Lecture 9 Notes (Geoscience 331)
Lecture 9 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This 6 page Class Notes was uploaded by Hannah James on Wednesday October 28, 2015. The Class Notes belongs to Geoscience 331 at University of Wisconsin - Madison taught by Huifang Xu in Fall 2015. Since its upload, it has received 28 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 10/28/15
I II Complex Silicates Tourmaline Bervl Lapis Lazuli Tanzanite and Zircon Lecture 9 Tourmaline Basic data Chemical Formula complex Mohs39 hardness 775 Crystal System Hexagonal Color Varies see varieties Fracture Conchoidal Specific Gravity 3325 Refractive Index 164168 Luster vitreous to resinous Interesting Property Piezoelectric and Pyroelectric becomes charged When heated Tourmaline is pyroelectirc and becomes charged When heated O Often picks up dust in display cases due to heat of display lights While also piezoelectric this property is not as sensitive as it is in quartz Tourmaline has many colors with multiple colors often being displayed in one crystal often found in pegmatites and igneousmetamorphic rocks sometimes stream gravels 0 Most often found in boronrich igneous rocks Strongly pleochroic strongest color seen along prism length or caxis Varieties O Elbaite pink green most often used as a gem Schorl black Dravite brown Verdelite green Rubellite pink red Achroite White Indicolite blue 0 00000000 000000 Tourmaline is not easy to synthesize because they are complex 0 Heat treatment must be done carefully as the crystal contains water and too much heat could destroy the crystal structure making it brittle Beryl Basic data 0 Chemical Formula Be3A12Si6018 Mohs39 hardness 758 Crystal System Hexagonal Color Varies see varieties Fracture Conchoidal Specific Gravity 236291 Refractive Index 157161 OOOOOO O O Luster vitreous Interesting Property Major source of Beryllium Beryl is a ring silicate silicate tetrahedral are linked to form rings provides hexagonal shape of crystal 0 Can form huge elongate crystals with a hexagonal cross section The formation of Beryl requires the source of beryllium occurs in hydrothermal deposits or pegmatites Emerald O Differ in price and size due to their depth or color and clarity I Green in emerald is due to chromium Cr impurities pale green due to iron Fe impurities that absorb red and transmit green light Most emeralds contain aws and are very brittle thus large emeralds are more rare than large diamonds and are sometimes more valuable I Flaws are often multiphase inclusions solid liquid and gas inclusions all within the gem To enhance emeralds many are coated with an oil with a similar refractive index hides cracks or surface scratches I Emerald cut was produced specifically to protect the gems reduces the chance it will crack or chip I The orientation of the cut can affect the color I Cut with table perpendicular to the length of crystal yellowish green I Cut with table parallel to the length of the crystal bluish green Aquamarine Latin for sea water 0 O O Aquamarine is usually awless Secondmost highly prized of the beryls emerald first The color for aquamarine is a pale to medium blue I Color is due to presence of iron Fe in two oxidation states ferric and ferrous and the charge transfer between these ions Most aquamarine jewelry is heat treated to produce a deeper blue most natural aquamarine is a very pale blue I Heat treatment involves conversion of ferric to ferrous iron Heliodor Golden Beryl s name present of the sun 0 Heliodor39s color is due to presence of iron and the transfer of electrical charge between the Iron and Oxygen ions within the crystal structure I Occasionally the color can be caused by presence of uranium impurities O Heliodor is rarer than aquamarine but less valued I The largest gem all 2054 carats of it is located at the Smithsonian Museum Morganite 0 Morganite is typically a soft pink or violet but sometimes can be a rare peach and salmon color I The color is due to the presence of manganese Mn 0 Red Beryl 0 Red Beryl gets its red color from Manganese impurities like morganinte 0 Most red beryl is not faceted as the crystals are often small and highly fractured 0 There is a significant trade in collecting red beryl crystals one of the reasons for this is that they are quite rare I Goshenite named after Goshen Massachusetts 0 Goshenite is a clear colorless variety of beryl 0 It is not often used in gemstones but it can be used in optics 0 Maxixe 0 Maxixe is a deep blue variety of beryl 0 The formation of this color involves irradiation effects and nitrate and carbonate groups in channels in the structure I Synthetics O Emerald has been synthesized since 1848 although the first facetable gems weren39t produced until 1934 0 It is generally not economic to synthesize aquamarine and other beryls I Some synthetic emeralds however can be produced With great clarity O The most common methods of synthesis are I Catham since 1940 I Gilson ux method I Linde hydrothermal method I Lechteitner hydrothermal overgrowth method 111 Zoisite Tanzanite 0 Basic data 0 Chemical Formula Ca2Al30SiO4Si207OH Mohs39 hardness 657 Crystal System Orhorhombic Color depends on varieties Fracture Conchoidal to uneven Specific Gravity 335 Refractive Index 17 Luster vitreous 0 Interesting Property strongly pleochroic 0000000 I Zoisite is a hydrous calcium aluminum silicate 0 Since 1967 tanzanite has become a very popular gem I Strong marketing by