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300 on the CAUSE of ASTERISM in STAR CORUNDUM K. Nesseu 300 MINERALOGICAL NOTES KnNNrov, G. C. (1950) Pressure-volume-temperature relations in water at elevated temperatures and pressures. Amer. f . Sci,.2481 540-564. Klnern, W. (1940) Kristallographische Untersuchungen am Aragonit unter besonderer Berucksichtigung des Vorkommens am Erzburg, Steirmark. N euesJakrb. Minad, Abt. ,4, Beil.-Bd. 75,465. M,LcDoNar.o, G. J. F. (1956) Experimental determination of calcite-aragonite equilibrium relations at elevated temperatures and pressures: Amer. Mineratr.4lr lM-756. Wnev, J. L., euo DeNrnr.s, F. (1957) Precipitation of calcite and aragonite: f . Amer. Chem. 50c.79,2031. THE AMERICAN MINERAI,OGIST, VOL. 53, JANUARY_FEBRUARY, 1968 ON THE CAUSEOF ASTERISMIN STAR CORUNDUM K. Nesseu, Bell TelephoneLaboratories, Inc. Murray HilI, Neu Jersey. INtnooucuoN It has beengenerally accepted that asterismin corundum (star rubies and sapphires)is causedby needlesor rutile (TiOz) aligned along crystal- lographic planes of the corundum (e.g. Webster, 1962). The evidence given ty Tait (1955) was: (1) the presenceof titanium, (2) the needle- like habit, and (3) the squarecross-section of the needles.Based on the analogy with rutilated quartz, and by eliminating octahedrite and brook- ite by the morphology, rutile was accordingly identified. This type of evidence,however, is explained equally well by either TiOr or the com- pound aluminum titanate Al2TiO6. ConuNrulr Gnowrn CousrrBnetroNs The technique for the laboratory growth of star rubies and sapphire has beenrevealed in patents by the Linde Division of the Union Carbide Corp. (Burdick et aI., 1949, 1954). Corundum is grown by the Verneuil technique with the addition of, preferably, 0.1 to 0.3 percent titanium oxide in the feed powder. The corundum as grown is clear, but develops asterismupon heat treatment at 1100 to 15000C(Webster, 1962; Burdick et al., 1949,1954) due to precipitation of the excesstitanium, the solu- bility of which is exceededat this temperature. The phase diagram of the AhOa:TiOz system has been studied by Bunting (1933) and by Lang, Fillmore and Maxwell (1952). The relevant parts of these studies are included in Figure 1. There is a slight uncer- tainty as to whether AlzTiOr melts congruently or incongruently near 18600Cin the shadedregion, but this doesnot affect the presentdiscus- sion. Similarly, there may be other forms of AlzTiOs besidesthe a form which is stable above 1820oC.and the 6 form stable below 1820oC.Ac- MINERALOGICAL NOTES 301 AL2O3+ LlQ. ou trUJ ALzOs+ ocAtaTLOs l F $ reo td .L UJ F Al2o3 + pAL2TL05 o20 40t60 AL2O3 AL2TL05 TLO2 Frc. 1. Phase relations in the Al rich side of the AlOr:TiO2 system after Lang, Fillmore and Maxwell (1952); uncertainties exist in the shaded region. cording to this phase diagram, Al2TiObis the phase that should separate out from the corundum when the solubility is exceededat temperatures near the melting point of corundum. Confusion,however, entersin the temperature region below 16000C. According to Lang, Fillmore and Maxwell (1952), B-AlrTiOr becomes unstable, dissociatinginto AIzOs*TiO2 again at lower temperatures. They reported90 to 98 percentAl2TiOb at 1600oC,and only 2 to 10 per- cent Al2TiO6at 1200-1350oC.Hamelin (1951)similarly statesthat in the presenceof 5 percent boric acid at 1250oCno AIzTiOs is formed after 1 hour, but complete conversionwas obtained at 1450oCin 10 minutes. Basedon theseresults the asterismin synthetic corundum would be ex- pected to originate from TiOz. In a study on the solubility of TiOz in Al2O3,Winkler, Sarver and Cutler (1966) however prepared ALTiO6 by firing at 14000Cfor 2 hours and obtained Al2TiOswith no unreacted AlzOa or TiOr. Yamaguchi (1944) also obtained AlrTiO6l at 1400oC.A quick check, heating an intimate mixture of AlzO: and TiOz at 1300oCfor 3 hours, produced essentially no Al2TiO5, consistent with Lang et al., 1 Yamaguchi gave the name tieilite to B-AltTiOs, but did not observe it in nature. 302 MINERALOGICALNOTES (1952).It may well be that small amounts of impurities can affect the stability of Al2TiO6,and accordingly the precipitate in synthetic star corundum cannot be predicted on the basisof thesestudies. Phase diagrams applying to the AlzOa-and TiOz-containing systems during the formation of natural star rubies and sapphires are not avail- able.Nor is it possibleto extrapolatethe alreadyuncertain melt and sin- ter conditions discussedabove to the magmatic, metamorphic, or hy- drothermal conditions during the growth of naturally occurring corun- dum at much lower temperatures. Accordingly, either TiOz or AI2TiOb could be expectedin natural asteriated corundum. The compound AhTiOs has an orthorhombic structure (8,9) (Bbmrn, 7617)isomorphous with pseudo-brookiteFezTiOs (Yamaguchi, 1944; Austin and Schwartz, 1953).Lattice parametersgiven by Austin and Schwartz(1953) are a:9.436, b:9.648, c:3.557, so that rectangular or near-squaremorphology is possible;needles have beenobserved (Lang et al., 1952). Accordingly needle morphology considerations are inade- quate for