American Mineralogist, Volume 58,pages 727-732, 1973
The Toughnessof Jade
Rrcnnno C. Bnlnr.
CeramicScienceSection,Moterial SciencesDepartment
RosEnr E. NrwNHnM, ANDJ. V. Blccens
Materials ResearchLaboratory
The PennsyluaniaState Uniuersity,UniuersityPark, Pennsyluania16802
Abstract
The mechanical properties related to the toughness of jade are measured for both jadeite
and nephrite. Fracture surface energies are an order of magnitude greater than most commercial ceramics about 120,000 ergs/cm' for jadeite and 225,00O ergslcm' for nephrite.
Iadeite is the harder of the two minerals, but nephrite is the tougher and the stronger.
Scanning electron microscopy of the fracture surfaces indicates that the exceptional toughness
results from the fibrous texture of nephrite and extensive transgranular cleavage of the blocky
microstructure of jadeite.
Introduction
jades
Ornamental
include two different minerals,
jadeite and nephrite. Nephrite is an amphibole in
the tremolite-actinolite
series,Ca2(Mg,Fe)s(Si+Orr)z
(OH)2, derived from alpine-typeperidotite-dunite
intrusives. The rarer jadeite is a pyroxene of composition NaAlSizOo,found in Burma as stream-worn
boulders.Jadeiteis the harder of the two minerals,
7 on Mohs' scalecomparedto 6Vz, aTthoughnephrite, "the axe stone," is generallyconsideredto be
tougher and more resistantto fracture.
Beauty and durability are the essentialattributes
of a gemstone.Durability requires both hardness
and toughnessin order to withstand abrasionand
impact. Hardnessand toughnessare physicallyirreversibleproperties,since their tests leave the material in a pennanently altered condition. Although
scientists and engineers generally describe these
propertiesin a rather qualitative fashion, there are,
in fact, specific quantitativemethods of measurement.To the mineralogist,hardnessmeansresistance
to scratchingand as suchmay be empirically defined
by the Mohs' scale.Hardnessmay alsobe measured
by indentationusing the Vickers or Brinell tests.
The three technqiuesusually agree on the relative
ranking of differentmaterials,althoughthe numerical
scalesappearquite different (Taylor, 1,949).
Qualitatively, toughnessmeans the resistanceto
breakage.Quantitatively,it can be describedby two
related parameters,the work of fracture or fracture
surface energy, and the fracture toughness(Tetelman and McEvily, 1967). Fracture surface energy
is the amount of energy required to create a unit
area of fracture surface. Fracture toughness,designated K1" for the openingmode of fracture common
to brittle materials.is a measureof the resistanceof
the material to unstable crack propagationor fracture. Fracture toughnessis derived by analyzingthe
stressdistribution at a crack's tip and is equal to the
square root of twice the product of the elastic
modulus and the fracture surface energy of the
material. Often, as is the casein this study, fracture
toughnessis reported as just K when the entire
fracture processmay not be an opening mode.
Among mineralogiststhere is no widely accepted
scale of toughness,although some minerals such as
jade are regarded as much tougher than others.
Becauseof their exceptionaltoughness,jade boulders are often resistant to breakagewith a hammer
and may be frequently found in the nearly pure
state,sinceweatheringhas worn away the surrounding minerals.Early man utilized jade extensivelyfor
making tools, finding it almost ideal to shapewithout fracture. This sentiment has been shared for
centuriesby Chinesecraftsmenwho have given the
world many exquisite jade carvings, shaped to extremely delicateforms. It is the purposeof this study
of severaljades
the toughness
to quantitativelyassess
and to compare them with synthetic materials and
with other minerals.
727
R. C. BRADT, R. E. NEWNHAM,
Experimental
The nephrite jades studied in this investigation
were obtained from several American dealers and
are thought to originate from Russia and Alaska,
while the jadeite specimenswere obtained from
streetshopsin Hong Kong, and are probably Burmese in origin. The nephrites are a translucent
mottledgreenwith a specificgravity of 3.01, indicating appreciableiron content. X-ray diftraction patterns are in good agreementwith the tremolite
pattern (Stempleand Brindley, 1960). The texture
(Fig. 1) of the matted fibers is characteristicof
nephrite. The fibers vary widely in thickness and
length,but are generallylessthan 10pmin diameter,
aspectratios from 10 to 50.
with length-to-diameter
In some regions,the fibers show a strong preferred
orientation, being grouped into orderly bundles,
while other regions exhibit a whirlpool-like appearanceof a more random nature.
