Lecture 12 Notes (Geoscience 331)
Lecture 12 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This 0 page Class Notes was uploaded by Hannah James on Friday November 20, 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 59 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 11/20/15
Diamonds and Precious Metals Elementgrv Gem0102v Lecture 1 Native elements are not often found naturally Most elements oxidize or combine with other elements to achieve a stable electron combination Diamonds 0 Basic Data 0 Chemical Formula C carbon Mohs39 hardness 10 hardest mineral Crystal System cubic Color clear color varieties see below Fracture good cleavage Specific Gravity 351 Refractive Index 242 very high Luster Adamantine 0 Interesting Property Most diamonds are over 1 billion years old 0000000 0 Diamonds are made entirely of carbon 0 Pure carbon at the earth39s surface is normally found as graphite I Graphite is the grey greasy mineral found in pencil quotleadsquot 0 Graphite has a hardness of only 1 essentially the opposite of diamond 0 The key to diamond39s superlative properties lies within the crystal structure 0 And where it forms deep in the earth is where the crystal structure takes shape 0 Diamond is the hardest substance known the only thing that scratches a diamond is another diamond 0 It has a very high RI and is a better conductor of heat than any metal 0 In graphite carbon is also strongly bonded to one another but this bond is only in two dimensional sheets each quotsheetquot can slide past each other relatively easily 0 In diamond each carbon atom is strongly bonded to four other carbon atoms I This produces a very strongly linked crystal lattice 0 The heat and pressure required to get carbon to form these strong bonds normally exists only deep within the earth39s mantle more than 150km 100mi below the surface 0 The high refractive index and dispersion give diamond excellent brilliance and fire the brilliant cut is usually used for diamonds because it emphasizes these properties 0 Despite it39s extreme hardness it is brittle 0 There are many planes of cleavage within the stone hitting a diamond with a hammer is a sure way to break it 0 Diamond cutters use these cleavage planes to minimize the grinding and polishing required to finish a stone 0 It also has resulted in unscrupulous gem buyers smashing a large diamond to quotprovequot it39s a lesser stone and essentially worthless I Then the crooked gem dealer can pick up the larger fragments and sell 0 Diamond formation 0 Most diamonds are very old I Inclusions tell us that most diamonds are at least a billion years old many being over 3 billion far older than most other gemstones 0 Since minerals are most stable at the temperature and pressure at which they form diamonds remain in the mantle for many many years millions to billions I Temperatures in this region of the mantle are around 9001300 degrees centigrade with pressures of 50 60 kilobars kB 0 From this you should get two things I 1 diamonds are unstable at the earth39s surface I 2 to make synthetic diamonds you need some very expensive machinery 0 Diamonds must be brought to the surface quickly in order to keep their crystalline structure bring a diamond up slowly and it turns to graphite I Explosive volcanic eruptions bring diamonds up to the surface fairly quickly 0 This may seem slow but near the surface this speed would increase to supersonic velocities as the overlying pressure decreases 0 These volcanoes are called kimberlite volcanoes and the rocks where most diamonds are found is called kimberlite I Term kimberlite pipe describes one of these quottubesquot of diamondrich rock 0 Kimberlite is an igneous rock rich in olivine and other mantle minerals I Olivines pyroxenes MgAl garnets and Mgrich mica make up most of these rocks diamonds occur as crystals within the remaining ground mass 0 As the kimberlite travels up to the surface diamonds are incorporated into the rock as it travels up to the surface 0 Lamproite is a rare igneous rock that contains a great deal of olivine and occasionally diamonds I Some metamorphic rocks have diamonds but this rare and very small diamond definitely not for gems and these metamorphic rocks must have formed deep under the earth I Even more rare are diamonds that form at the earth39s surface due to meteorite impacts about the only place at the surface where you can get enough temperature and pressure 0 Finding diamonds requires finding the geologic structures mainly kimberlite craters and then mining them I They can be found in alluvial deposits as the diamonds weather out and collect in the stream gravels I It is most likely that the first discovery of diamonds were from these alluvial deposits 0 The advent of modern industrial mining techniques has allowed for some truly amazing quotholesinthegroundquot in a search for diamonds Diamonds as gems O The use of diamonds as gems dates back nearly 3000 years to ancient India the Hope Diamond is from India I Interestingly enough Australia is the world39s leading producer of diamonds by volume I Australia 39 Russia 15 Zaire 15 and South Africa 10 make up most of the world39s