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The Symmetry of Vesuvianite: Optics and Diffraction

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The Symmetry of Vesuvianite: Optics and Diffraction 6L7 The Canadian Mineralogist Vol.3l, pp. 617-635 (1,993) THESYMMETRY OF VESUVIANITE LEEA. GROAT Departmentof GeologicalSciences, University of British Columbia,Vancouver, British ColunbfuV6T 17A FRANKC. HAWTIIORNE Departtnentof GeolagicalSciences, University of Maninba" Wilmipeg,Manitoba R3T2N2 T. SCOTTERCIT Mineral SciencesSection" Canadian Museam of Narure,Otnwa" Ontaio KIP 6P4 ANDREWPI.ITNIS Deparwunt of Eanh Sciences,University of Carnbridge,Cambridge CB2 3EO England ASsTRAcr The vesuvianitestrustur€ hss ideal symmetryP$/nnc, but manyvesuvianite samples show physicalproperties that indicate deviationsfrom this syffnefly. Here we examine76 samplesof vesuvianitefrom approximately50 different localities, and focus on their physical and chemicalproperties that are affectedby symmetry.Tbree groups are recognized,on the basis of optical properties:(i) normal crystalsof vesuvianiteshow uniform extinction and small (G-5") 2V; (ii) blocky crystalsof vesu- vianite show inegularly shapedareas of variablebirefringence in a (001) section,with 2V valuesin the range5-35'; (iii) sec- tor-zonedcrystals of vesuvianiteshow {001}, {101} and {100} sectorswith low (-5'), intermediate(2G-35") and high (40-60') valuesof 2V, respectively;some crystals may be more complex,with { 110} zones.X-ray precessionphotographs of fragments from each of the sectors show the number and intensity of the "glide-violating" reflections to increasein the sequence{ 101} -+ {001 } + { 100}; in addition, deviationsfrom AlmnnI-ate symmetryalso were apparent.Difhrse streaking is associatedwith the'\,iolahng" reflections.Possible derivative space-groupscan be derived from the P$lnrc pwent grotp using groupreduction techniques.This showsthat severalspace groups previously suggestedare not possible.In addition, sev- eral ofthe derivative space-groupscan be eliminated,as the observedoptic orientationsare not compatiblewith thesesyrnme- tries. A combinationof optical and X-ray-dilfraction evidenceindicates that the symmetryof vesuvianiteis P2ln (or Pn),We suggestthat there is a continuous(or near-continuous)ferroelastic phase transition betweena high-temperatureP4lnnc stntc- ture and a low-temperafineP2Jn or Pn structure.Differential order of cationsover pai$ of sitesin the channelsof tle structure cannot drive the transition to ^F2llnsymmetry, as the associatedorder-parameter does not transform as the active irreducible relnesentationof this transition.The variety of optical types of vesuvianiteare a result of differeut relationshipsbetween the temperatureinterval of crystallizationand the temperatureof the phasetransition. Keywords:vesuvianite, symmetry, ferroelastic phase transition, channel, degree of order. Sol"naarns La structue de la v6suvianitepossede une sym6trie idlzIe PLlnnc, mais les propri6t6sphysiques de plusieurs6chantillons t6moignentd'une sym6trie diff6rente.Nous examinonsici soixante-seize6chantillons provenant de cinquantelocalit6s diff6- rentes,en portantnotre attentionsur les propri6t6sphysiques et chimiquesqui sont affect6espar la symdtrie.Nous r6partissons les &hantillons en fois groupesselon leurs propri6t6soptiques: i) cristaux nonnau( montrantun angle d'extinction unifonne et un angle 2V faible (G-5"); ii) cristaux en blocs montrant des zonesd forme irr6gulihre, de bir6fringencevariable dansune section(001), avec2V entr:e5 et 35o,et iii) cristaux h zonationen secteurs,ayant des secteua {001}, { 101} et { 100} avec2V faible (-5), interm6diaire(20-35), et 6lev6 (40-600),respectivement. Certains de ces cristaux sont m6meplus complexes,et montrent des zones {110}. D'aprbs les clichds de pr6cession(en diffraction X) de fragmentsprovenant de chacun de ces secteurs,le nombreet l'intensitd desr6flexions en violation desplans de glissementaugmentent dans l'ordre { 101} + {001 } + { 100}. De plus, les &arts d la sym€tie deI,aue4lnmm sont dvidents.L'dtirement destaches est li6 i la pr6sencede r6flex- ions qui causentles vioLations.Irs groupesspatiaux d6rivatifs possiblesd6coulent du groupeparent P4lnnc par techniquesde r6duction de groupe.Plusieurs groupes spatiaux propos6s ant6rieuement ne sont pas des choix possibles.De plus, plusieun des groupesspatiaux d6rivatifs peuvent6tre dliminds i causede leur incompatibili€ avec les orientationsoptiques observ6es. Une combinaisondes 6videncesoptiques et en difhaction X montreque la sym6triede la v6suvianiteserajrt run (ou Pz). Nous croyonsqu'il existeune transitionfeno6lastique continue (ou presque)entre une structue P4lnnc stableI temp6rature6lev6e et une structureHJn or: Pz stableA faible temp6ratue. Une mise en ordre diffdrentielle des cationsimpliquant des paires de 618 sites dansles canauxde la structurene saurait6tre responsabled'une transition i la sym6trieP2ln, pwce que le parambtrede mise en ordre associ6ne se transformepas dansla reprdsentationactive irr6ductible de cette transition. I-,avai6tf des types optiquesde la v6suvianiter6sulte des relationsentre I'intervalle de tempdraturede cristallisationet la tempdraturede l'inver- sion. (fraduit par la R6daction) Mots-cl6s:v6suvianite, sym6trie, transition de phaseferro-6lastique, canal, degr6 d'ordre. INIRoDucrroN completehomogeneity in al1major elements(Ca" Si, Al, Mg, Fe) (Arem 1973).