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Download the Scanned American Mineralogist, Volume 63, pages 316-325, 1978 Biaxialitvin'isometric' and'dimetric' crvstals EucrNn E. Foono'nNo BRropoRn A. Mrlls Department of Geology, Stanford Uniuersity S tanfo rd, C al ifornia 94 305 Abstract In an attempt to explain the anomalousoptical propertiesof certain cubic, tetragonal,and hexagonal(trigonal) mineral species,variations in2V, composition,and space-groupsymme- try have been determinedfor eight different minerals inctuding garnet (And-Gross), rutile, jeremejevite [A16850lu(F,OH)r],apatite, kleinite [Hg,N(Cl,SO,).xH,O], beryl, quartz, and elbaite. The space-groupsymmetries of the specimens,determined by single-crystalX-ray techniques,showed no deviationsfrom previously-observedcrystallographic symmetry, yet significantdeviations from isometricand uniaxial optical characterwere observed.All speci- mensexcept rutile were found to be compositionallyzoned, but no systematiccorrelation was found betweenelemental zoning and 2/measured along the sametraverses The anomalous optical propertiesare thought to be due to strain induced by chemicalsubstitutions and/or defectsoccurring during crystal growth, by rapid temperatureor pressurequenches, or by mechanicaldeformation. Introduction but jeremejevite,quartz, and tourmaline have cen- Numerous observationsof anomalously biaxial trosymmetricstructures. These eight mineralswere cubic, tetragonal,and hexagonal(including trigonal) chosenbecause of the range of crystallographicsym- substanceshave been reported over the past half- metry, crystal structure, and varying chemistry that century. Various hypotheses have been offered to they display. explain the biaxiality of these materials,including The proposedcauses of observedanomalous opti- internal stresses(Deer et al., 1966),twinning (Win- cal charactercan be consideredconceptually as being chelland Winchell,1951; Turner, 1975a, b), and me- primary or secondary in nature, depending on chanical deformation (Johannsen, 1918; Turner. whether they originated during or after crystal 1975a).These proposedcauses of biaxiality need not growth. A primary causefor an anomalousoptical affectthe space-groupsymmetry as determinedby X- axialangle, in an otherwiseisotropic or uniaxialmin- ray diffraction. Thus, cubic, tetragonal,or hexagonal eral, involvesthe generationof internal stressespro- mineralsdisplaying anomalous optics commonly dis- duced by a range of chemicalsubstitutions or by play cubic, tetragonal, or hexagonal space-group defectsoccurring during crystal growth. The compo- symmetry. sitionally-inducedstresses could be especiallylarge From the many mineralsexhibiting anomalous op- near contacts between zones of sharply contrasting tical properties,the following have been chosen for composition,and may resultin a2V in theseregions further study: (l) garnet (cubic); (2) rutile (tetrag- togetherwith related changesin refractiveindices. A onal); (3) jeremejevite, A16BbOlu(F,OH)s(hexa- possiblesecondary cause of anomalousbiaxiality in gonal); (4) apatite (hexagonal); (5) kleinite, compositionally-zonedcrystals involves rapid pres- HgrN(Cl,SO,).xH,O (hexagonal);(6) beryl (hexa- sure and temperaturechanges, subsequent to crystal- gonal), includingvarieties goshenite, morganite, and lization of the mineral considered,which produce emerald; (7) quartz (trigonal), variety amethyst;and internal stressesand anomalousoptical properties. (8) tourmaline(trigonal), variety schorl-elbaite.All The effectof internal stressesproduced by any of the above mechanismson the magnitudeof anomalous I Presentaddress: Stop 905, U.S. Geological Survey, Denver 2lns and refractive indices is at present unknown. Federal Center, Lakewood, Colorado 80225. The purposeof this study is to examinethe rangein 0003-004x/78/0304-03 I 6502.00 3 I 6 FOORD AND MILLS BIAXIALITY 317 anomalousoptical properties of theseeight minerals, Table L Minerals analyzedby electronmicroprobe; elements and most likely causedby chemicalzonation, primary standards growth features,or secondarypressure-temperature Analyzed elements (staodatds) effects. Tourmaline Fe, Mn, Ca, Ti, Si, A1, Mg, I, Na Experimentaldetails Zn (pure netal) All opticalobservations were made with a monoc- Beryl ular Zeisspolarizing microscope utilizing a 3200'K emerald ar rn,r,,r:l.h,nhirc\ (tungsten)light source,an 8X ocular, and a 40X gosheni Le-morganite Na, Cs (natural beryls) (N.A. objective 0.85). A calibratedsingle-line mi- Utah red beryl Fo an.i"r.l L6n.rira\ crometerocular (6X) was used for measurementof Mn, Zn, Sn (pure metals) t1 (svnEneEacr1v-, isogyreseparation. For all mineralsexamined opti- -l cally, doubly-polished(3-micron polish) plates rang- Si, A1, K (natural otthocfase) Mg (synEhetic Mgo) ing from 0.03 to 1.00 mm thick were used.Single Na, Cs (natural