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Unformatted text preview: Lecture 6 The Silicates: Quartz and other varieties of SiO2 I. May the Quartz be with you As discussed in an earlier lecture, the most abundant elements in the earth's crust are Oxygen and Silicon. It should be no surprise to you that silicates are the largest and most varied group of minerals (and the largest group of gem minerals as well). All silicates are based on the silicon-oxygen tetrahedron . The Si-O tetrahedron (a silcon ion tetrahedrally coordinated with four oxygen ions = SiO4 ) is the building block for all silicates. Most other silicates have other ions in their structure, but quartz is just SiO2. Why Si02 and not Si04? Because to be charge-balanced, not all the tetrahedrons are complete - thus it is SiO2. In its pure form, quartz is colorless. Impurities of other ions create the many shades that we associate with quartz. 1. Basic Data: Chemical Formula: SiO2 Mohs' hardness 7 Crystal System Hexagonal Color Many (see varieties) Fracture Conchoidal Specific Gravity 2.65 Refractive Index Low: 1.54 Luster: Glassy (vitreous) Interesting Property: Piezoelectric 2. Comments: Quartz is found in igneous, sedimentary and metamorphic rocks. It is hard, lacks cleavage, and is chemicaly stable. Thus, quartz survives weathering at the earth's surface and can readily be found as pebbles in streams and sand grains at the beach. In some sedimentary and metamorphic rocks it is the only mineral present (sandstone and quartzite, respectively). Flint (a fine-grained variety of quartz) was the first mineral fashioned into tools and weapons. It was undoubtedly the first mineral used as a gemstone (it's colored, crystalline varieties). The earliest written work on minerals, On Stones, written 2300 years ago by Theophrastus discusses quartz and its varieties in more detail than any other stone. As a mineral, only copper and iron have played a more important role in shaping society today. Note the hardness of 7. Quartz is a very common mineral, and a major component of sand, silt and dust. It makes sense that most valuable gemstones should be harder than 7 to resist abrasion. Quartz has a very low index of refraction - it does not bend light significantly. The fire and brilliance of a gemstone stems from a high RI, which makes quartz a relatively unpopular gemstone (especially clear crystals). Several varieties of quartz, however, have desirable colors, that make them semi-precious stones. Quartz gemstones. They are (clockwise from lower left) Rose quartz, citrine, smokey quartz, citrine, rock crystal (clear), and Amethyst (center). 3. Varieties: Quartz is divided into two broad categories: coarsely crystalline quartz and cryptocrystalline quartz (crypto=small, hidden). Geolgists usually refer to coarsely crystalline quartz as " quartz ," while the cryptocrystalline varieties are referred to as chalcedony . All have essentially the same chemical composition and crystal structure. They differ in methods of formation, grain size, and impurities that give rise to their specific color properties. Opal is size, and impurities that give rise to their specific color properties....
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This note was uploaded on 03/27/2008 for the course GEO 331 taught by Professor Huifang during the Spring '07 term at University of Wisconsin.
- Spring '07