VNUJournalofScience,EarthSciences23(2007)152‐158 152 Characteristicsofcorundums fromPhuocHiepoccurrence(QuangNamProvince) NguyenNgocKhoi,NguyTuyetNhung, NguyenThiMinhThuyet*,PhanVanQuynh CollegeofScience,VNU Received24June2007 Abstract. Phuoc Hiep corundum occurrence has been discovered recently in the Central partof Vietnam (Quang Nam Province). Common protoliths in the region are high‐grade metamorphic rocks(typicallyofamphibolite‐granulitefacies),suchasgarnet‐bearingamphibolites,gneissesand schists,intruded byigneous formationsof differentcompositionand age.To date, the rubies and fancysapphireshavebeenfoundinsecondary,mostlyalluvialdeposits. Thecorundumcrystalsoccurasbroken,frommoderate‐towell‐rounded,fragments,butremains oftheoriginalhabitsareoftenpresent.Theirsizerangesfrom3×5mmto20×30mm,sometimesto 100×120mm. Mostofcorundumpiecesareofpurplishpinktopurplishred,withasmallprop ortion oflighttodarkblue. 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),colorzoningandsecondary,fluid,ʺfingerprintʺinclusions. Based on the similarity in tectonic settings, geological environments, gemological and geochemical characteristics of corundums, Phuoc Hiep occurrence can be grouped into the metapelite‐hostedtypeofcorundumdepositsinVietnam,whichincludeTanHuongandTrucLau depositsintheNorthVietnam. Keywords:Corundum;Geologicalsettings;Crystallography;Gemologicalproperties;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 _______ * Correspondingauthor.Tel.:84‐4‐5587061. E‐mail:nmthuyet@vnu.edu.vn importance. The main ruby and sapphire miningareasinVietnamare: •LucYen,TanHuongandTrucLauinthe north; •QuyChauinthecentre; •DakTon,MaLam,DaBaninthesouth. Phuoc Hiep occurrence, which has been discovered recently, is located in Phuoc Son District, Quang Nam Province, approximately 70kmfromsouthwestofDaNangCity(Central NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 153 partofVietnam). Until now, no any data have been publishedonthecorundumsfromthisarea. 2.Geologicalsettings The locations, where rubies and sapphires were discovered, coincide with the tectonic originvalleys.Thesevalleysoccurasdepression 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 anditstributaries. 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(KhamDucFormation‐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,granitogneissesofChuLaiComplex andultramaficrocksofHiepDucComplexwere intrudedbymagmaticrocksofDienBinhComplex and younger magmatic formations, forming metasomatic zones with the characteristic, talc‐ tremolite‐act imolite‐asbestos‐serpentine‐garnet‐ ruby‐sapphire, mineralassemblage(Fig.1). All the geologic formations in the area belongtoKhamDuczonewhichextendsalong east‐west direction and which is influenced by stagesoftectonicactivitiesfrom LateProterozoic 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.GeneralizedgeologicalmapofPhuocHiepoccurrence. NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 154 3.CharacteristicsofPhuocHiepcorundum For implementing this research project, we surveyed Phuoc Hiep occurrencein November of2005year.Duringthis time,we hadselected corundumsamplesfromThuBonRiveralluvial sedimentsusingasiftin gpanandalsopurchased fromlocalindividualgemdiggers(Fig.2). Fig.2.LotsofPhuocHiepcorundumsselected fromthegemlocality. 3.1.Crystallographyandvisualappearance Most of the corundum pieces appeared rounded or tumbled, with no indication of the original crystal form. The crystals occur as brokenfragments,theoriginalhabits rarelyare 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). Thesizerangesfrom3×5mmto20×30mm, sometimesto100×120mm. Fig.3.Corundumcrystals fromPhuocHiepoccurrence. Fig.4.Lozenge‐shapedPhuocHiep corundumfragments. Fig.5.Differentcolorvarieties ofPhuocHiepcorundums. Majority ofstones, whichare selectedfrom this locality, are pink to light pink with violetish or brownish overcastted colorations. NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 155 Themaincolorvarietiesareredtopink,blueto light blue, some stones show irregular color distributions (color zoning, color spots, etc, ). Othercolorsaregrey,colorlessorsmoky(Fig.5). Almost all stones show low to medium transparency, although transparent pieces are found during the trip to this area. Low transparencyisduetohighfracturingofstones. 3.2.GemologicalProperties 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.MicroscopicFeatures 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(colorpatches,color spots,etc., see Fig. 5). Color of these patches may be blue to bluish,greytocolorless. 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 highfracturing(andlowtransparency)ofthese corundum swherefract ur esoccuralongtwinning planes parallel to rhombohedral faces of the crystalsaftertheirformation.Thisfeatureisvery similar to that of corundums from Tan Huong andTrucLaudepositsinNorthVietnam[6,7]. c.Inclusions Duetolowtransparencyofmostcorundum pieces,itwasverydifficulttoidentifyallkinds of inclusions. Nevertheless, mica and chlorite inclusions(Fig.7) havebeenidentifiedinsome corundum crystals. Frequently encountered are secondary fluid inclusions along cracks (ʺfingerprint sʺ), andʺiron stainsʺ as golden‐ brownishrustedpatches. 3.2.2.ChemicalProperties Qualitative X‐ray fluorescence analysis of somecorundumsamplesfromPhuocHieparea hasrevealedthepresenceofCr, FeandTias the main coloring trace elements. Because in Vietnam we have no facility for quantitative EDXRFanalysis,theircontentsfordifferentcolor varieties have been analyzed by EPMA method (Table2). Asitcanbeseenfromthistable,thepinkto redvarietycontainsgenerallyhighercontentof chromium (0.002‐0.276, av. 0.136wt%), some samples contain high content of iron, whereas Ti,V,Mnarefoundinminoramounts.Theblue to light blue variety appears to contain higher ironcontent(0.090‐0.964,av.0.327wt%). Table1.StandardgemologicalpropertiesofPhuocHiepcorundums ColorVariety Property Pinktolightred Bluetolightblue Pleochroism Definite,yellowishpink/violetishpink Definitetoweak,blue/greenishblue Fluorescence Long‐waveUV Short‐waveUV Definite,lightred Inerttoveryweak Inert Inert Absorptionspectra Chromium Cannotbeevaluated Refractiveindex 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 Specificgravity 3.90‐4.03 3.92‐4.02 NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 156 Table2.AveragecontentsoftraceelementsinPhuocHiepcorundumsbyEPMAanalysis Traceelements Colorvariety FeO Ga 2 O Ti 2 O V 2 O 5 Cr 2 O 3 MnO Pinktolightred (27points/3samples) 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 Bluetolightblue (11points/3samples) 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.DeformationtwinninginPhuocHiep corundum(nicol:+;d=1.2mm). chlorite mica Fig.7.ChloriteandmicainclusionsinPhuocHiep corundum(nicol:+;d=1.2mm). 3.2.3.SpectroscopicFeatures TorevealspectroscopicpropertiesofPhuoc 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 absorptionspectraofdifferentcolorvarietiesof PhuocHiepcorundumsareshown inFig.8‐10. Fig.8.UV‐VIS‐NIRabsorptionspectraobtainedfrom redtopinkvariety(samplePH4). Fig.9.UV‐VIS‐NIRabsorptionspectraobtainedfrom redtopinkvarietywithbluespot(samplePH6). Fig.10.UV‐VIS‐NIRabsorptionspectraobtained frombluespot(samplePH6). NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 157 Pink tored variety shows variousshades of red, pink, violetish to orangey pink that cause slightly different absorption patterns. Sample PH4 (pure pi nk) appears to show absorption peakscausedonlybyCr 3+ at412,556and694nm (Fig.9), while samplePH6 (lightredwith blue spot)showspeakscausedbothbyCr 3+ (412,556 and694nm)andFe 3+ (388nm,seeFig.9). WiththesamesamplePH6wealsoobtained spectrum for blue spot (Fig. 10). Besides the absorption pattern which contains bands and peakstypicalforbluesapphiresfromelsewhere (Fe 2+ /Ti 4+ band at 565 nm, Fe 3+ /Fe 3+ peak at 450 nm) we can see also an absorption peak causedbyCr 3+ at412nm. 3.3.Mineralassemblages Until now, no any outcrops of corundum‐ bearing hard rocks have been found in this area. Nevertheless, we have purchased some samplesof corundumsin matrixfromthelocal gem dealers (Fig. 11). The typical minera l assemblage,whichwecoulddetermineinthese specimens, is corundum, plagioclase and mica (Fig.11,12). Fig.11.Corundum,muscovite,plagioclase assemblage(d=1.5cm). 4.Discussionandconclusion The most prominent features of rubiesand sapphires from Phuoc Hiep occurrence are red to blue color, low to medium transparency, mediumto bigsize,lamellarand polysynthetic (deformation) twinning and secondary, fluid, ʺfingerprintʺinclusions. Fig.12.Corundum,plagioclaseassemblage (d=0.6mm,nicol:+). Based on the characteristics in tectonic settings,geologicalenvironmentandproperties of corundums, Phuoc Hiep occurrence can be grouped into the metapelite‐hosted type of corundumdepositsinVietnam[1,2,5,8]. Although Phuoc Hiep ruby and sapphires havelowtomediumquality,whicharesuitable 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 heattreatment technologyis applied[9]. Our results are only initial. Systematic and comprehensiveinvestigationshouldbedonein anearfuture. 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 NguyenNgocKhoi etal./VNUJournalofScience,EarthSciences 23(2007)152‐158 158 QGTD 05.01ʺEstablishment 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‐sillimaniteGneissfromnearPolgahawela, Ceylon,GeologicalMagazineXCVII(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‐ mineralsofVietnam,Hanoi,2003. [3] R.W. Hughes, Corundum: ruby and sapphire, WhiteLotus,1991. [4] R.E. Kane et al, Ruby and fancy sapphire from Vietnam,Gem&Gemology27(1991)136. [5] Nguyen Viet Y et al, On the forming origin of sapphireandrubyinVietnam,JournalofGeology B/23(2004)110(inVietnamese). [6] PhamVanLong,NguyenNgocKhoi,TranKim 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(inVietnamese). [8] G.J. Simandl, S. Paradis, Corundum in Alumina‐rich Metasediments, in Selected British Columbia Mineral Deposit Profiles, Volume 3, IndustrialMinerals, 1999. [9] T. Themelis, The heat treatment of ruby and sapphire,GemlabInc.,USA,1992. . VNUJournal of Science,EarthSciences23(2007)152‐158 152 Characteristics of corundums from Phuoc Hiep occurrence (Quang Nam Province) NguyenNgocKhoi,NguyTuyetNhung, NguyenThiMinhThuyet*,PhanVanQuynh College of Science,VNU Received24June2007 Abstract. Phuoc . Characteristics of corundums from Phuoc Hiep occurrence (Quang Nam Province) NguyenNgocKhoi,NguyTuyetNhung, NguyenThiMinhThuyet*,PhanVanQuynh College of Science,VNU Received24June2007 Abstract. Phuoc Hiep corundum occurrence has been discovered recently in the Central part of Vietnam (Quang Nam Province). . tectonic settings,geologicalenvironmentandproperties of corundums, Phuoc Hiep occurrence can be grouped into the metapelite‐hosted type of corundumdepositsinVietnam[1,2,5,8]. Although Phuoc Hiep ruby and