diamond production I You can for a small fee search for diamonds at Crater of Diamonds State Park in Arkansas it39s about the only place in the world where the public can search inside a kimberlite deposit 0 This is the only true diamond quotminequot in the US 0 Diamonds are rareup to 250 tons of kimberlite must be processed to find a single carat of gem quality diamond Of all the diamonds mined in the world about 20 of these are gem quality 0 The remainders are used industrially as coatings abrasives and polishes heat sinks and other tools More than 75 of all gem quality diamonds find their way to the Central Selling Organization DeBeers in London 0 Here they are sorted and graded to be sold to diamond cutters around the world DeBeers owns many large mining operations and buys many rough stones on the open market 0 Every few weeks quotsightholdersquot those getting disbursements of these diamonds are allowed to view their allotments I This is an incredibly controlled distribution system and all the information on a diamond39s origin is kept confidential I This also makes it very difficult to know the provenance source of any particular stone difficult for those who do not wish to support illicit activities by purchasing diamond 0 It should be noted that this is generally true for natural stones there are always exceptions DeBeers takes great lengths to denote synthetic stones as such something which in recent years has become harder to do In ancient India diamonds were not cut they believed altering the stone would destroy its magical properties 0 Simple cutting of diamonds started in Europe around 1300 AD In the middle of the 17th century Cardinal Mazarin of France is credited with creating the ancestral shape to the modern brilliant cut 0 This was a cushionstyle cut with 34 facets that better showed the brilliance and fire of diamond I It is more likely that the French Cardinal sponsored the work and didn39t actually cut the stone himself In 1919 Marcel Tolkowsky worked out the precise angles and proportions for the modern brilliant cut 0 These 58 facets provide an optimum balance between fire and brilliance The cutting process typically removes about half the carat weight of the uncut stone so careful planning is required to maximize the amount of diamond used The tradition of a diamond engagement ring dates back to 1477 when Archduke Maxmillian of Austria gave a diamond ring to Pricess Mary of Burgundy 0 Aggressive marketing after World War II has led to its current status While in their purest form diamonds are colorless most diamonds contain some amount of impurities 0 Most commonly nitrogen substitutes for carbon in small amounts and colors the diamond yellow as N increases the stone becomes more brown I Most yellow diamonds are less valuable than their clear counterparts unless the yellow is particularly deep and pure of color 0 Most colored diamonds are termed Fancy Diamonds 0 When small amounts of boron substitute for carbon diamonds take a blue color like the Hope Diamond 0 Red and pink colors are a result of defects of the crystal lattice 0 Green diamonds can be produced through irradiation Not all colored diamonds are less valuable than their colorless counterparts 0 While a awless colorless diamond sells for 25000 50000 per carat depending on total size and quality of cut a red diamond sold at a 1987 auction went for nearly one million dollars a carat O A 13carat blue diamond sold for more than 500000 per carat in the early nineties Diamonds and the 4 C s Making the Grade F1 vvs1vvs2vvs3vs1 vs2 s1 s2 11 12 13 Flawless Imperfect O FFlawless VVSVery Very Slightly included VSVery Slightly included SISlightly Imperfect IImperfect O Inclusions and cracks reduce a diamond s clarity I There are several ways of dealing with them I Oiling cracks and drilling inclusions then bleaching the darker inclusion is the primary method of treatment for less than perfect stones Descriptive Brilliance terms II 0 quotfirst piquequot inclusions readily recognizable at 10X magnification not significantly diminishing brilliance 0 quotsecond piquequot larger inclusions can be seen with naked eye 0 quotthird piquequot many large inclusions diminishing brilliance 0 Color blueishwhite gt white gt silver gt yellow GIA Carer Gr ding I l w a w f s a x b l id in l uzl 61 ll 7 r 7 5 wit 7 35 719 5111 E E E H I i L M H 0 9 his quotr u H w aimless Hear Colorless Faimt Fe luW New Light fellow Ligh ellnw Color Grading O DI will appear colorless to an untrained eye regardless of size 0 J L small stones will appear colorless to an untrained eye large stones will be tinted O MZ will appear tinted to an untrained eye regardless of size 0 Anything deeper yellow than Z will be termed quotfancy yellowquot Cut descriptions VERY GOOD GOOD MEDIUM POOR Precious Metals Platinum Gold Silver plus Copper Iron Nickel etc as a group are responsible for much of our development as a species 0 Iron and copper have had the most significant affect on our society today in terms of volume used anyway 0 Aluminum is also a major metal but the value of Platinum Gold and Silver has always been high While not gemstones