v

Occurrence

Topaz is commonly associated with silicic igneous rocks of the granite and rhyolite type. It typically crystallizes in granitic pegmatites or in vapor cavities in rhyolite lava flows including those at Topaz Mountain in western Utah and Chivinar in South America. It can be found with fluorite and cassiterite in various areas including the Ural and Ilmen mountains of Russia, in Afghanistan, Sri Lanka, Czech Republic, Germany, Norway, Pakistan, Italy, Sweden, Japan, Brazil, Mexico; Flinders Island, Australia; Nigeria and the United States.

Uses Area

The name “topaz” and lots of language editions had been used for yellowish gemstones for at the least two thousand years. At that point yellowish gem stones have been known as “topaz” in lots of elements of the world. Many of the earliest gem buyers did no longer recognise that these yellowish stones were virtually exclusive materials.

Then, approximately two hundred years ago, people who traded in gems started out to recognize that those yellowish gemstones might be topaz, quartz, beryl, olivine, sapphire, or one among many different minerals. They additionally learned that topaz passed off in a huge range of colors other than yellow.

If you visited a jewelry store fifty years ago and asked to see topaz, you will possibly be shown gemstones that had been inside the colour range of yellow, orange, and brown. Starting within the Nineteen Seventies and Eighties, the most not unusual color that you might be proven commenced to be blue. This blue color was generally produced with the aid of remedies that converted colorless topaz into a greater marketable gemstone.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Dana, J. D. (1864). Manual of Mineralogy… Wiley.
  • Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
  • Mindat.org. (2019): Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
Cite this article as: Geology Science. (2019). Topaz. [online] Available at: http://geologyscience.com/minerals/topaz/ [11th November 2019 ]
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