The jadeites vary in color from a pale green to
icy white, with numerous glints of reflected light
Fto. l. Photomicrographsof nephrite (top) and jadeite.
Note the matted fibrous texture of the nephrite, and compare it with the blocky grain texture of the jadeite.
AND I. V. BIGGERS
indicatingthe presenceof large crystallites(Fig. 1).
The individual crystallitesize is much larger than
the nephrite fibers, generallywithin the 50 to 100p.m
range.The grains exhibit a blocky morphologywhen
compared to the nephrite fibers. Jadeite densities
are about 10 percentgreaterthan nephrite,generally
about 3.33 gms/cm3.The X-ray diffractionpatterns
are identical with those reported for NaAlSi2O6
(Yoder, 1950).
To comparetoughnesson a quantitativebasis,it
is necessaryto measurecertain mechanicalproperties such as elastic modulus and fracture surface
'1.f8"
energy. Strengths are measured by breaking
square bars in three point flexure over a 0.4 inch
span with an Instron testing machine operated at
0.02 inchesper minute cross-headspeed.Fracture
strength, o1, is calculated from the standard engineering-mechanics
formula:
"':*k'
(r)
where P is the breaking load, and L, b, and h are
the specimen'slength, breadth, and height dimensions (Marin, 1962).
The elastc moduli are measured by a flexural
resonancetechniqueon /q" squareby 3" long bars
employing a Nametre Model XII B acoustic spectrometer. This involves vibrating the sample in a
flexural mode with an oscillator and observing the
natural resonanceof the specimen.The resonant
frequenciesdepend on the sample dimensionsand
elasticmodulus (Spinnerand Teft, 1961), and are
usually in the audio-frequencyrange. Damping capacities of the jades are determinedfrom the resonance peaks by the bandwidthmethod (Kennedy,
t963).
Fracture surface energies are evaluated by the
work of fracturetechnique(Nakayama,1965). The
work required to propagate a stable crack over a
specific area is measured using rectangular bars
similar to thoseused in the strengthmeasurements.
The bars are notched at the midsection with a
0.010' diamond blade, leaving a triangular cross
section for the fracture area. After fracturing the
bars in a non-catastrophic,stable fashion on the
speedof 0.002 inches
testingmachineat a crosshead
per minute, the work, or energy, of fracture is determined by integration of the force-displacement
curve. Surfacefracture energy is then calculatedby
dividing the work by twice the triangular cross-
729
TOUGHNESS OF ]ADE
section area, since two surfacesare formed during
the fracture process.A comparisonof this technique
with other methodshas been given (Coppola and
Bradt, 1972). The fracture surface energy and the
strengthresults are reported as the averageswith the
95 percentconfidencelimits as describedby the "t"
distribution. Thin-sections of jadeite and nephrite
were also examined optically on a standard petrographic microscopeand the fracture surfacesphotographed with a scanning electron microscope after
coating the samples with a thin layer of vapordepositedgold to promote conductivity.
Tesr.s l. Mechanical Properties of Jades
J a d e1 t e
Mqdulus,
Efastic
(dynes,/cn')
E
Danplng
qtrength,
Fracture
(dynes,/cm')
or,
t.3 x 1012
2,0 x Lo12
-?
1 x 10 -
-?
1.5 x 10 -
z.tz+0.55 x :ro9
1.02+0.21x109
Fracture surlace Energy, 226,000+155,000
1r, (er|s/cmz )
121'000+32,000
Fracture Tqughness, K^'
( d y n e s , zc m - J / 2 )
7.1
7.7 x 108
x
R
10-
Scanningelectronmicrographsof the fracture surfaceof jadeite (Fig. 2) reveala very high percentage
of transgranularcleavagefractures.That is, when a
crackpropagatesthroughjadeite,it doesnot proceed
in an intergranularfashion following the boundaries
between the grains, rather it proceeds directly
through the grains in a transgranularfracture mode.