(v) Somevesuvianite Vesuvianite is a common rock-forming and acces- crystals show a lamellar $tructurethat, in Arem's sory silicate mineral. In most texts on mineralogyand opinion,is dueto twinning. optical mineralogy, it is describedas tetragonal,with Much work has beenpublished on the optical prop- the occasionalcomment that optically anomalous erties of vesuvianiteoand there is sufficient repro- and biaxial varieties also are known. In fact, there are ducibility from locality to locality as studiedby differ- major uncertaintiesin the symmetry, structureand ent investigatorsto indicate that the differences chemistryof vesuvianite,uncertainties that have in optical propertiesare real. resistedconsiderable effort to clarify and understand them. As part of a generalinvestigation of vesuvianite Dffiaction evidence (Groat et al. L992a,b), we here examinethe structural aud physical characteristicsthat pertain to the question Warren & Modell (1931) solvedthe crystal struc- of symmefy and the factorsthat affect it. ture of vesuvianite in the space-grotp P4lnnc. T\is symmetry was assumedto be correct until the work Pnsuous WoRK of Arem & Bumham (1969). Using precessionphoto- graphy, theseauthors showed that one or more of the There aretwo principal typesof experimentalinfor- glide-plane extinction criteria f.or P4lnnc symmetry mation that have been used in the derivation of are violated in many samplesfrom a variety of local- the symmetry of vesuvianite: optics and diffraction. ities; thus a variety of space-groupswere proposed We will review the publishedwork in some detail, (Table 1). Vesuvianitesamples from severallocalities as it indicatessome of the ways in which the problem have P4lnnc symmetry; for some other localities, tle canbe approachedfrom new directions. symmetry is reduced.The space-grorpP4lnmm ww proposed,with the deviations from P4lnnc symmetry Optical evidence characterizedas'ostrong" and "weak". Violations of the z-glide perpendicularto [001] also were Vesuvianitecommonly occurs as very large and observedin three samples,giving the possible space- well-developedeuhedral crystals that exhibit tetra- groupsP4lmmm, P42m, P4m2, P4mm and P422; for gonal morphology. Nevertheless,examination one samplefrom Asbestos,Quebec, a positive piezo- in tansmitted light showsthem commonlyto be biax- electric responseeliminated P\lmrnm as a possibility ial, and some show complex sector-zoning.These featureswere discussedin detail by Arem (1973), and it is apparentfrom his review that vesuvianiteshows BB 1. !On-!4/@ SPACBm BEPolm I]' InSVXOUSslgDrtg an extremely wide variation in optical properties: "no two idocrasespecimgns seemed to give the same sPeoa crouP (:.969) results". TmFortantoptical featuresof vesuvianiteare ?4/@ - 'a6e8" Bl&L Ia&6, Qr4- ed & Bcnb& &larai,e, Qe. summarizedas follows: (i) Somevesuvianite samples h61, Qb. are uniaxial. (ii) Many vesuvianitesamples are biaxial, hll!.U€, E - t4/@ "wah' Fod& S@, IelY w\th 2V varying in the range 0-62". (ii| Some vesu- Tel@!L, [oFey vianite crystals are sector-zoned,the sectorsbeing F e6l1ttr. N &bostoB , qra . related to the bounding crystal faces.The sectorsare Etodr$agh, P6&l€b subtendedby a specific face at the surfaceof the crys- 8e gad& Co., & Poltstbos , C!6€c€ SaP6sls A (aggF (1980) lal, and sectorsmay be idenffied by the Miller indices A1a Ptedrcsr, IEly C1@opD6ttl & kt (1983) of the subtendingface (or form). (iv) Some vesu- Asbas@, qu€. A1168 i hrnbe (1983a, b) vianite crystals pattem (if Edo uiIk, E show an iregular zonal cut fr Ede nl1s, w A11e (1985) normal to the Z axis), with the different zonesextin- &b6sFs, Qu6. Ft*gateld at al. (1986b) guishing in different positions; thesecrystals show 8e balb &. , 6 lLESerald ot aI. (1987) TIIE SYMMETRY OF YESIIVIANITE 6L9 P4ln P4nc The proposeddifferences in the various domains are OR OR associatedwith different schemesof order at the partly occupiedI(1) and X(4) sites in the channelsof the o(10)A Yo)A o00)A x4)B o(ro)A x(4)B q10)A Y(1)A o(10)B x(4)A o(r0)B Y(l)B o00)B Y0)B o00)B x(4)A structure. These and previous argumentshave not x{4)A O0o)B Y(l)B O(r0)B x{4)A o(10)B Y(r)B O(r0)B specifically addressedthe non-tetragonalaspects Y(l)A o(1OA x(4)B o(r0)A Yo)A o(10)A x(4)B o0o)A ofthe vesuvianitestructure; this is donehere. ABCD E:esRnfiI.nAL Fto. 1. Possibilitiesfor
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