Báo cáo "Characteristics of corundums from Phuoc Hiep occurrence (Quang Nam Province) " potx

VNUJournalofScience,EarthSciences23(2007)152‐158 152 Characteristicsofcorundums fromPhuocHiepoccurrence(QuangNamProvince) NguyenNgocKhoi,NguyTuyetNhung, NguyenThiMinhThuyet*,PhanVanQuynh CollegeofScience,VNU Received24June2007 Abstract. Phuoc Hiep corundum occurrence has been discovered recently in the Central partof Vietnam (Quang Nam Province). Common protoliths in the region are high‐grade metamorphic rocks(typicallyofamphibolite‐granulitefacies),suchasgarnet‐bearingamphibolites,gneissesand schists,intruded byigneous formationsof differentcompositionand age.To date,  the rubies and fancysapphireshavebeenfoundinsecondary,mostlyalluvialdeposits. Thecorundumcrystalsoccurasbroken,frommoderate‐towell‐rounded,fragments,butremains oftheoriginalhabitsareoftenpresent.Theirsizerangesfrom3×5mmto20×30mm,sometimesto 100×120mm. Mostofcorundumpiecesareofpurplishpinktopurplishred,withasmallprop ortion oflighttodarkblue. For the first time, the gemological characteristics of Phuoc Hiep corundums are described in detail in this paper. Common gemological properties of Phuoc Hiep corundums are normal for rubies and sapphires  elsewhere in the world. The most notable features are lamellar and polysynthetic (deformation) twinning, low transparency (do to high fracturing along twinning planes),colorzoningandsecondary,fluid,ʺfingerprintʺinclusions. Based on the similarity in tectonic settings, geological environments, gemological and geochemical characteristics of corundums, Phuoc Hiep occurrence can be grouped into the metapelite‐hostedtypeofcorundumdepositsinVietnam,whichincludeTanHuongandTrucLau depositsintheNorthVietnam. Keywords:Corundum;Geologicalsettings;Crystallography;Gemologicalproperties;Mineral assemblage. 1.Introduction *  Vietnam now is considered as a country with big potential for gemstones. Many kinds of gemstones have been found and mined in different areas of the country. Among these gemstones, ruby and sapphire are of most _______ * Correspondingauthor.Tel.:84‐4‐5587061. E‐mail:nmthuyet@vnu.edu.vn importance. The main ruby and sapphire miningareasinVietnamare: •LucYen,TanHuongandTrucLauinthe north; •QuyChauinthecentre; •DakTon,MaLam,DaBaninthesouth. Phuoc Hiep occurrence, which has been discovered recently, is located in Phuoc Son  District, Quang Nam Province, approximately 70kmfromsouthwestofDaNangCity(Central NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 153 partofVietnam). Until now, no any data have been publishedonthecorundumsfromthisarea. 2.Geologicalsettings The locations, where rubies and sapphires were discovered, coincide with the tectonic originvalleys.Thesevalleysoccurasdepression belts along the fault systems. Investigation works in the area have shown that rubies and sapphires are found in ground, alluvial and first‐order terrace sediments of Thu Bon River anditstributaries. The area of the Phuoc Hiep occurrence is  where are distributed metamorphosed volcano‐ sediment ary formations, which include garnet‐ bearing amphibolites, biotite plagiogneisses, quartz‐plagioclas‐biotite schists with disthene, quartz ‐fe ld spa r‐biotite‐pyroxene‐garnet,actinolite ‐tremolite, talc‐tremolite schists, tremolite marbles(KhamDucFormation‐PR 2 kd).These rocks were metamorphosed to amphibolite‐ granulite facies with widespread development of migmatization processes. The magmatic rocks include granitogneisses (Chu Lai Complex‐γPR 2 cl), apodunites, apoperidotites, apopyroxenites (Hiep Duc Complex‐δPZ hd), granodiorites, leucogranites, pegmatites (Dien Binh Complex‐γδPZ 1 db) and younger formations. The metamorphic rocks of Kham Duc Formation,granitogneissesofChuLaiComplex andultramaficrocksofHiepDucComplexwere intrudedbymagmaticrocksofDienBinhComplex and younger magmatic formations, forming metasomatic zones with the characteristic, talc‐ tremolite‐act imolite‐asbestos‐serpentine‐garnet‐ ruby‐sapphire, mineralassemblage(Fig.1). All the geologic formations in the area belongtoKhamDuczonewhichextendsalong east‐west direction and which is influenced by stagesoftectonicactivitiesfrom LateProterozoic until present. The gemstone occurrence is controlled by main, northwest‐southeast, fault systems (oldest) and meridian fault  systems (youngest). Milonitization zone + + + + + + + Plagioclasite Plagioclasite Phuoc Son town LAO T h u B o n r i v e r (Chu Lai granite) P o k o f a u l t Mica schist Mica schist Phuoc Hiep corundum area Sericite schist β Q z z z Fig.1.GeneralizedgeologicalmapofPhuocHiepoccurrence. NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 154 3.CharacteristicsofPhuocHiepcorundum For implementing this research project, we surveyed Phuoc Hiep occurrencein November of2005year.Duringthis time,we hadselected corundumsamplesfromThuBonRiveralluvial sedimentsusingasiftin gpanandalsopurchased fromlocalindividualgemdiggers(Fig.2).    Fig.2.LotsofPhuocHiepcorundumsselected fromthegemlocality. 3.1.Crystallographyandvisualappearance Most of the corundum pieces appeared rounded or tumbled, with no indication of the original crystal form. The crystals occur as  brokenfragments,theoriginalhabits rarelyare observed wh ich resemble short hexagonal prisms (Fig. 3). Some pieces are lozenge‐ shaped, broken fragments, formed parallel to  rhombohedral faces of the crystals (due to deformation)(Fig.4). Thesizerangesfrom3×5mmto20×30mm, sometimesto100×120mm.  Fig.3.Corundumcrystals fromPhuocHiepoccurrence.  Fig.4.Lozenge‐shapedPhuocHiep corundumfragments.  Fig.5.Differentcolorvarieties ofPhuocHiepcorundums. Majority ofstones, whichare selectedfrom this locality, are pink to light pink  with violetish or brownish overcastted colorations. NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 155 Themaincolorvarietiesareredtopink,blueto light blue, some stones show irregular color distributions (color zoning, color spots, etc, ). Othercolorsaregrey,colorlessorsmoky(Fig.5). Almost all stones show low to medium transparency, although transparent pieces are found during the trip to this area. Low transparencyisduetohighfracturingofstones. 3.2.GemologicalProperties For the first time, gemological properties of two ma in color varieties of Phuoc Hiep corundums are given (Table 1). They were found to be consistent with corundum in general[3,4,6,7]. 3.2.1.MicroscopicFeatures a.Color zoning Color zoning is not a typical feature of Phuoc Hiep corundums. In some pieces it is observed in the form of irregular color distribution(colorpatches,color spots,etc., see Fig. 5). Color of these patches may be blue to bluish,greytocolorless. b.Twinning Deformation twinning (Fig. 6), which occurred after corundum crystals had formed, is very characteristic for corundums from this area. This phenomenon is the main cause of highfracturing(andlowtransparency)ofthese corundum swherefract ur esoccuralongtwinning planes parallel to rhombohedral faces of the crystalsaftertheirformation.Thisfeatureisvery  similar to that of corundums from Tan Huong andTrucLaudepositsinNorthVietnam[6,7]. c.Inclusions Duetolowtransparencyofmostcorundum pieces,itwasverydifficulttoidentifyallkinds of inclusions. Nevertheless, mica and chlorite inclusions(Fig.7) havebeenidentifiedinsome corundum crystals. Frequently encountered are secondary fluid  inclusions along cracks (ʺfingerprint sʺ), andʺiron stainsʺ as golden‐ brownishrustedpatches. 