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Chemical Classification of Gem Garnets Deepti

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Chemical Classification of Gem Garnets Deepti Chemical Classification of Gem Garnets Deepti Maharaj 29183376 GLY 890 Dissertation November 2015 Department of Geology University of Pretoria ABSTRACT Gem garnet chemistry is described using the end-members pyrope, almandine, spessartine, andradite, grossular, and uvarovite. The large variation in garnets makes classification difficult. Garnet gemstones are typically classified by means of their refractive indices, together with specific gravity. However, the range of refractive indices for gem garnets can be restrictive and unreliable in gemstone classification. Chemical classification is generally the most accurate means of classification and was used in this thesis by means of a portable XRF for the non-destructive chemical analyses of 1513 garnet gemstones. Colour, refractive index and magnetic susceptibility were also determined. The garnets were divided into two species: ugrandite (uvarovite, grossular and andradite) and pyralspite (pyrope, spessartine and almandine). The chemistry of the pyralspite species was very diverse with a large range in end-member proportions producing extensive solid solutions between end-members. A diverse range in colour, refractive index, and magnetic susceptibility was also observed in the pyralspite species. However, no distinction based on refractive index and magnetic susceptibility could be made in the pyralspite species. Distinctive chemistries were observed in the ugrandite species, which correlates with the magnetic susceptibility and refractive index. Colour was the excepton as no relationship between colour and chemistry was observed. The samples with unusual compositions were reported such as pyrope-andradite, spessartine-grossular, and almandine-grossular. This study concluded that refractive index, magnetic susceptibility and colour should not be used in isolation because this can lead to misinterpretation. Rather, analytical techniques, if available, should be used. i TABLE OF CONTENTS 1. Introduction .................................................................................................................... 1 2. Literature Review ........................................................................................................... 3 2.1. Garnet ..................................................................................................................... 3 2.1.1. Garnet production .......................................................................................... 10 2.2. Gem Classification Schemes ................................................................................ 10 2.2.1. Webster ......................................................................................................... 10 2.2.2. Anderson ....................................................................................................... 12 2.2.3. Stockton and Manson .................................................................................... 12 2.3. Colour in Gem Garnets ......................................................................................... 14 2.3.1. Colour in garnets gemstones as a means of classification ............................. 14 2.3.2. Colour change ............................................................................................... 14 2.4. Magnetic Susceptibility ......................................................................................... 15 2.4.1. Previous uses of magnets in gemology .......................................................... 16 2.4.2. Garnet magnetism ......................................................................................... 17 2.5. Gem Garnets ........................................................................................................ 17 2.5.1. Uvarovite ....................................................................................................... 17 2.5.2. Almandine ...................................................................................................... 17 2.5.2.1. Localities ................................................................................................. 18 2.5.2.1.1. Tanzania .............................................................................................. 18 2.5.2.1.2. America................................................................................................ 18 2.5.2.1.3. Canada ................................................................................................ 18 2.5.2.1.4. Other localities ..................................................................................... 19 2.5.3. Andradite ....................................................................................................... 19 2.5.3.1. Demantoid .............................................................................................. 19 2.5.3.1.1. History ................................................................................................. 20 2.5.3.1.2. Localities .............................................................................................. 20 2.5.3.1.2.1. Russia ........................................................................................... 20 2.5.3.1.2.2. Val Malenco, Italy .......................................................................... 21 2.5.3.1.2.3. Kerman Province, Iran and Kaghan Valley, Pakistan ..................... 22 2.5.3.1.2.4. Erongo Province, Namibia ............................................................. 22 2.5.3.1.2.5. Antetezambato, Madagascar ......................................................... 22 2.5.3.1.2.6. Other localities ............................................................................... 23 2.5.3.1.2.7. Comparison of different localities ................................................... 24 2.5.4. Grossular ....................................................................................................... 25 2.5.4.1. Tsavorite ................................................................................................. 26 2.5.4.1.1. Localities .............................................................................................. 27 ii 2.5.4.1.1.1. Kenya ............................................................................................ 27 2.5.4.1.1.2. Tanzania ........................................................................................ 27 2.5.4.1.1.3. Madagascar ................................................................................... 28 2.5.4.1.1.4. Pakistan ......................................................................................... 28 2.5.4.1.1.5. East Antarctica .............................................................................. 28 2.5.4.1.1.6. Other localities ............................................................................... 28 2.5.4.1.2. Tsavorite geochemistry ........................................................................ 29 2.5.4.2. Hessonite ................................................................................................ 29 2.5.4.2.1. Localities .............................................................................................. 30 2.5.5. Grossular-andradite ....................................................................................... 30 2.5.5.1. Mali garnet .............................................................................................. 30 2.5.5.1.1. Localities .............................................................................................. 30 2.5.6. Pyrope ........................................................................................................... 31 2.5.6.1. History .................................................................................................... 31 2.5.6.2. Properties ............................................................................................... 32 2.5.6.3. Localities ................................................................................................. 32 2.5.6.3.1. Czechoslovakia .................................................................................... 32 2.5.6.3.2. Italy ...................................................................................................... 33 2.5.6.3.3. East Africa ........................................................................................... 33 2.5.6.3.4. Canada ................................................................................................ 33 2.5.6.3.5. China ................................................................................................... 33 2.5.6.3.6. America................................................................................................ 34 2.5.7. Pyrope-almandine .......................................................................................... 34 2.5.7.1. Rhodolite ................................................................................................ 34 2.5.7.1.1. History ................................................................................................. 34 2.5.7.1.2. Properties ............................................................................................ 34 2.5.7.1.3. Localities .............................................................................................. 35 2.5.7.1.3.1. East Africa ..................................................................................... 35 2.5.7.1.3.2. Canada .......................................................................................... 35 2.5.7.1.3.3.
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