Tiffany amp Company Who coined the term quotTanzanitequot from Tanzania the country of origin has made zoisite an important gemstone 0 The most valuable tanzanite gems have a deep sapphire blue With an intense violet highlight vanadium V is the source of the color 0 Tanzanite is strongly pleochroic and can appear blue violet or red depending on its orientation I Cut stones are oriented to take advantage of the best colors 0 Nearly all Tanzanite is heattreated to improve the violet color and removes unwanted green and yellow tints within the stone 0 Zoisite forms as a reulst of regional metamorphism I Tanzanite occurs in metamorphosed limestones source of calcium near the contact with schists the source of aluminum and silica O The hardness of tanzanite is a bit low they require a little more care to keep from scratching since they are more vulnerable to dust and general wear IV Lapis Lazuli 0 Basic data 0 Moh s hardness 55 Cyrstal System Cubic thus isotropic Color Blue Specific Gravity 2729 Refractive Index 150 Luster VitreousGreasy 0 Interesting Property it is a felspathoid somewhat feldsparlike 0 Lapis Lazuli is normally a mixture of three minerals O Lazurite very complex blue mineral Calcite calcium carbonate which is white and Pyrite an iron sulfide that is whitegold in color I Lazurite is the essential ingredient of lapis lazuli and is the mineral that gives it blue color 0 The best quality material contains less calcite and pyrite 0 Lapis lazuli is a contact metamorphic rock with varying compositionphysical properties 0 It usually forms by contact metamorphism of limestones 0 Treatmentenhancement 0 Usually carved or polished as beads and cabochons I It is opaque so cutting facets doesn39t enhance the appearance 0 Lapis lazuli is not as quotrobustquot a gemstone as most others I It is relatively soft 55 and heat acids and strong bases can destroy the mineralogy of the stone care should be taken in its storage and display 0 Lapis lazuli is sometimes dyed or coated with wax to enhance less valuable specimens I However acetone or a hot needle poked into the surface will often reveal these enhancements 00000 0 Lapis is a very complex mineral aggregate so a true synthetic is nearly impossible to make 0 However there are simulants that mimic the appearance of the stone I Like all simulants attention to physical properties will usually reveal the fakery 0 Examples of simulants 0 Swiss Lapis Dyed jasper quartz is often used as an imitation I Lacks pyrite inclusions and has an R1 of 154 I Also jasper is going to be harder than true lapis O Sintered synthetic spinel I Has good color but has a much higher RI and SG 1725 35 respectively I Somtimes metallic gold or copper is included during synthesis producing specular metallic re ections imitating the pyrite O Gilson imitation lapis lazuli I A complex synthetic mix of synthetic ultramarine hydrous zinc phosphates and sometimes pyrite I The RI and SG of this synthetic stone are nearly identical to true lapis but the hardness is only 3 I Hydrochloric acid also attacks this simulant by discoloring the material to white and producing a quotrotten eggquot smell V Zircon 0 Basic data 0 Chemical Formula ZrSiO4 Mohs39 hardness 75 Crystal System Tetragonal Color usually brown Fracture Conchoidal Specific Gravity 468 high Refractive Index 192196 Luster Adamantine 0 Interesting Property oldest mineral grain ever found was a zircon 0000000 0 Zircon is commonly found in alluvial deposits 0 It is commonly formed in granite but large gemquality crystals are quite rare 0 Range in color from brown redbrown green to yellow 0 Virtually all zircons as gems are heat treated 0 Heating in an oxygenfree oven produces a blue color while subsequent heating in air produces yellow color shows how the oxidation state of the chromophores can be altered to produce the desired color I This method can also produce colorless stones 0 This has allowed zircon to be used as a diamond simulant which has given zircon an undeserved quotcheapquot reputation I Zircon is a zirconium silicate but there are many trace impurities within the stone 0 Halfnium is common along with uranium and thorium I These are responsible for the wide varieties of color among zircons O Uranium decays it s unstable and leaves a variety of products including lead I This radioactive decay produces defects in the crystal structure I Crates a cloudy appearance and eventually destroys the crystal structure completely 0 These noncrystalline zircons are referred to as metamict and are usually a brittle pale green unsuitable for gemstones unless heated I Heating will restore the damaged crystal structure Zircons used as a diamond simulant are not as hard and are more brittle than real diamonds O Often facet edges will show small chips and the facet surfaces can dull with increased wear Zircon is also a major source of the element zirconium 0 Zirconium is quite corrosionresistant and very strong even at high temperatures this makes it a vital resource in the nuclear power industry Cubic Zirconia is not a silicate but a zirconium oxide that has a cubic crystal structure 0 Production of this artificial gem began as a hightemperature ceramic I The zirconium oxide has small amounts of calcium magnesium and yttrium added to stabilize the crystal structure The oldest mineral grain ever found was a zircon 0 Researchers here at the UW Geology department described a zircon crystal 44 billion years old