Mineralogy and Geochemistry of Gem Vanadian-chromian Grossulars(Tsavorites)from East Africa
-
摘要: 东非（包括肯尼亚、坦桑尼亚和马达加斯加）铬钒钙铝榴石（察沃石）均赋存于新元古代莫桑比克变质带的钙质硅酸盐石墨片麻岩中。矿物学研究表明，东非察沃石为宝石级钒（±铬）钙铝榴石，以w（V2O3）≈0.20%或w（Cr2O3）≈0.30%为分界线可划分为钒钙铝榴石、铬钙铝榴石和铬钒钙铝榴石3类，其中以钒钙铝榴石为主。东非察沃石的晶体化学式为（Ca2.792～3.005Fe0～0.013Mn0.008～0.080Mg0.022～0.078）2.948～3.110(Al1.787～1.968 Ti0.005～0.028 Cr0.001～0.072 Fe0～0.080V0.005～0.163)1.890～2.052 [Al0～0.288 Si2.712～3.017] O12,可简写成Ca3(Al,Cr,Fe,V)2[Si04]3;其端员组分为Grs87.61～96.79 Gld0.16～5.66Uvr0.02～2.46 Pyr0.73～2.68 Sps0.28～2.75Adr0.00～2.71Alm0.00～0.43,表明它由钙铝榴石(87.61%～96.79%)、钙钒榴石(0.16%～5.66%)和钙铬榴石(0.02%～2.46%)组成。以往研究表明,东非不同国家所产的察沃石之间 V/Cr 质量分数比值存在明显差异,它们的化学成分是可变的,但同一矿床的 V/Cr 质量分数比值通常是不变的;并且它们还具有3 组一致的δ18O值,依次为9.5‰~11.0‰、11.6‰~14.8‰和15.5‰~21.1‰,利用O同位素与 V-Cr-Mn微量元素量相结合的方法,可鉴别其地质和地理来源。Abstract: The gem vanadian(chromian)grossulars from East Africa(e.g.Kenya,Tanzania and Madagascar),also known as Tsavorites,are usually hosted in calc-silicate graphitic gneiss of the Neoproterozoic Mozambique Metamorphic Belt.According to the divisions at w(V2O3)≈0.20%or w(Cr2O3)≈0.30%,the tsavorites from East African can be classified into three types,which are vanadian grossulars,chromium grossulars and vanadian-chromium grossulars.Mineralogical analysis shows that the tsavorite from East Africa is mainly aluminum calc-silicate in composition,with crystallochemical formula of(Ca2.792~3.005Fe0~0.013 Mn0.008~0.080Mg0.022~0.078)2.948~3.110 (Al1.787~1.968Ti0.005~0.028Cr0.001~0.072Fe0~0.080V0.005~0.163)1.890~2.052[Al0~0.288Si2.712~3.017O12],abbreviated as Ca3(Al,Cr,Fe,V)2[SiO4]3.The end-member components of the tsavorite from East Africa are Grs87.61~96.79Gld0.16~5.66Uvr0.02~2.46Pyr0.73~2.68 Sps0.28~2.75Adr0.00~2.71Alm0.00~0.43,suggesting a composition of grossular(87.61%to 96.79%)with a small amount of goldmanite(0.16%to 5.66%),and uvarovite(0.02%to 2.46%).Previous research suggested that there was an obvious difference in V/Cr ratio among the tsavorites from different countries in East Africa.Although the chemical composition of tsavorites is variable,V/Cr ratios remain constant for a given deposit.In addition,three consistent sets ofδ18 O values,from 9.5‰to 11.0‰,from 11.6‰to 14.5‰,and from 15.5‰to 21.1‰,have been recognized for the tsavorites from Tanzania,Kenya,and Madagascar,respectively.The geological and geographical sources of tsavorites can be identified by the combined research on oxygen isotope compositions and V-Cr-Mn quantification.
-
Keywords:
- tsavorite /
- vanadian-chromian grossular /
- mineralogy /
- geochemistry /
- locality identification /
- East Africa
-
-
[1] Bridges C R.Green grossularite garnets(tsavorites)in East Africa[J].Gems & Gemology,1974,14(10):290-296.
[2] Switzer G S.Composition of Green Garnet from Tanzania and Kenya[J].Gems & Gemology,1974(14):296-297.
[3] Wise R W.Tsavorite garnet,king of African gems[J].Asia Precious,1998,6(3):26-28.
[4] Giuliani G,Fallick A E,Feneyrol J,et al.18 O/16 O and V/Cr ratios in gem tsavorites from the Neoproterozoic Mozambique metamorphic belt:A clue towards their origins?[J].Mineralium Deposita,2011,46(7):671-676.
