American Mineralogist, Volume 58,pages 727-732, 1973 The Toughnessof Jade Rrcnnno C. Bnlnr. CeramicScience Section, Moterial SciencesDepartment RosEnr E. NrwNHnM, ANDJ. V. Blccens Materials ResearchLaboratory The PennsyluaniaState Uniuersity,Uniuersity Park, 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 com- mercial 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 surface energy, and the fracture toughness(Tetel- Ornamental jades include two different minerals, man and McEvily, 1967). Fracture surface energy jadeite and nephrite. Nephrite is an amphibole in is the amount of energy required to create a unit the tremolite-actinoliteseries, Ca2(Mg,Fe) s(Si+Orr )z area of fracture surface. Fracture toughness,desig- (OH)2, derived from alpine-typeperidotite-dunite nated K1" for the openingmode of fracture common intrusives. The rarer jadeite is a pyroxene of com- to brittle materials.is a measureof the resistanceof position NaAlSizOo,found in Burma as stream-worn the material to unstable crack propagationor frac- boulders.Jadeite is the harder of the two minerals, ture. Fracture toughnessis derived by analyzingthe 7 on Mohs' scalecompared to 6Vz, aTthoughneph- stressdistribution at a crack's tip and is equal to the rite, "the axe stone," is generallyconsidered to be square root of twice the product of the elastic tougher and more resistantto fracture. modulus and the fracture surface energy of the Beauty and durability are the essentialattributes material. Often, as is the casein this study, fracture of a gemstone.Durability requires both hardness toughnessis reported as just K when the entire and toughnessin order to withstand abrasionand fracture processmay not be an opening mode. impact. Hardnessand toughnessare physicallyir- Among mineralogiststhere is no widely accepted reversibleproperties, since their tests leave the ma- scale of toughness,although some minerals such as terial in a pennanently altered condition. Although jade are regarded as much tougher than others. scientists and engineers generally describe these Becauseof their exceptionaltoughness, jade bould- propertiesin a rather qualitative fashion, there are, ers are often resistant to breakagewith a hammer in fact, specific quantitativemethods of measure- and may be frequently found in the nearly pure ment.To the mineralogist,hardness means resistance state, sinceweathering has worn away the surround- to scratchingand as such may be empirically defined ing minerals.Early man utilized jade extensivelyfor by the Mohs' scale.Hardness may alsobe measured making tools, finding it almost ideal to shapewith- by indentationusing the Vickers or Brinell tests. out fracture. This sentiment has been shared for The three technqiuesusually agree on the relative centuriesby Chinesecraftsmen who have given the ranking of differentmaterials, although the numerical world many exquisite jade carvings, shaped to ex- scalesappear quite different (Taylor, 1,949). tremely delicateforms. It is the purposeof this study Qualitatively, toughnessmeans the resistanceto to quantitativelyassess the toughnessof severaljades breakage.Quantitatively, it can be describedby two and to compare them with synthetic materials and related parameters,the work of fracture or fracture with other minerals. 727 R. C. BRADT, R. E. NEWNHAM, AND I. V. BIGGERS Experimental indicatingthe presenceof large crystallites(Fig. 1). The nephrite jades studied in this investigation The individual crystallitesize is much larger than were obtained from several American dealers and the nephrite fibers, generallywithin the 50 to 100p.m are thought to originate from Russia and Alaska, range.The grains exhibit a blocky morphologywhen while the jadeite specimenswere obtained from comparedto the nephrite fibers. Jadeite densities generally streetshops in Hong Kong, and are probably Bur- are about 10 percentgreater than nephrite, patterns mese in origin. The nephrites are a translucent about 3.33 gms/cm3.The X-ray diffraction mottledgreen with a specificgravity of 3.01, indicat- are identical with those reported for NaAlSi2O6 (Yoder, ing appreciableiron content. X-ray diftraction pat- 1950). quantitative terns are in good agreementwith the tremolite To comparetoughness on a basis,it