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J the Journal of L \ A, If» R j The Journal of l \ A, if» r Volumemmoie 27 Noemmoiemmoi. 3 oVolum e 2 •-••^ppsgiiiWJigpy^PW 5I>us8rtlne ? '"? ^" Gansof samets Somaliland'JoiiKiliUiiKl InJ/J NlsertajJJ£i2/J:i ,WT'^ T/j'i rJ;j;j/]'iif]ü;j The cau.eof j'iüJYüCtoj/rii'ir colour of alexandrlte. The Gemmological Association and Gem Testing Laboratory of Great Britain Gemmological Association and Gem Testing Laboratory of Great Britain 27 Greville Street, London EC1N 8TN Tel: 020 7404 3334 Fax: 020 7404 8843 e-mail: [email protected] Website: www.gagtl.ac.uk/gagtl President: Professor A.T. Collins Vice-Presidents: E.M. Bruton, A.E. Farn, R.A. Howie, D.G. Kent, R.K. Mitchell Honorary Fellows: Chen Zhonghui, R.A. Howie, R.T. Liddicoat Jnr, K. Nassau Honorary Life Members: H. Bank, D.J. Callaghan, E.A. Jobbins, H. Tillander Council of Management: T.J. Davidson, N.W. Deeks, R.R. Harding, I. Mercer, J. Monnickendam, M.J. O'Donoghue, E. Stern, I. Thomson, V.P Watson Members' Council: A.J. Allnutt, P. Dwyer-Hickey, S.A. Everitt, A.G. Good, J. Greatwood, B. Jackson, L. Music, J.B. Nelson, P.G. Read, P.J. Wates, C.H. Winter Branch Chairmen: Midlands - G.M. Green, North West -1. Knight, Scottish - B. Jackson Examiners: A.J. Allnutt, M.Sc, Ph.D., FGA, L. Bartlett, B.Sc, M.Phil., FGA, DGA, E.M. Bruton, FGA, DGA, S. Coelho, B.Sc, FGA, DGA, Prof. A.T. Collins, B.Sc, Ph.D, A.G. Good, FGA, DGA, J. Greatwood, FGA, G.M. Howe, FGA, DGA, Li Li Ping, FGA, DGA, B. Jackson, FGA, DGA, G.H. Jones, B.Sc, Ph.D., FGA, M. Newton, B.Sc, D.Phil., C.J.E. Oldershaw, B.Sc (Hons), FGA, DGA, H.L. Plumb, B.Sc, FGA, DGA, R.D. Ross, B.Sc, FGA, DGA, P.A. Sadler, B.Sc, FGA, DGA, E. Stern, FGA, DGA, S.M. Stocklmayer, B.Sc. (Hons), FGA, Prof. I. Sunagawa, D.Sc, M. Tilley, GG, FGA, CM. Woodward, B.Sc, FGA, DGA The Journal of Gemmology Editor: Dr R.R. Harding Assistant Editors: M.J. O'Donoghue, P.G. Read Associate Editors: Dr C.E.S. Arps (Leiden), G. Bosshart (Zurich), Prof. A.T. Collins (London), Dr J.W. Harris (Glasgow), Prof. R.A. Howie (Derbyshire), Dr J.M. Ogden (Hildesheim), Prof. A.H. Rankin (Kingston upon Thames), Dr J.E. Shigley (Carlsbad), Prof. D.C. Smith (Paris), E. Stern (London), Prof. I. Sunagawa (Tokyo), Dr M. Superchi (Milan), CM. Woodward (London) Production Editor: M.A. Burland Vol 27, No. 3, July 2000 ISSN: 1355-4565 AA neneww finfindd ooff spessartinspessartinee garnetgarnetss iinn NigeriNigeriaa ThomaThomass LinLindd anandd UlricUlrichh HenHennn GermanGerman GemmologicalGemmological Association,Association, Idar-Oberstein,Idar-Oberstein, GermanyGermany ABSTRACT:ABSTRACT: ThThee physico-chemicaphysico-chemical l anandd microscopimicroscopicc featurefeaturess ooff spessartinspessartinee garnetgarnetss frofromm aa neneww occurrencoccurrencee iinn NigeriNigeriaa araree presentedpresented. ChemicaChemical l analyseanalysess shoshoww thathat t thesthesee garnetgarnetss havhavee aa ververyy highighh spessartinspessartinee componencomponent t ooff 89-9589-95%% witwithh varyinvaryingg smalsmall l amountamountss ooff pyroppyropee anandd almandinealmandine. VariatioVariationn ooff ththee almandinalmandinee componencomponent t betweebetweenn 11%% anandd 66%% iiss responsiblresponsiblee