Geo331 lect9

Geo331 lect9 - Lecture 9: Complex silicates: tourmaline,...

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Lecture 9: Complex silicates: tourmaline, beryl, lapis lazuli, tanzanite, and zircon I. Tourmaline 1. Basic Data: Chemical Formula: complex (see below) Mohs' hardness 7-7.5 Crystal System Hexagonal Color Varies: see varieties Fracture Conchoidal Specific Gravity 3-3.25 Refractive Index 1.64-1.68 Luster: vitreous to resinous Interesting Property: Peizoelectric and Pyroelectric Tourmaline is pyroelectric: it develops an electrical charge when heated. This can cause crystals to collect dust in display cases, since the warmth of the display lighting is often enough to induce a charge in the crystal. Tourmaline is also piezoelectric (see quartz). This property is not as sensitive as quartz, however, and so tourmaline is rarely used for more sensitve instrumentation - however, pressure gauges to measure the force of nuclear explosions contain some parts made of tourmaline. Elbaite crystal courtesy F. John Barlow mineral collection, MH photo. The structure of tourmaline is exceptionally complex - and an important source of the element Boron: (Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4 Tourmaline also exhibits an amazing array of colors and often many colors are present in a single crystal. Ranges from blue to aqua, aqua to green, green to red, and red to colorless . Tourmaline is often found in pegmatites and igneous and metamorphic rocks. Occasionally it is found in stream gravels. But the vast majority of tourmaline comes from boron-rich igneous rocks. A variety of elbaite gemstones.
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Major sources for tourmaline include Elba (Italy), Brazil, Ural Mountains (Russia), and many US localities (including Wisconsin). It is also strongly pleochroic; strongest color seen along prism length (c-axis). Most gems are cut with the c-axis running up and down the cut gem. Varieties: Elbaite : pink - green (most often used as a gem) Schorl : black Dravite : brown Verdelite : green Rubellite : pink - red Achroite : white Indicolite : blue Other notes: Tourmalines are complex - they are not easy to synthesize, and the results usually cost much more than natural gems. Heat treatment must be done carefully, as the gems contain water in their structure and too much heat will destroy the crystal structure. If done correctly, heat treatment should change darker shades to a more attractive paler shade. Excess heat can make crystals brittle - so they should be stored with care as well. Irradiation produces brilliant pink and red shades, with occasional yellow or orange shades - the radiation probably changes the oxidation state of Fe or Mn ions. Tourmaline is pleochroic, and sometimes displays chatoyancy. II. Beryl Red Beryl crystal, courtesy F. John Barlow mineral collection, MH photo. 1. Basic Data:
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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 Wisconsin.

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Geo331 lect9 - Lecture 9: Complex silicates: tourmaline,...

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