3.2.2.ChemicalProperties Qualitative X‐ray fluorescence analysis of somecorundumsamplesfromPhuocHieparea hasrevealedthepresenceofCr, FeandTias the main coloring trace elements.  Because in Vietnam we have no  facility for quantitative EDXRFanalysis,theircontentsfordifferentcolor varieties have been analyzed by EPMA method (Table2). Asitcanbeseenfromthistable,thepinkto redvarietycontainsgenerallyhighercontentof chromium (0.002‐0.276, av. 0.136wt%), some samples contain high content of iron, whereas Ti,V,Mnarefoundinminoramounts.Theblue to light blue variety appears to contain higher ironcontent(0.090‐0.964,av.0.327wt%). Table1.StandardgemologicalpropertiesofPhuocHiepcorundums ColorVariety Property Pinktolightred Bluetolightblue Pleochroism Definite,yellowishpink/violetishpink Definitetoweak,blue/greenishblue Fluorescence Long‐waveUV Short‐waveUV  Definite,lightred Inerttoveryweak  Inert Inert Absorptionspectra Chromium Cannotbeevaluated Refractiveindex n o  n e   1.768‐1.774 1.760‐1.766  1.770‐1.772 1.762‐1.764 Birefringence 0.008‐0.010 0.008‐0.009 Specificgravity 3.90‐4.03 3.92‐4.02 NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 156 Table2.AveragecontentsoftraceelementsinPhuocHiepcorundumsbyEPMAanalysis Traceelements Colorvariety FeO Ga 2 O Ti 2 O V 2 O 5  Cr 2 O 3  MnO Pinktolightred (27points/3samples) 0.087‐0.175 Mean:0.125 0‐0.027 Mean:0.003 0.006‐0.043 Mean:0.020 0‐0.087 Mean:0.005 0.002‐0.276 Mean:0.136 0‐0.021 Mean:0.006  Bluetolightblue (11points/3samples) 0.090‐0.964 Mean:0.327 0‐0.021 Mean:0.005 0.001‐0.482 Mean:0.096 0.001‐022 Mean:0.006 0.006‐0.528 Mean:0.177 0‐0.011 Mean:0.007  Fig.6.DeformationtwinninginPhuocHiep corundum(nicol:+;d=1.2mm). chlorite mica  Fig.7.ChloriteandmicainclusionsinPhuocHiep corundum(nicol:+;d=1.2mm). 3.2.3.SpectroscopicFeatures TorevealspectroscopicpropertiesofPhuoc Hiep corundums, we have investigated their absorption spectra using Shimadzu UV‐VIS spectrophotometer (model UV‐2450) supplied by Center for Material Sciences (College of Science, VNU). The characteristic UV‐VIS absorptionspectraofdifferentcolorvarietiesof PhuocHiepcorundumsareshown inFig.8‐10.  Fig.8.UV‐VIS‐NIRabsorptionspectraobtainedfrom redtopinkvariety(samplePH4).  Fig.9.UV‐VIS‐NIRabsorptionspectraobtainedfrom redtopinkvarietywithbluespot(samplePH6).  Fig.10.UV‐VIS‐NIRabsorptionspectraobtained frombluespot(samplePH6). NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 157 Pink tored variety shows variousshades of red, pink, violetish to orangey pink that cause slightly different absorption patterns. Sample PH4 (pure pi nk) appears to show absorption peakscausedonlybyCr 3+ at412,556and694nm  (Fig.9), while samplePH6 (lightredwith blue spot)showspeakscausedbothbyCr 3+ (412,556 and694nm)andFe 3+ (388nm,seeFig.9). WiththesamesamplePH6wealsoobtained spectrum for blue spot (Fig. 10). Besides the absorption pattern which contains bands and peakstypicalforbluesapphiresfromelsewhere (Fe 2+ /Ti 4+  band at 565 nm, Fe 3+ /Fe 3+  peak at 450 nm) we can see also an absorption peak causedbyCr 3+ at412nm. 3.3.Mineralassemblages Until now, no any outcrops of corundum‐ bearing hard rocks have been found in this area. Nevertheless, we have purchased some samplesof corundumsin matrixfromthelocal gem dealers (Fig. 11). The typical minera l assemblage,whichwecoulddetermineinthese specimens, is corundum, plagioclase and mica (Fig.11,12).  