pattern (Stempleand Brindley, 1960). The texture is necessaryto measurecertain mechanicalprop- erties such as elastic modulus and fracture surface (Fig. 1) of the matted fibers is characteristicof '1.f8" nephrite. The fibers vary widely in thickness and energy. Strengths are measured by breaking length,but are generallyless than 10pmin diameter, square bars in three point flexure over a 0.4 inch with length-to-diameteraspect ratios from 10 to 50. span with an Instron testing machine operated at In some regions,the fibers show a strong preferred 0.02 inchesper minute cross-headspeed. Fracture orientation, being grouped into orderly bundles, strength, o1, is calculated from the standard engi- while other regions exhibit a whirlpool-like appear- neering-mechanicsformula : anceof a more randomnature. The jadeites vary in color from a pale green to (r) icy white, with numerous glints of reflected light "':*k' where P is the breaking load, and L, b, and h are the specimen'slength, breadth, and height dimen- sions (Marin, 1962). The elastc moduli are measured by a flexural resonancetechnique on /q" squareby 3" long bars employing a Nametre Model XII B acoustic spec- trometer. 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 ca- pacities of the jades are determinedfrom the reso- nance 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 midsectionwith 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 testingmachine at a crossheadspeed of 0.002 inches per minute, the work, or energy, of fracture is de- Fto. l. Photomicrographsof nephrite (top) and jadeite. termined by integration of the force-displacement Note the matted fibrous texture of the nephrite, and com- curve. Surfacefracture energy is then calculatedby pare it with the blocky grain texture of the jadeite. dividing the work by twice the triangular cross- TOUGHNESS OF ]ADE 729 section area, since two surfacesare formed during Tesr.s l. Mechanical Properties of Jades the fracture process.A comparisonof this technique with other methodshas been given (Coppola and Jade1t e 1972). The fracture surface energy and the Efastic Mqdulus, E t.3 x 1012 2,0 x Lo12 Bradt, (dynes,/cn') strengthresults are reported as the averageswith the -?- -?- Danplng 1 x 10 1.5 x 10 95 percentconfidence limits as describedby the "t" Fracture qtrength, or, z.tz+0.55 x :ro9 1.02+0.21x109 distribution. Thin-sections of jadeite and nephrite (dynes,/cm') were also examined optically on a standard petro- Fracture surlace Energy, 226,000+155,000 121'000+32,000 (er|s/cmz 1 r, ) graphic microscopeand the fracture surfacesphoto- R Fracture Tqughness, K^' 7.7 x 108 7.1 x 10- graphed with a scanning electron microscope after ( dynes,z cm- J/ 2 ) coating the samples with a thin layer of vapor- depositedgold to promote conductivity. Scanningelectron micrographs of the fracture sur- Results and Discussion faceof jadeite(Fig. 2) reveala very high percentage of transgranularcleavage fractures. That is, when a The mechanicalproperties of jadeiteand nephrite crackpropagates through jadeite, it doesnot proceed are listed in Table 1. Although jadeite has the in an intergranular fashion following the boundaries higher elastic modulus, nephrite is superior in between the grains, rather it proceeds directly strength,o1, fracture surfaceenergy, ond fracture 7r, through the grains in a transgranularfracture mode. toughness, IQ, substantiating the general opinion Cleavagestep patternsare especiallyobvious at the that nephrite is tougher than jadeite. The confidence higher magnification,indicating that the fracture is limits for the fracture surface energies and the almost wholly restricted to certain crystallographic toughnessesare comparable to that observed for planes.The elongatednature of some of thesecleav- most brittle materials, with the exception of the age stepsstrongly suggeststhat the fracture is pref- fracture surface energies of the nephrite. A wide erentially occurring parallel to the pyroxene chains, range of scatter was observedon these specimens, probably {110} planes.It is this extensivetrans- with occasionalvalues near 400,000ergs/cm'; how- granular cleavagefracture mode that imparts high ever, no obvious correlationswith place of origin or toughnessto jadeite.