beryls) crystalswere cut normal to the c axis (hexagonalor Jerenej ev iie F6 rnrf,'r.l ham.iii.l trigonal and tetragonal). The garnet (grossular- Si (synthetic SiO^) andradite) was examined in a doubly-polished A1 (syntheEic A1^0^) petrographicthin sectionof 0.03mm thickness.Mea- I (syntheric LII) surementsof 2V weremade by Tobi's method(Tobi, KfeiniEe al /cwnrhcri..:l.mFl\ r956). ilo q fn,i"7.1.inn.hrrl X-ray precessionphotographs were obtained for ApatiEe Ca, Sl, P, Ce, Y, I (naEural apatite) tourmaline,beryl, jeremejevite, quartz, and kleinite. Mn, I'e (pure metals) For tourmaline,beryl, and quartzno deviationsfrom Garne t previously-reportedsymmetry were observed.The gros sular-andrad ite Ca, Si, A1, Fe, Mn, Ti, Mg spacegroups for jeremejeviteand kleinite are cur- (nacuraf garnets) Rutile rently being redetermined.No violations of hexa- I gonal symmetryin eitherwere observed. Quartz J An ARL-EMX-SM microprobe was used for quantitativeelement determinations. In all casesac- celeratingvoltage was l5 kV, samplecurrent was 0.04 Grande) has been examinedin most detail (Foord, to 0.06 microamperes,and the spot sizewas about 2 1976). microns.Analyzed elements and standardsfor each "Pocket" tourmaline crystals show concentric mineralare listedin Table l. Raw data werereduced growth zoning with or without additional sectorzon- using the MAGIC IV computer program (Colby, ing and twinning on {101l}. The basal portions 1968).Microprobe scanningtraverses were made at (roots) of thesecrystals are usually very homoge- points separatedby 0.01 to 0.5 mm with l0-second neous over a large region (mm scale) and are per- counting times.2Z determinationswere made along fectly uniaxial. Extinction positionsare sharp, and the sametraverses at intervalsof 0.I to 0.5 mm. All basal sectionsshow minimum birefringenceat all measurementswere taken at 23o + 2"C. positionsof rotation of the microscopestage. In eu- hedral "pocket" crystals,chemical zonation becomes Data significant,as doesvarying degrees of biaxiality.This biaxiality is generally most pronounced at color Tourmaline boundaries(pink-green, pink-colorless, or others) Of the eight mineralsexamined (Table 2), tourma- where the steepest chemical gradients occur. Al- line receivedby far the most attention. "Pocket" though there is an apparent correlation between2V tourmalinesfrom the Mesa Grande, Pala, and Ra- variationand elementalzonation within tourmaline, mona districtsin San Diego County, California; from there is no consistentrelationship betweenany one the Newry and Paris districts, Oxford County, elementand 2V variation (Figs. 1a and 2a). Secon- Maine; and from the state of Minas Gerais, Brazil, dary or later episodesof tourmaline growth (pencils) have been examined to evaluatethe relationshipbe- result in material which is also strongly composition- tween compositionalzonation and optical character. ally zoned.These crystals, although flawlessor nearly Tourmaline from the Himalaya dike system (Mesa so, show large variations in 2 Z and optic-figurequal- FOORD AND MILLS BIAXIALITY Table 2. Compositions,space groups, and anomalousoptical propertiesof mineralsexamined Anornalousoptical Mineral Conposition Space group n?^narfi ac Garnet ornqqrr'l ar- CarFerSi,O' andradite Ca3Al2Si -LA,5d birefringence, 2/ 3012 | ^-. Rutile Tio2 P4/nwn DlretTusence I c. zv Jeremejevite At6B5Ols(F'0H) 3 Ca, (Poo) (F, oH) , P6 '/n Kleinite HgrN(Cl , S0O)-cHrO Beryl goshenite - Be-A1^Si-0.^ J Z b 16 norganite enerald Utah red beryl :: Quartz amethyst sio2 P3.2 or PS^2 rl t4 Tounnaline schorl - Na(Fe, Mg) (OH,F) ,A1UBSSi602 7 4 RSn elbaite Na(Li ,Al (0H,F) ) 3Al6B3Si6o27 4 ity. However, strain shadowsand patchy extinction crystalsshowing strain shadows and mosaicas well as are often noticeable, especially near small cracks. lamellartextures show no observablerelationship be- Occasionally, the extinction pattern resemblesthe tween the azimuth of the optical axial plane and grid twinning seenin microcline.Crystals in which crystallographicsymmetry (Figs. lb, 2b). The lack of domains of differingcomposition are in sharpcontact correlation is probably related to the domain size with one another (e.g. dendritic growth) frequently being examined and the presenceof irregularly dis- show microcline-likegrid twinning. In portions of tributed internal stresses.Primary developmentof crystals(tourmaline as well as others) where the la- internal stressesand consequentdevelopment of pri- meflar pattern is present, 2V is clearly decreased. mary 2V may be due to growth imperfections and Source-imagedistortion techniques(Wagner et al., growth characteristicsof the crystals,e.g., screwdis- 197l) showed the existenceof domain structure in locations and growth hillocks (spirals). These fea- color-zoned tourmalines from Brazil.
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