Cleavagestep patternsare especiallyobviousat the
higher magnification,indicatingthat the fracture is
almost wholly restricted to certain crystallographic
planes.The elongatednature of someof thesecleavage stepsstrongly suggeststhat the fracture is preferentially occurring parallel to the pyroxene chains,
probably {110} planes. It is this extensivetransgranular cleavagefracture mode that imparts high
toughnessto jadeite.
Nephrite has a vastly different appearingfracture
topography (Fig. 3). Fibers and bundlesof fibers
can be seen protruding from the surface,even at
low magnification.The random orientation of in-
Results and Discussion
The mechanicalpropertiesof jadeiteand nephrite
are listed in Table 1. Although jadeite has the
higher elastic modulus, nephrite is superior in
strength,o1,fracture surfaceenergy,7r, ond fracture
toughness,IQ, substantiating the general opinion
that nephrite is tougher than jadeite. The confidence
limits for the fracture surface energies and the
toughnessesare comparable to that observed for
most brittle materials, with the exception of the
fracture surface energiesof the nephrite. A wide
range of scatter was observedon these specimens,
with occasionalvaluesnear 400,000ergs/cm'; however, no obvious correlationswith place of origin or
structural featureswere apparent.The jade fracture
surfaceenergiesand calculatedfracture toughnesses
are compared with other polycrystalline and single
crystal mineralsin Table 2. It is apparentthat jade
is deservingof its reputation as a very tough material. Comparing these results with the fracture
Tesr-e 2. Comparison of Toughness
surface energiesof other materials (Duga, 1969)
indicatesthat jade is about an order of magnitude
Fracture
FractureSurface
tougherthan most ceramicmaterials.
Toughnessr (^
Energyr y.
Jade mineralsare chain silicates,and the difference in their averagefracture surface energiessug121,000
7f x 107
Jade 1te
geststhat perhapsa network silicate, such as quartz
226,0OO
77 x Io7
Nephrlt e
or glass,might be even tougher.This conceptproves
4,32A
7 xto7
Quart z i te
quite erroneous,for the reported fracture surface ( l , l l e d e r h o r n , 1 9 6 9 b )
energiesof quartzite(seeTable 2) and glass(Wie- Alunlna
15,000-50,ooo
35-64 x :'O7
(Gutshal1 & Gross, 1969)
derhorn, I969c) are only about 5,000 ergs/cm2,or
20,000
35 x Io7
Maqnesla
less. On the same scale of fracture toughnessas ( C l a r k e e t a 1 . , 1 9 6 6 )
Table 2, both glassand quartz are only about 7 x
tr x to7
Mlca ( 001)
10'dyne cm-3/2.
Thesecomparisonssuggestthat the ( B r y a n t e t a l . , 1 9 6 3 )
r,030
high fracture surface energy and fracture toughness Q u a r t z ( l o T o )
5xto7
(Brace and Walsh, 1962)
jade
to
of
are not directly related the atomic bond600
7xIa7
corundum (lofl)
ing per se, but are apparentlyrelated to texture and ( t / 1 e d e r h o r n , 1 9 6 9 a )
L,2oo
9 x 107
the restrictionswhich it imposeson crack propaga- P e r l c l a s e ( 1 0 0 )
(westwood and Go1dhelm, 1963)
tion.
730
R. C, BRADT, R. E. NEWNHAM,
AND I. V. BIGGERS
tions; however,a correlation of thesetextural parameterswith toughnessundoubtedlyexists,and is probably the main criterion by which jade carvers are
able to select particularly "tough" jade specimens.
While the exact contribution of a fibrous texture to
increasedtoughnessis not well understood,it has
*4
FIc. 2. Scanning electron micrographs of the fracture
surfaces of jadeite. Note the extensive transgranular cleavage characteristics of the crack propagation.
dividual fibers and the wide distribution of fiber
sizesbecomesapparentat higher magnification.It is
these variations in fiber and fiber bundle sizes and
in orientationsthat are probably responsiblefor the
variations in fracture energy reported in Table 1.
No attempt was made to characterizethe nephrites
on the basis of fiber or fiber bundle size distribu-
Frc. 3. Scanning electron micrographs of the fracture
surfaces of nephrite. Note the fibrous nature of the fracture surface, the difference in fiber sizes, the occasional
bundles of fibers, and the regions of random orientation.
73r
TOUGHNESS OF JADL
recently become a well documentedphenomenon,
having been reported in fiber composites (Lynch
and Kershaw, 1972), in solidified eutectic alloys
(Piekarski and Helmer, 1972), in silicon nitride
(Lange, 1972) and in silicon carbide (Coppola and
Bradt, 1972\.
There appearto be severalmechanismsby which
transgranularcleavage,such as evidencedin the
jadeite, and the fibrous texture, such as that of the
nephrite, might increasefracture surfaceenergy and
enhance toughness.It almost seems contradictory
to considercleavageas a tough process,particularly
with referenceto mica. However, mica is a single
crystalwhere the cleavageplanestraversethe entire
specimen,whereas the cleavageplanes in polycrystalline jadeite traverse only a single grain' A
propagatingcrack in polycrystallinejadeite must
change direction to a differently oriented cleavage
plane each time it crossesa grain boundary. This
directionalchangecan be up to 47o in jadeite and
very likely causes secondary cracks or fractures
which consumeadditional energy. (Ffiedman et a:1,
1972) These directionalchangesalso result in a
zig-zagfracture path that yields a roughenedfracture
surface, so that the real fracture surface area is
greater than the apparent fracture surface area.
(Hoagland et al, l97I) This increaseof surface
area, probably only about 50 percent, does not
appearto be as important as secondarycrackingor
crack-branchingin jadeites.
Nephrite fractures in accordancewith its fibrous
texture, a structural feature that has already been
cited to enhancetoughness.Severalfactorsmay be
responsiblefor this high fracture surfaceenergy,one
of which is undoubtedly the high ratio of real-toapparent fracture-surfacearea. This contribution is
most certainly greater in nephrite than in jadeite.
However,probably more important are the frictional
effects when individual fibers or bundles of fibers
are extractedduring fracture.Furthermore,there is
little doubt that the propagatingcracks in nephrite
are often forced to make drastic directional changes
to permit the fiber pullouts, so secondary crack
branchingalmostcertainlyoccurs.Fresentlyit is not
possibleto define clearly which processcontributes
most to the jade'shigh fracturesurfaceenergies,but
its reputationas a
it is clear that jade fully deserves
very tough mineral.
In regard to the other mechanical properties of
jade, the lower elasticmodulusof nephriteis almost
certainly a result of its lower density;however,its
magnitude is still comparable to commercial ceramics and about twice that of common glass. It
rock has a
appears(Fig. 1) that the jadeite-bearing
secondaryphase between the jadeite grains, acting
as a bond between crystallites; however, no secondary phaseswere identified in the X-ray patterns
or thin sections, and the relatively high modulus
and low damping suggestthat any secondaryphase
is a strong, coherentone. The relatively low damping suggestsgood structural integrity in both jades,
with relatively little po'rosity.
The strengthsof the two jades exceedmost commerically available ceramics;however, they are not
comparableto the ultra high strength of hot'pressed
oxidesand nitrides usedfor cutting tools and turbine
vanes.The Griffith relation, a, : k\/ Ett/c, offers
an explanationof the strengths.This equationrelates
the fracture strength or, to the elastic modulus E,
the fracture surface energy 71, &[d the flaw sizn c,
through a proportionality constant k. (Kingery,
1960).From the data in Table l, it is apparent that
nephrite has the higher fracture surfaceenergy and
strength,eventhough the modulusis lower. However,
the quantity t/Ett in the Griffith equation accounts
for only about l0 percentof the observeddifferences
in the jade fracture strengths.The remainder may
be attributed to the smallerflaw sizein the nephrite.
Flaw size is generallyrelated to the size of microtextural features,and in this casethe nephrite fiber
diameter is considerably smaller than the jadeite
grain size.
Acknowledgment
The authors would like to acknowledge the contributions
of G. Gardopee, T. Cline, R. Wolfe, and J' Lebiedzik of the
Pennsylvania State University for assistance in the experimental work, and Hok Shing Liu of Chung Hing Mining'
Hong Kong, and the Cheng-Kung University, Tainan, Taiwan, ROC, and N. Lambert for assistancein obtaining the
jades.
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Manuscript receiued, October 2, 1972;
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