Identification Methods of Sri Lankan Corundum in Comparison to Other
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Independent Project at the Department of Earth Sciences Självständigt arbete vid Institutionen för geovetenskaper 2017: 5 Identification Methods of Sri Lankan Corundum in Comparison to Other Common Gemstones Identifikationsmetoder av korund från Sri Lanka och jämförelse med andra vanligt förekommande ädelstenar Erik Axel Kilbo Pehrson DEPARTMENT OF EARTH SCIENCES INSTITUTIONEN FÖR GEOVETENSKAPER Independent Project at the Department of Earth Sciences Självständigt arbete vid Institutionen för geovetenskaper 2017: 5 Identification Methods of Sri Lankan Corundum in Comparison to Other Common Gemstones Identifikationsmetoder av korund från Sri Lanka och jämförelse med andra vanligt förekommande ädelstenar Erik Axel Kilbo Pehrson Copyright © Erik Axel Kilbo Pehrson Published at Department of Earth Sciences, Uppsala University (www.geo.uu.se), Uppsala, 2017 Sammanfattning Identifikationsmetoder av korund från Sri Lanka och jämförelse med andra vanligt förekommande ädelstenar Erik Axel Kilbo Pehrson Sri Lanka är ett av de ledande exportländerna av högkvalitativ ädelkorund (Al2O3) som bildats i landets prekambriska bakgrund. Hög metamorfos av silikat- och karbonatrika bergarter har skapat rätt miljö för mineralet att bildas. Vittrad berggrund har transporterats och avsatts av fluviala processer och ädelstensförande grus finns i huvudsakligen i Sri Lankas Highlandkomplex. För att kunna avgöra om en ädelsten är korund eller ett annat mineral används olika tekniker och instrument. Denna rapport fokuserar huvudsakligen på instrumentella metoder som mäter fysikaliska egenskaper hos mineral för att därigenom kunna få en definitiv mineralidentifikation. Inom gemologi är det absolut nödvändigt med mineralidentifikation för att bestämma det kommersiella värdet av en ädelsten. Även faktorer som transparens, färg och inneslutningar hos en ädelsten måste undersökas vid värdering. Syntetiska och eller värmebehandlade ädelstenar har blivit allt mer vanliga genom utvecklig av avancerad teknik och att skilja dessa från naturliga stenar är också viktigt. Genom undersökning av inneslutningar och den kemiska sammansättningen av en ädelsten kan dess ursprung och i vissa fall de geologiska bildningsprocesserna fastställas. Nyckelord: korund, ädelstenar, identifikation, Sri Lanka Självständigt arbete i geovetenskap, 1GV029, 15 hp, 2017 Handledare: Karin Högdahl och M. C. de J. Jayasekare Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, 752 36 Uppsala (www.geo.uu.se) Hela publikationen finns tillgänglig på www.diva-portal.org Abstract Identification Methods of Sri Lankan Corundum in Comparison to Other Common Gemstones Erik Axel Kilbo Pehrson Sri Lanka is one of the leading countries in export of high quality corundum (Al2O3) gemstones due to the Precambrian rocks geological background. Combined high- grade metamorphic silicates and carbonate rocks makes perfect conditions for crystallization of this mineral. Weathered and eroded rocks have been transported by fluvial processes and formed gem gravels that are located throughout the country but mainly in the Highland complex of Sri Lanka. In order to determine whether a gemstone is corundum or any other gem mineral different techniques and instrumental methods are used. This report focuses on instrumental methods used to measure the minerals physical properties in order to get a definite mineral identification. Mineral identification in gemological circles is critical for determining the commercial value of a specimen. Factors like transparency and color of and inclusions in gemstone have to be taken into account when evaluating the value of a specimen. It also needs to be distinguished from artificial gemstones and heat- treated specimens that in recent years have become more common due to more advanced techniques. By examine inclusions and the chemical composition of a rough or facetted specimen the origin and process can be determined. Keywords: corundum, gemstones, identification, Sri Lanka Independent Project in Earth Science, 1GV029, 15 credits, 2017 Supervisors: Karin Högdahl and M. C. de J. Jayasekare Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala (www.geo.uu.se) The whole document is available at www.diva-portal.org Table of Contents 1 Introduction……………………………………………………………………………1 1.1 Aim of study…………………..………………………………………………....1 2 Background……………………………………………………………………………2 2.1. Geology and gem-mining in Sri Lanka…………………………….……2 2.1.1 Exploration for gemstones in Sri Lanka.………....………………….....3 2.2 Description of gemstones…….………………………………..…………….5 2.3 Optical Characteristics………………………………………………………..6 2.3.1 Pleochorism…………………………………………...……………......7 2.3.2 Asterism and chatoyancy…………….……………………………..