themselves most gems are set with metal to make jewelry 0 The technological applications of precious metals are still being developed but their use as a valuable decorative element has been around for at least 6000 years Precious metals are usually distinguished by their stability they do not react with much in their environment 0 They are also very ductile these metals can be bent and pulled into many shapes 0 Also they are quite dense and are excellent conductors of heat and electricity Platinum 0 Platinum gets its name from the Spanish word for silver plata I It was in Columbia South America in the 18th century where it was first discovered 0 Platinum weathered out of the volcanic rocks and was unearthed from the placer deposits in streams washing down from the mountains 0 At first Spaniards thought it was quotimmaturequot gold and threw it back into the river to ripen I 19th century Russia became a large source of platinum 0 Platinum has been used as an alternative to silver gold in jewelry for centuries Silver 0 0 It is very resistant to corrosion and has a high melting point I This makes it ideal for many industrial applications especially in the refining of petrochemicals and in automobile catalytic converters Platinum is more valuable per ounce than gold I It is a highly demanded metal because it is a good catalyst nanometer size particles used in many places like muf ers to oxidize carbon monoxide into carbon dioxide and hydrogenbased fuel cells Platinum occurs primarily in basaltic magmas rich in olivine and pyroxene I The presence of sulphur will allow platinum along with iron nickel and copper to concentrate within these basalts Since platinum is resistant to chemical weathering as the surrounding rock erodes nuggets of platinum will weather out of the rock and wind up in alluvial deposits Gold is probably the most famous metal in the world I However knowledge of how it forms is hard to come by Gold is found in placer deposits or by digging it out of the ground I Washing away the lighter stream gravels will sometimes reveal heavier gold nuggets at the bottom of a miner39s pan Gold has a very high specific gravity I This density allows it to sit on the bottom while the lighter minerals are washed away Gold formation is the result of both igneous and hydrothermal processes I Gold crystals are rare and rounded nuggets are more common primarily because it is so soft Gold is also measured by a quotcarrotquot of sorts I 24 Karat gold is pure gold but this is too soft to be useful in most jewelry Most jewelry is sold as an alloy of gold and another metal for example Nickel and Palladium is used in white gold I Copper and silver are also common alloy metals Gold is primarily found associated with milky quartz in hydrothermal veins near igneous rocks solutions of these metals are deposited in veins within country rocks I South Africa is the world39s largest producer of gold followed by the US Canada Australia and Russia Silver is another metal found deposited by hydrothermal processes often associated with copper It39s crystal growth often occurs as fine quotwiresquot or dendritic branching shapes I While removed from most coins the last US coins to use silver in normal circulation were 1964 quarters and halfdollars it continues to see limited use in currency Jewelry and household items are still made using silver 0 Most silver is used by the photography industry where silver salts are used in film emulsions because silver halide turns dark when exposed to light with the increase in digital photography this is probably a declining statistic 0 Sterling silver is an alloy of silver and copper usually about 5 copper I It is a little harder than pure silver 0 Sometimes silver is quotwatered downquot with extra copper 0 Copper 0 Just a few hundred miles north in the Upper Peninsula of Michigan lies the world39s largest deposit of native copper 0 Copper was an important metal to native Americans who used the metal for spear points I In ancient Egypt an alloy of copper and tin bronze was instrumental in the tools used to construct the pyramids 0 Copper is widely used for electronics coinage and as a major alloy of many metals 0 Native copper is usually associated with basalt lava ows I Large deposits of copper were precipitated hydrothermally within large vesicles of these lava deposits 0 Silver is commonly found in minor quantities with copper I The geology museum has a 1300 pound copper nugget from the Upper Peninsula of Michigan 0 Iron Aluminum 0 0 Minnesota Wisconsin and Michigan are located along one of the world39s largest iron deposits I These thick deposits of hematite and magnetite are related to ancient bacteria fixing oxygen and iron nearly 2 billion years ago The actual biologicalchemical processes are still debated I There are also thick deposits in other countries like Australia Aluminum is a very useful metal today Most aluminum comes from deposits of bauxite a complex hydrous aluminum oxide I Bauxite is found in tropical soils derived from basalt I The energy required to separate aluminum from oxygen is tremendous
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