[5] Mercier A,Moine B,Delorme J,et al.A note on a new occurrence of vanadian grossular garnet from Madagascar[J].Journal of Gemology,1997(25):391-393.
[6] Pardieu V,Hughes R W.Tsavorite-the untamed beauty[J].InColor,2008(9):12-20.
[7] Gübelin E J,Weibel M.Green vanadium grossular garnet from Lualenyi,near Voi,Kenya[J].Lapidary Journal,1975(29):202-414,424-426.
[8] Kane R E,Kampf A R,Krupp H.Well-formed tsavorite gem crystals from Tanzania[J].Gems & Gemology,1990,26(2):142-148.
[9] Muije P,Muije C S,Muije L E.Colorless and Green Grossularite From Tanzania[J].Gems & Gemology,1979(16):162-173.
[10] Adamo I,Diella V,Pezzotta F.Tsavorite and other Grossulars from Itrafo,Madagascar[J].Gems & Gemology,2012,48(3):178-187.
[11] Feneyrol J,Giuliani G,Ohnenstetter D,et al.New aspects and perspectives on tsavorite deposits[J].Ore Geology Reviews,2013(53):1-25.
[12] Key R M,Hill P G.Further evidence for the controls on the growth of vanadium grossular garnets in Kenya[J].Journal of Gemology,1989,21(7):412-422.
[13] Osanai Y,Ueno T,Tsuchiya N,et al.Finding of vanadium-bearing garnet from the Sør Rondane Mountains,East Antarctica[J].Antarctic Record,1990(34):279-291.
[14] Jackson B.Vanadian grossular garnet(tsavorite)from Pakistan[J].Journal of Gemmology.1992,23(2):67-70.
[15] Emerson E,Darley J.Featured Gem News International:Tsavorite and other green garnets reportedly from Afghanistan[EB/OL].Gems & Gemology,2010,(2):1.
[16] Themelis T.Gems and mines of Mogok[M].Thailand:Ted Themelis,2008:352.
[17] Uher P,Kovacik M,Kubis M,et al.Metamorphic vanadian-chromian silicate mineralization in carbon-rich amphibole schists from the MaléKarpaty Mountains,Western Carpathians,Slovakia[J].American Mineralogist,2008,93(1):63-73.
[18] Benkerrou C,Fonteilles M.Vanadian garnets in calcareous metapelites and skarns at Coat-an-Noz,Belle-Isle-en-Terre(Côtes du Nord),France[J].American Mineralogist,1989,74(7-8):852-858.
[19] Wight W,Grice J D.Grossular garnet from the Jeffrey mine,Asbestos,Quebec,Canada[J].Journal of Gemology,1982(18):126-130.
[20] Pan Y M,Fleet M E.Mineral chemistry and geochemistry of vanadian silicates in the Hemlo gold deposit,Ontario,Canada[J].Contributions to Mineralogy and Petrology,1992,109(4):511-525.
[21] Powell W G,Pattison D R M,Johnston P.Metamorphic history of the Hemlo gold deposit from Al2SiO5mineral assemblages,with implications for the timing of mineralization[J].Canadian Journal of Earth Sciences,1999,36(1):33-46.
[22] Mueller A G,Delor C P.Goldmanite-rich garnet in skarn veins,Southern Cross greenstone belt,Yilgarn block,Western Australia[J].Mineralogical Magazine,1991,55(4):617—620.
[23] Simonet,C,Okundi S,Masai P.General setting of coloured gemstone deposits in the Mozambique Belt of Kenyapreliminary considerations[M].Nairobi:Proceedings 9th Conference of the Geological Society of Kenya,2000:123-138.
[24] Malisa E P,Muhongo S.Tectonic setting of gemstone mineralization in the Proterozoic metamorphic terrane of the Mozambique Belt in Tanzania[J].Precambrian Research,1990,46(1-2):167-176.
[25] Feneyrol J,Giuliani G,Ohnenstetter D,et al.Lithostratigraphic and structural controls of‘tsavorite’deposits at Lemshuku,Merelani area,Tanzania[J].Comptes Rendus Geoscience,2010,342(10):778-785.
[26] Suwa K,Suzuki K,Agata T.Vanadium grossular from the Mozambique metamorphic rocks,south Kenya[J].Journal of Southeast Asian Earth Science,1996,14(3-4):299-308.
[27] Gübelin E J,Weibel M.Gruner vanadium grossular,Kenya[J].Lapis,1977(4):17-21.
[28] Kievlenko E Y.Geology of gems[M].USA:Ocean Pictures Ltd.,2003:185.
计量
- 文章访问数: 119
- HTML全文浏览量: 22
- PDF下载量: 24
下载:
