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