Fig.11.Corundum,muscovite,plagioclase assemblage(d=1.5cm). 4.Discussionandconclusion The most prominent features of rubiesand sapphires from Phuoc Hiep occurrence are red to blue color, low to medium transparency, mediumto bigsize,lamellarand polysynthetic (deformation) twinning and secondary, fluid, ʺfingerprintʺinclusions.  Fig.12.Corundum,plagioclaseassemblage (d=0.6mm,nicol:+). Based on the characteristics in tectonic settings,geologicalenvironmentandproperties of corundums, Phuoc Hiep occurrence can be grouped into the metapelite‐hosted type of corundumdepositsinVietnam[1,2,5,8]. Although Phuoc Hiep ruby and sapphires havelowtomediumquality,whicharesuitable mostly for cabochons, but  with big size and quantity, they can be considered as a potential source of gem material for Vietnam, especially when appropriate heattreatment technologyis applied[9]. Our results are only initial. Systematic and comprehensiveinvestigationshouldbedonein anearfuture. Acknowledgements This paper presents the results of implementation of the Projects within the  framework of the Fundamental Research Program funded by Vietnam Ministry of Science and Technology, and the Key Project NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 158 QGTD 05.01ʺEstablishment of some gemstone deposit types having industrial potential in Vietnamʺ, funded by Vietnam National University,Hanoi. References [1] P.G. Cooray, P.S. Kumarapeli, Corundum in Biotite‐sillimaniteGneissfromnearPolgahawela, Ceylon,GeologicalMagazineXCVII(1960)480. [2] Ch.A. Hauzenberger, T. Hager, W. Hofmeister, V.X. Quang, G.W.A. Rohan Fernando, Origin and formation of gem quality corundum from Vietnam, Proceedings of the 2 nd  International Workshop on Geo‐ and Material‐Science on Gem‐ mineralsofVietnam,Hanoi,2003. [3] R.W. Hughes, Corundum: ruby and sapphire, WhiteLotus,1991. [4] R.E. Kane et al, Ruby and fancy sapphire from Vietnam,Gem&Gemology27(1991)136. [5] Nguyen Viet Y et al, On the  forming origin of sapphireandrubyinVietnam,JournalofGeology B/23(2004)110(inVietnamese). [6] PhamVanLong,NguyenNgocKhoi,TranKim Hai, Typomorphic characteristics of Vietnam rubies and sapphires, Final report of the Scientific Project, Vietnam Ministry of Industry, 2001 (in Vietnamese). [7] Pham Van Long, Research on the crystallo‐ mineralogical and gemological chara c teristics of rubies and sapphires from Luc Yen and Quy Chau deposits, PhD. Thesis, College of Science, VNU, 2003(inVietnamese). [8] G.J. Simandl, S. Paradis, Corundum in Alumina‐rich Metasediments, in Selected British Columbia Mineral Deposit Profiles, Volume 3, IndustrialMinerals, 1999. [9] T. Themelis, The heat treatment of ruby and sapphire,GemlabInc.,USA,1992. . VNUJournal of Science,EarthSciences23(2007)152‐158 152 Characteristics of corundums  from Phuoc Hiep occurrence (Quang Nam Province) NguyenNgocKhoi,NguyTuyetNhung, NguyenThiMinhThuyet*,PhanVanQuynh College of Science,VNU Received24June2007 Abstract. Phuoc . Characteristics of corundums  from Phuoc Hiep occurrence (Quang Nam Province) NguyenNgocKhoi,NguyTuyetNhung, NguyenThiMinhThuyet*,PhanVanQuynh College of Science,VNU Received24June2007 Abstract. Phuoc Hiep corundum occurrence has been discovered recently in the Central part of Vietnam (Quang Nam Province). . tectonic settings,geologicalenvironmentandproperties of corundums,  Phuoc Hiep occurrence can be grouped into the metapelite‐hosted type of corundumdepositsinVietnam[1,2,5,8]. Although Phuoc Hiep ruby and