foforr aa variatiovariationn ooff colourcolourss betweebetweenn yellowyellow, , goldegoldenn yelloyelloww anandd brown-orangebrown-orange. AbsorptioAbsorptionn spectrspectraa ooff ththee yelloyelloww anandd goldegoldenn yelloyelloww specimenspecimenss (typ(typee aa spectraspectra)) araree ververyy similasimilarr ttoo thosthosee ooff spessartinspessartinee frofromm KunenKunenee (Namibia)(Namibia), , whilwhilee thosthosee ooff brown-orangbrown-orangee coloucolourr (typ(typee bb spectraspectra)) araree similasimilarr ttoo thosthosee frofromm RamonRamonaa (USA)(USA). ThThee maimainn inclusioninclusionss iinn ththee NigeriaNigeriann spessartinespessartiness araree healehealedd crackcrackss representinrepresentingg thithinn liquid-filleliquid-filledd cavitiecavitiess oorr fingerprint-typfingerprint-typee feathersfeathers. Keywords:Keywords: spessartinspessartinee garnetgarnet, , NigeriaNigeria, , physico-chemicaphysico-chemical l propertiesproperties, , internainternal l characteristiccharacteristicss 129 IntroductioIntroductionn spessartinspessartinee garnetsgarnets. ThesThesee gegemm garnetgarnetss werweree firsfirst t designedesignedd aass 'Kunen'Kunenee spessartinespessartine' ' afteafterr pessartinpessartinee garnetgarnetss ooff gegemm qualitqualityy araree ththee boundarboundaryy riveriverr betweebetweenn NamibiNamibiaa anandd relativelrelativelyy rarraree comparecomparedd ttoo otheotherr AngolaAngola, , bubut t becambecamee widelwidelyy knowknownn iinn ththee SSspeciespeciess ooff ththee garnegarnet t solisolidd solutiosolutionn jewellerjewelleryy industrindustryy undeunderr ththee tradtradee namnamee seriesseries. BotBothh quantitquantityy anandd qualitqualityy ooff ththee 'mandari'mandarinn garnet'garnet'. materiamaterial l frofromm differendifferent t sourcesourcess sucsuchh aass RamonRamonaa CountyCounty, , CaliforniaCalifornia, , MacoMaconn CountyCounty, , VirginiaVirginia, , MinaMinass GeraisGerais, , BrazilBrazil, , MadagascarMadagascar, , SrSrii LankLankaa anandd KenyaKenya, , havhavee beebeenn ververyy erraticerratic, , anandd thereforthereforee spessartinspessartinee garnetgarnetss havhavee beebeenn aa collector'collector'ss gemstongemstonee ratheratherr thathann aa stonstonee suitablsuitablee foforr jewellerjewelleryy industrindustryy purposespurposes. ThiThiss wholwholee picturpicturee changedchanged, , whewhenn iinn 19919922 aann occurrencoccurrencee ooff spessartinspessartinee garnegarnet t wawass discoverediscoveredd oonn ththee northernorthernn edgedgee ooff NamibiaNamibia, , neanearr ththee bordeborderr witwithh AngolAngolaa (Lin(Lindd etet ah,ah, 1993,1994)1993,1994). ThThee materiamaterial l frofromm thithiss occurrencoccurrencee iiss ooff aa finefine, , brighbright t orangorangee coloucolourr whicwhichh iiss easileasilyy FigureFigure 1:1: Cut Cut spessartinespessartine garnet garnet ofof 2.68 2.68 ctctfrom from distinguishedistinguishedd frofromm alall l hitherthithertoo knowknownn aa new new occurrenceoccurrence in in Nigeria.Nigeria. ©© Gemmologica Gemmological l AssociatioAssociationn anandd GeGemm TestinTestingg Laborator Laboratoryy oof f GreaGreat t BritaiBritainn ISSNISSN: : 1355-4561355-45655 Production from this source has been Composition continuous, and