distinguishing between TiOr and Al2TiOs in asteriated corun- dum. Expr'ntltBNrer Pale blue star sapphiresfrom Ceylon,obtained from the International Gem Corp.,New York, and syntheticstar ruby from the Linde Company, were examinedby optical microscopy,electron microscopy, and by elec- tron probe microanalysis. Optical examination showed the typical arrangement of three sets of needle-likeinclusions in the basal plane as illustrated by Tait (1955). The needleswere square to rectangular in cross-sectionlin some cases they appearedto be empty (hollow). A number of irregularly shapedand placedinclusions were also observed in the natural material,but werenot further investigated.Needle diameters ranged from a few tens of microns in diameter down. In the synthetic material the needleswere much smaller and could not be used for further examination. Attempts were made to take plastic replicas from polished and frac- tured surfacesfor electron microscopy in the hope of pulling out a needle and obtaining electron diffraction patterns. A number of attempts did not yield any usable replicas. Electron probe microanalysiswas performed on a Cambridge Micro- scaninstrument. The sapphire sampleswere polished on diamond powder on polystyrene,ending up with |-p diamond, vapor coatedwith carbon in vacuum, and analyzed for the AlKar Iine by use of a gypsum crystal and for the TiKar line by using a lithium fluoride crystal. A check was also run acrossthe whole spectralrange of the instrument but no other MINERALOGICAL NOTES Tanr,r l. Er,ncrnoN Bpau MrcnolNAlysrs CoNCENTRATToNSAs INTENsrry Rerros ColtpaRED To rrrE Punr Mnrer Titanium Contentr: Calculated for A1:TiOs o.257 Found in inclusion 0 329 Calculated for TiOz 0 607 Aluminum Contmt: Calculated for AlzOs o.332 Found in bulk of corundum o.254 Calculated for Al:TiO; 0 134 Found in inclusion 0.052 Calculated for TiO: 0.0 elementswere detected, either in the inclusions or in the bulk of the sapphires. Examined in titanium radiation inclusionsshowed up as intenseemis- sion regionswhen they were on the surface,but more weakly when they were below the surfaceas establishedby optical microscopy.Accurate measurementswere performed on an inclusion which was approximately 30 micronsby 20 micronsin size,the largestfound. Referencestandards of pure aluminum and titanium were used to give the concentration value of Table 1 in terms of intensity ratios. The calculatedintensity ratios were obtained correctingonly for X-ray absorption(Birks, 1963).Fluo- rescenceand atomic number correctionsare exDectedto be of minor im- portance. DrscussroN In the electronprobe microanalysisof small, thin specimensthere is the danger of spuriousreadings from picking up radiation from the ma- trix above,to the sides,or from below the specimen.Optical examina- tion showedthat there was no corundum overlaying the inclusion.The approximately flat regions obtained in Figure 2 confirm that the sur- rounding corundum was ilot contributing. The slight slope,however, in- dicated that the inclusion was probably quite thin and varying in thick- ness,so that the titanium reading obtained would be on the low side. Accordingly the values of Table 1 clearly rule out the possibility of Al2- TiOs and confirm TiOz. The Al reading from the'inclusion is expected to be on the low side due to the strong absorptionof the Al radiation by the TiOz. Since the inclusion is thin, some Al radiation is expectedto be observedeven from TiOz. The inclusions in the svnthetic ruby available were found .to be too 304 MINERALOGICAL NOTES TL t E o z lrl Fz oto?o304050 MICRONS Frc. 2. Electron probe microanalysiscurves across inclusion in Ceylon star sapphiretaken with Al and Ti radiation. small for analysis, but on the basis of the phase diagram considerations discussed could be either TiOz or AlzTiOr,. The possibility remains that natural star corundum which grew under different conditions from the sample here examined could contain AlzTiOr needles. Acr<nowr.nocunNts The author is grateful for experimentalassistance by Miss S. E. Koonce,H. Scheiber, and particularly Mrs. L D. Payne for the electronprobe analysiswork. RrrnruNcns AusrrN,A. E. arqpC. M. Scnwetrz (1953)ActaCrystallogr.6,St2-13. Brnxs, L. S. (1963)Electron Probe MicroanaLysis. Interscience Publishers, New York, p. t25 fr. BuNrrwc,E. N. (1933)J. Res.U. S. Not. Bur. Slanil.ll,719-725. Bnnorcr, J. N. lNn J. W. Gr-nNN(194q A. S. Pat. 2,488,507Nov. 15, 1949. R. A. Joxrs (1954)U. S. Pot.2,690,062,Sept.28,1954. HeurrrN, M. (1951)Bull' Cuam. Soc.(France) ll, lit-16. MINERAINACAL NOTES 30s Lanc, S. M., C. L. Frr,uonr erqoL. H. Mexwul (1952)J. Ras,U. ^S.Nat. Bur. Sl'anil.48, 298-3t2. Tart, A. S. (1955)J. Gemology5,65-72. Wnrsrnn, R. (1962)Gems,YoI.1, Butterworttrs, Washington, D. C., p. 307. Wrxrmn, E. R., J. F. SnnvnneNo I. B. Curr,nn,/. Amer.Ceram. Soc.49r 63+-637. Yauecucut, G. (1944)f . Jopan Cerum.Assn.52, G7 , THE AMERICAN MINERALOGIST, VOL. 53, JANUARY.FEBRUARY, 1968 THE INFRARED SPECTRUMOF DAWSONITE Parnrcre A. Esrap ANDCLARENcE KARR, Jx., Morgantown Cool Research Center, Bureau of Mines, U.S.
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