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Lesson 21 - Lesson 21 Pegmatite Gems Topaz and Spodumene...

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Lesson 21: Pegmatite Gems - Topaz and Spodumene Introduction In this Lesson, we'll learn about two other common pegmatite gems: topaz and spodumene. Topaz has been known throughout antiquity, however, it is thought that this word was applied to a variety of gemstones such as peridot, aquamarine, or citrine. Its name likely comes from the island of Zabargad, formerly called Topazos. Interestingly, Zabargad is a classic source for gem quality peridot not topaz. Both the names of spodumene's gem varieties, kunzite (pink) and hiddenite (green), originate from individuals. A.E. Hidden recognized gem quality green spodumene in North Carolina at the end of the 19 th century and G.F. Kunz was a famous American mineralogist who was lucky enough to have a gem named after him! Essential readings and topics for this lesson in your textbook Topic Pages Topaz 306 - 307 Spodumene 273 What is Topaz and What are its Basic Qualities? True topaz is an aluminosilicate mineral containing fluorine (F); often, appreciable hydroxyl groups (OH) - replace F. It is part of the orthorhombic crystal system and usually forms prismatic crystals with an eight sided cross-section (similar in shape to a lozenge) that are terminated in a wedge-like fashion. Striations are common along the length of the crystal. A perfect basal cleavage makes this mineral difficult to work with in jewellery, however, it has a good hardness of 8, placing it
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above quartz and tourmaline but below corundum on the Mohs scale. It is fairly dense with a SG of ~3.5. Topaz crystals can reach considerable sizes and single crystals up to 10 m long and 3 m across have been found, weighing up to ~350 tonnes! Of course, these crystals would not be of gem quality. Gemmy ones have been found up to several hundred kilograms. These make for particularly large cut stones, often in the thousands of carats! The largest cut topaz is from Ouro Preto, Brazil, weighs 22,892.5 carats and is hosted in the Smithsonian Institution National Museum of Natural History. Although from Brazil, it boasts the name "American Golden Topaz". The American Golden Topaz. Photo courtesy of Smithsonian Institution National Museum of Natural History . What is its Chemistry and Crystal Structure? Topaz's chemical formula is Al 2 SiO 4 (F, OH) 2 . The Si in the structure is tetrahedrally coordinated (has four oxygen anions around it) and the Al is octahedrally coordinated (has six anions around it). What is unusual is that the anions of the Al octahedron is a mix of four oxygen and two fluorine atoms. Limited chemical variation is seen in topaz but some Cr has been noted in pink samples.
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Crystal structure of topaz looking just oblique down the c axis. The red tetrahedron are silicon surrounded by four oxygen, the green octahedron are aluminum surrounded by four oxygen atoms (red) and two fluorine atoms (black).
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