Lecture 14 Notes (Geoscience 331)
Lecture 14 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
Popular in Geology
This page Class Notes was uploaded by Hannah James on Monday November 30, 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 22 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
Reviews for Lecture 14 Notes (Geoscience 331)
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 11/30/15
Synthetics and Simulants A Case of Mistaken Identitv Lecture 14 1 Synthetic gemstones 0 Gem synthesis is the process of growing gem crystals in a laboratory 0 Chemically synthetic gems are identical to their natural counterparts I Synthetic diamond is identical to real diamond in chemistry carbon and crystallography each carbon is linked to four other carbons in cubic symmetry 0 Synthetic gemstones can be told apart from natural stones 0 The key is to know what to look for I Sometimes these can be visible to the naked eye I Other features will require a 10X magnifying loupe I Some require precise elemental analysis or Xray crystallography 0 Some obvious signs of a synthetic gemstone I Curved growth lines Curved striations in a gemstone are obvious signs that it has been formed artificially I As a seed crystal is rotated new material is added to the outside the curved striae are a result of this rotation not found in natural stones I Hourglass Color zonation Sometimes the color zones will expand outward from a visible quotseedquot crystal 0 This seed is used as a nucleus of new growth synthetic gems are very symmetrical in this growth often produce an hourglass shape I Lack of natural inclusions Synthetic gems are often quottoo good to be truequot 0 They lack any notable inclusions which would be nearly impossible naturally 0 If inclusions are present they are often elements or chemicals that aren39t present in nature I Flux inclusions Flux is a growth medium that contains the chemicals necessary to form a gemstone crystal 0 The crystal then grows in this medium 0 Sometimes the synthetic crystal will have bits of ux trapped inside its structure 0 These ux inclusions are often combinations of things that are never or at least rarely found in nature I Often these ux inclusions can be used to point to a specific process sometimes even a specific lab 0 The trouble is if the inclusions are deep within a stone analyzing those materials can be damaging to the stone I Expansion Cracks Some synthetic gemstones crack as they cool more quickly than they would naturally I These cracks have a unique shape and often follow curved growth lines I Odd Growth structures Sometimes there will be oddly shaped bubbles indicating artificial growth 0 Synthesis Techniques Melt Growth 0 The most common method of gemstone synthesis usually involves high temperatures and molten gem material I As this material cools crystals of the synthetic gem forms 0 Verneuil I Sometimes referred to as Flame Fusion this is a very common method for creating synthetic corundum 0 This was first used on a commercial scale in 1902 I In this process powdered alumina Aluminum Oxide drops onto a boule while under a very hot 2200 degrees C hydrogen ame O Czochralski I Often referred to as quotcrystal pullingquot the Czochralzki process is sort of an inverted Verneuil process I The seed crystal is dipped into a crucible of molten gem material and slowly pulled upward 0 The material crystallizes on the end of this seed resulting in a continually growing synthetic crystal 0 Growth from Solution 0 Another process includes a saturated liquid solution of the intended gem material 0 At first the temperature is quite high increasing the solubility of the material I Then the temperature is lowered which decreases the solubility of the material I This supersaturated solution then precipitates the gem material 0 If you recall high school chemistry the same can be done with sugar I Heat water and continually add sugar water can dissolve a ridiculous amount of sugar I Then cool the sugar solution and suspend a string into the sugar solution 0 Sugar crystals will precipitate onto the string 0 At room temperature gem materials do not form I But by dramatically increasing the pressure and temperature gems can be synthesized this way 0 This is usually referred to as hydrothermal growth 0 Flux Growth I In this process the chemical components are mixed with a ux agent and melted in a nonreactive crucible often platinum or iridium 0 After thorough mixing the concoction is left to cool I Since the ux and the gem material crystallize at different temperatures the gemstone crystallizes first 0 The ux solidifies surrounding the gem material I Once cooled the whole blob is tossed into a solvent the ux dissolves away leaving the synthetic gem crystals 0 Commonly synthesized gemstones O Commonly synthesized gems include emerald rubies and sapphires 0 Synthetic quartz is usually grown by a hydrothermal process and is most often used in the electronics industry 0 Some gemstones are too expensive to synthesize since there is not enough demand for natural stones to offset the cost of production I Aquamarine and topaz are good examples a synthetic stone costs much more than their natural counterparts the chemical structures of these minerals are very complex 0 Diamonds I Synthetic diamonds are often yellowish in color rarely used for gem purposes more commonly used as diamond grit for industrial purposes 0 Modern synthesis of thin film diamond has other industrial applications 0 A 5 mm diamond 05 carat takes over a week to grow I Synthetic diamonds can sometimes be distinguished from natural diamonds by the presence of ux inclusions Ni Al or Fe 0 These materials were in the container with the Carbon necessary to make the diamond and are not abundant in naturally occurring diamonds II Simulants 0 Simulants simulate the appearance of diamond 0 The distinction between a synthetic diamond manmade diamond consisting of carbon atoms arranged in the typical diamond structure and a diamond simulant not a carbon compound with the diamond structure is VERY important 0 In order of increasing RI the most common simulants are 0 1 YAG yttrium aluminum garnet O 2 GGG gadolinium gallium garnet O 3 CZ cubic zirconia O 4 Strontium titanate This rhyme can be used to memorize the common diamond simulants in the above order You go crazy staring at diamonds 0 Again Simulants look alikes with a different chemical makeup are not the same as synthetics synthesized by humans but have the same chemical structure 0 Simulants are distinguished from diamonds using measurement or observation of various properties such as 0 Basic Physical Properties RI SG Dispersion Hardness I Diamond has a very high refractive index most simulants do not except for Strontium Titanate I R1 is responsible for the quotread through effect I Diamond is 10 on Mohs39 scale harder than any other material Silicon Carbide SiC is 95 but is almost never used as a gemstone I Re ection pattern Since simulants often have a different RI than real diamond they will re ect light differently etc I Shadow patterns When immersed in liquids with differing RI39s the simulant materials will have significantly different appearances This works because of the difference in refractive index between the immersed gem and the liquid Light passing from one one substance to another will refract 0 The more closely two materials39 RI39s are the less the light will refract If two substances are nearly identical in R1 the light will bend little or not at all and there won39t be much to see 0 However if there is a large difference between substances there will be a definitive quotoutlinequot of the gem the light is being bent and therefore some areas show up as dark or light see above photo The diamond simulant with the RI most similar to methylene iodide is barely visible while the diamond shows an outline O This quotoutlinequot is referred to as relief the higher the difference in R1 the greater the relief Immersion in a liquid of a known R1 is a straightforward and low tech way of identifying transparent minerals 0 Some of the chemicals are rather nasty and there are other ways such as a refractometer that don39t make as much of a mess I However as a quick visual test they work quite well 0 Conversely If you had a liquid with a high R1 of about 24 and placed both a low RI gem such as quartz and a diamond into it the quartz gem would show a strong outline but the diamond would be barely visible because the relative difference in R1 is greater between the liquid and quartz than between the liquid and diamond 0 quotRead through effectquot I Most gems will allow light to pass from the table out to the pavilion Diamonds do not I Thus when looking through the table facet a slight tilting of the gemstone will cause a dark quotwindowquot to appear in most diamond stimulants O Other simulants Strontium Titanate will however pass this test but there are several other properties that easily differentiate it from diamonds O Diaspore alphaAIOOH mixed With dye and epoxy to mimic turquoise in certain market Its color and hardness are very similar to those of real turquoise
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'