…7 2.4 Crystal system ……………………………………………………...……..…...8 2.5 Corundum ………………………………………………………………………..9 2.5.1 Sri Lankan corundum …………………………………………………….....10 3 Instrumental identification methods……………………..……………….11 3.1 Specific gravity…………………………………………..……………………..11 3.2 Refractometry and refraction index………………………………………12 3.2.1 Immersive method………………...………………………..……………...13 3.3 Fluorescene as identificational aid………………………………..……...14 3.4 Inclusions in corundum..………………………..……………………..…….16 3.4.1 Origin of inclusions……………………...……..…………………..……..…..16 4 3. Energy dispersive x-ray spectrometer (EDS)…………..…………….18 4 Synthetic corundum and heat-treatment……….……...…..……........18 4.1 Heat-treatment………………………………………..………………………..19 5 Analysis on Sri Lankan corundum………..………………………………20 5.1 Results……………………………………………………………………………21 5.2 EDS analysis……………………………………………………………………22 6 Discussion……………………………………..…………………………………….26 6.1 Easily accessed methods………………………………..…………………26 6.2 EDS analysis……………………………………..……………………..……..27 7 Acknowledgments……………………..…………………………………………29 8 References ……………………………………..…………………………………..30 9 Appendix: Tables and figures ……………… ………………………..…...32 1. Introduction Corundum is a rather common mineral found worldwide and it can, in some cases, be classified as a gemstone. A gemstone can be described as a mineral with attractive visual appearance durable and high lustre, making it suitable for jewelry and collecting purposes. Sri Lanka produces large amounts of high quality gemstones originating from metamorphic rocks that have provided the right condition of formation. Components for crystallization of corundum is desilication, where the silica being pumped out by silica-deficient fluids. However, the less mobile alumina remains in the protolith and recrystallize as corundum or as other aluminium-rich oxides. Although most natural corundum are not of gem-quality (Simonet et al., 2008). Gem quality corundum is divided into two types related to their color as either ruby or sapphire. Both types have the same chemical composition of Al2O3 ,but incorporation of small quantities of other elements substituting for Al, changes the color and transparency of the stone (Ariyaratna, 2013). For example ruby has traces of chromium, leading to a red color, where the shade depends on the percentage of Cr and a pinkish variety is known as padparascha in Sri Lanka. Blue and yellow types yield their colors from contamination of Fe, Ti and or V. Identifying gemstones from each other or from synthetic varieties often requires different analytical methods, measuring the physical and chemical properties of the sample (O’Donoghue, 2015). Some of the most commonly used techniques and instruments to identify corundum gemstones are: crystal system classification, refractometry, spectroscopy, specific gravity, microchemical-SEM (scanning electron microscope), fluorescence and analyses of inclusions and other impurities in the specimen (O’Donoghue, 2015). It should be noted that all of these methods are not needed in order to identify a specific gem. SEM analyses with either EDS or WDS detectors measures the chemical composition of the sample and minerals with a simple composition are usually readily identified. The instruments mentioned above besides the SEM are ”in field and easy to access” methods (Reed, 1993). 1.1 Aim of the study The aim of this study is to describe and compare different analytical methods used for gem-mineral identification. Although Sri Lankan corundum is the chosen mineral the results can be applied for most gemological minerals. 1 2. Background 2.1 Geology and gem-mining in Sri Lanka About 90% of the bedrock in Sri Lankan is dominated by high-grade Archean to early Paleoproterozoic gneisses metamorphosed in the Neoproterozoic era. Before break- up it was a part of the supercontinent Gondwana and its geology and mineral deposits are similar to those of Madagascar, India, Australia and Antarctica (Gunaratna, 1995). The metamorphic rocks are a major source of corundum gem minerals, since they provide the right conditions of formation. Factors that control growth of corundum are pressure, temperature, presence of fluids, and the mineralogy and chemistry of the host rock. Corundum crystallizes in silica-poor alumina-rich environment by metamorphic reactions in mainly closed systems (Simonet et al., 2008). In short, the components for crystallization of corundum undergoes a desilication, where the silica being pumped out by silica-deficient fluids. However, the less mobile alumina remains in the protolith and recrystallize as corundum or as other alumina- rich oxides. Although most crystallized corundum are usually not of gem-quality (Simonet et al. 2008). The basement rock of Sri Lanka has been divided into four major lithotectonic units, the Wanni Complex, Highland Complex,