has made spessartine Garnets are a group of minerals with garnets popular among gem dealers and extended isomorphous solid solution jewellers. Unfortunately, sizes of cut stones between its end members. Although solid generally are small, and faceted gems of solution of five and more components is more than 1 ct are already relatively rare. For common, usually 80% of the crystal is built sizes of 2-3 cts and up, a regular supply up of only two or three main constituents, cannot be maintained, even at high prices. and all garnets can be broadly subdivided Recently, a new source of gem quality into the two solid solution series ugrandite garnets of fine yellow-orange colour was (with end members uvarovite, grossularite discovered in Nigeria, West Africa (Bank et and andradite) and pyralspite (with end al., 1999). The material from this new members pyrope, almandine and occurrence complements the range of sizes spessartine). available for fine orange spessartine garnets, Pyralspite garnets with spessartine being as sizes of 2-3 ct and more can now be the main component are called spessartine in supplied to jewellers (Figure 1). Table II: Microprobe analyses of spessartine garnets Refractive index and specific gravity from Nigeria. Refractive indices of 20 specimens were Hgr324 ligr325 Ugr326 Hgr327 measured using a Rayner refractometer and fluid with an RI of 1.81 and the results varied wt% between 1.801 and 1.803. Specific gravities MgO 0.67 0.99 0.96 0.85 measured using the hydrostatic method 129 varied between 4.15 and 4.22. FeO 0.62 2.77 1.44 2.44 CaO 0.27 0.09 0.19 0.35 Table I compares values of measured gemmological data for spessartine garnets MnO 40.38 37.67 39.36 37.76 from Nigeria with values for spessartine garnets from other occurrences described in 21.21 21.40 21.48 the literature. A1203 21.36 Table I: RI values and SG values for gem quality Fe203 0.00 0.00 0.00 0.00 spessartine garnets from Nigeria compared with v2o3 0.00 0.00 0.00 0.00 published work. Cr203 0.07 0.03 0.05 0.02 Locality n SG 6 Nigeria 1.801-1.803 4.15-4.22 Ti02 0.07 0.01 0.08 0.09 Rutherford Mine, 1.802(3) - Si02 36.68 37.04 36.55 37.02 Virginia1 Total 99.97 100.00 99.99 100.01 Taita Hills, Kenya2 1.795-1.809 - Minas Gerais, Brazil3 1.803-1.805 4.15 Mol% Ramona, Calif.4 >1.81 4.17 Pyrope 2.8 4.1 3.9 3.5 Kunene, Namibia5 1.801-1.803 4.15-4.22 Almandine 1.4 6.5 3.3 5.7 1. Sinkankas and Ried, 1966 Spessartine 95.0 89.1 92.1 89.7 2. Medenbach et al, 1978 3. Bank et al, 1970 Grossular 0.6 0.2 0.4 1.0 4. Tisdall, 1962 5. Lind et al, 1993,1994 Uvarovite 0.2 0.1 0.2 0.1 6. this work /. Gemm., 2000, 27, 3, 129-132 the trade. Gem-quality spessartine garnets are usually members of two mainly binary Relevant sector of ternary pyralspite diagram solid-solution series: for spessartine from Nigeria • spessartine var. sources 1. spessartine-almandine series (pyrope A spessartine Nigeria (this paper) and grossular < 10%); • spessartine Namibia (Kunene) 2. spessartine-pyrope series (almandine and grossular < 15%); Sp 100% (see Sinkankas and Ried, 1966, Bank et al, 1970, Jobbins et al, 1977, Medenbach et al, 1978, and Lind et al, 1993,1994). Compositions of spessartines
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