849 The CanadianM ineralog ist Vol. 33,pp. 849-858(1995)
THECRYSTALCHEMISTRY OFSi = AI SUBSTITUTION IN TOURMALINE DANIEL J. MacDONALD ANDFRANK C. IIAWTHORNE Departrnent of Geological ManitobaR3T2N2 Sciences, University of Maninba Winnipeg, ABSTRACI Thestructuresof ninegem-qualitycrystalsof V-bearingvite,a=15.95,c=7.17 A y= 153043,R3*,havebeenrefined to R indices of.-227o using graphite-monochromated MoKcl X-radiation; the crystalsusedfor the X-ray dala collection were analyzedusing an electronmicroprobe.The Si content of thesecrystals is significantly less than 6 atomsper formula unit; zrssignment of talAl sufficient to fill the Ji site resultsin a linear relationshipbetween and t4lAI content.Examinationof recent results of stucture refinement for tourmalile shows no well-defined relationship between 44> and constituent Lczf;:on radius. Conversely,there is a well-developedlinear relationshipbetween and constifirentl-cation radius. Site-scatteringrefinement showsF to be strongly to completely orderedat the O(1) site. There is no significant positional disorderat the O(1) or O(2) sites. Keywords:tourmaline,uvite, crystal-stuctue refinement,site populations,chenical analysis,Si = Al substitution. Solvtr\a^ARE Nous avonsaffin6 la sffucturede neuf cristauxgemmesd'uvitevanadifdre, a - L5.95,c *7.17 A, y= 1530A3, R3rz,jusqu'i un r6siduft d'environ2.2Voen utilisant un rayonnementX de MoKcr (monochromatisationau graphite).Les cristaux qui ont servi pour la collection des donn6esont 6t6 analysdspar la suite avec une microsonde6lectronique.La tenew en Si de ces cristauxest infdrieure d 6 atomespar unit6 formulaire; l'atfribution de talAl pour remplir le site Si mdned une relation lin6aire entre et la teneuren t41A1. Une 6tudedes r6sultatsd'affrnementsr6centsde la sructure de la tourmaline ddmontre l'absenced'unerelationbien d6finie entre et le rayon deI'atomequi setrouvedansle siteZ. Au contrahe,noustrouvons une relation lin6aire bien ddvelopp€eentre et le rayon de I'ion occupantle site L Un affinementde la dispersiondes rayonsX par les divers sitesdd.montreque les atomesF ont une forte tendanced occuperle site O(l). Il n'y a pas de d6sordre de position importantimpliquant les sitesO(1) et O(2). (Traduit par la R6daction) Mots-cl6s:tourmaline,uvite, affinement de la structurecristalline, occupationdes sites, compositionchimique, substitution si=A1.
Intnooucnolt
introducedconcemingthe valencestateof Fe. In this way, we initially dispense with one of the more As part of a generalstudy on the crystal chemistry intractable problems of tle crystal chemistry of of tourmaline. here we examine the structural tourmaline; this can be addressedonce we have chemistry of uvite. We are attempting to derive a better understandingof the crystal chemisty of this relations betweenmean bond-lengthand constituent- goup. cation radius for all cation sites in the tourmaline structure(Hawthorneet al. L993,Bums et al. 1994). E:
