467
Canadinn Mineralogist Vol. 31, pp.467-470 (1993)
COMPOSITENATROLITE-MESOLITE CRVSTALS FROM THECOLUMBIA RIVER BASALT GROUP, CI.ARKSTON, WASHINGTON
MICKEYE. GTINTER Depamnentof Geologyand Geological Engineering, University of ldaho,Moscow, Idaha 83843, U.S.A.
CHARLES R. KNOWLES Idaho StateGeological Survey, Universiry of ldaho, Moscow,Idaho 83M3, U.S.A.
D. KATESCHALCK
Deparxnentof Geology,Washington Snte University,Pullman, Washington99164, U.S.A.
ABSTRAC"I
Chemicalcomposition, optical properties,and X-ray-diffraction photographsconfirm the occurrenceof "single" crystalsof natrolitecapped by mesolitein cavitiesin the ColumbiaRiver BasaltGroup near Clarkston, Washington. The indicesof refraction forthe natrolitetip areo 1.4781(3),B 1.4814(3),and T 1.4895(3),with a measured2Vrof 62.9',andtheindex ofrefraction for the near-isotropicmesolite tip is 1.5055(3).Chemical analysis by electronmicroprobe shows the natroliteand mesolite portions to be neartheirideal compositions.Single-crystal X-ray diffraction showsatripling of , for the mesoliteportion of the compositecrystal. Calciumand sodium do not readilyexchange in natroliteand mesolite;thus, the formationof Ca-fteenatrolite at the beginningof growth and Ca-richmesolite at the end of growth indicatesa changein the environmentduring crystalgrowth.
Keywords:natrolite, mesolite, physical properties, Columbia River BasaltGroup, Clarkston, Washinglon.
SoNaNaarne
lrs donn6eschimiques, les propri6t6soptiques, et les clich6sde diffraction X confirmentla pdsencede cristaux"uniques" de natrolite avec surcroissancede mdsolite dans les cavitds d'un basaltedu groupe de Columbia River, prbs de Clarkston,au Washington.l,es indicesde r6fractionde I'extr6mit6compos6e de natrolitesont: o 1.4781(3),B 1.4814(3),y 1.4895(3),2Vz= 62.9'.L'indice derdfraction pour I'extr6mit6 presque isotrope compos6e de natrolite est 1.5055(3).Des analyses d la microsonde 6lectroniquemontrent que natroliteet m6solitepossbdent une compositionpresque iddale. Une dtudepar diffraction X sur cristal unique ddmontreun riplage de b pour la portion du cristal compositefaite de mesolite.k calcium et le sodium ne sont pas facilement6changeables dans ces deux min6raux.la formation d'abord d'une natrolite sansC4 suivie d'une surcroissancede m6soliteriche en Ca"rdsulterait d'un changementdu milieu de croissance.
(Iraduit par la R6daction)
Mots-clds:natrolite, m6solitg propri6t6sphysiques, groupe de basaltesde ColumbiaRiver, Washingon.
INTRoDUcnoN the courseof this study, we discoveredthat this is not the only reported occurrenceof intergrown natrolite- (1992)provided The zeolites used in this study were originally groupzeolites. Tschernich a reviewof collected for use in an optical mineralogy class. A the known occurrences.The structureofboth natrolite group crystal observedin cross-polarizedlight exhibits retar- and mesolitehave been refined in space FdA, (1984) dationand indicesofrefraction indicative ofnatrolite; respectivelyby Artioli et al. and,Artiob et al. however, one end of the crystal is nearly isotropic (1986). Chemically, natrolite [Na1u(AltuSi2aO66) .16H2Ol Grg. 1A). The index of refraction also is higher for differsfrom mesolite[Na16Ca16(Ala6Si72O240) the isotropic end, nearer to the accepted value .64H2Olby an orderedsubstitulion of 2 Na = Ca + H2O for mesolite.To confirm theseobservations, we col- (Gotlardi& Galli 1985),which causes a triplingof Dfor lected both chemical and X-rav-diffraction data. In mesolite. 468 THE CANADIAN MINERALOGIST
Frc. 1. A. Thin-sectionphobgraph in cross-polarizedlight showingbirefringent natrolite terminated with near-isotropicmesolite tips. Chemicalanalyses were performed on the natroliteand mesoliteportions of both of thesecrystals and yieldednear-ideal end-memberchemical formulas (Table l ). B. Concentrationof Ca at the near-isotropic(mesolite) tip showingan abruptchange in Cacontent at the suturebetween natrolite and mesolite.
Occunnnucn methodGloss 1981,Sv et aI. 1987).For the natrolite tip, the indicesare a 1.4781(3),F 1.481a(3)and t The faults and folds that disruptthe basaltsin the 1.4895(3).The mesolitetip is nearlyisotropic, and its vicinity of Lewiston,Idaho, define what is refenedto as singleindex ofrefraction is 1.5055(3). the Lewiston strucfire (Hooper et al. 1985). Thrs structurebrought the ImnahaBasalt, the oldestforma- CHEMIcALCoMPoslrIoN tion in ColumbiaRiver BasaltGroup, to the surfaceon the SnakeRiver approximatelyl0 kilometerswest of Chemicalcomposition (Table I ) wasdetermined with Clarkston,Washington (on the north sideof the river, at an ARL-EMX-SM electranmicroprobe operating at 15 river mile marker131). In the areaof thesestructural kV and 20 nA. The electronbeam was defocusedto disturbances.the ImnahaBasalt contains zeolites (na- approximately 20 pm. Amelia albite and synthetic trolite,mesolite, chabazite, and erionite), which occur in anorthitewere used as standards. All datawere corrected filled vesicles.No thoroughmineralogical study has for drift and deadtime, and Bence& Albee (1968) beendone in thisarea. The zeolite-containing amygdules matrix corrections were applied. Three spots were weatherout of the basaltand are easilycollected. The analyzedon eachof the natrolite and mesoliteportions singlecrystals of natrolitecapped by mesoliteoccur in the amygdules.The crystalsare approximately 50 pm in cross-sectionand 400 pm in length,with the first 350 TABLE 1. CIIEIIICAI, COMPOSITION OF NATROLITN-MESOLITE CRYSTALS SEOWN IN FICUBE 1A pm composedof natrolite,and the tip, approximately50 pm, composedof mesolite.The amygdules are approxi- large crystal s@ll clyatal mately20 mm in diameterwith 3 mm walls. mtrclits @lite FIEIItE @olite
OPTTCAL PROPERTTES srq wt.g 47.8 48.4 49.0 \ot 25.3 21.4 24.9 24.1 CeO 0.0 9.5 0.0 9.6 The compositecrystals were first noticed with a N"zo 15.5 5.2 15.0 5.2 polarizingJightmicroscope. When viewed in cross-po- H2Ot 9.5 r2.4 9.5 12.4 larizedlight, the natrolitetip is birefringent(first-order auEber of @tlols bsed o! 80 (ed 240) atoru of oxyge! red to second-orderblue), and the mesolitetip seems sl 24.8 25.0(?5,0) 25.0 24.4 (73.2) near-isotropic.hecise optical data were collectedby 14.8 {44.4) 75.0 15.3(45.9) spindle-stagemethods (Bloss 1981).Extinction data 0.0 5.2 (15.8) o.o 5.5 (16.5) Na 5.2 (15.6) 14.9 5.4 (16.2) collectedfor thenatrolite tip of thecrystal yielded a2V, Hzo 16.0 21.3(64.0) 16.0 21.3(64.0) of 62.9(5)', as determinedby the computerprogram (B EXCALIBR loss1 98 1, Gunt er etal. 1988).Indices of I weight pereut Il2O @l@lateal frcE idel fomla (Ross et al. refractionwere determined usins the double-variation 1992) . COMPOSITENATROLITE-MESOLITE CRYSTALS 469 of the two crystalsshown in Figure lA: eachchemical minerals,Ca scansand back-scatteredelectron (BSE) compositionreported is the average. imageswere performed. The BSEimages and Ca scans were produced on a Cameca Camebax automated Cercuv SceNerqoBSE lMece electron-microprobeoperating at 15kV and20 nA using an 8-pm beam.The samesamples were usedfor the To verify further the chemicalvariation within these chemicalanalysis (Fig. 1A),Ca scans @ig. lB), andBSE A.
natrolite
so lite
Frc. 2. A. Sketch showing lhe crystal orientation with respect to the incident X-ray beam, which is paralle,lto a, the precession axis. B. Diffraction pattern D*c* showing the (022) and (062) spots of natrolite. C. Diffraction pattnm b' c^ showing the (022) and (062) spots of mesolite, confirming its relationship to natrolite by a tripling of &. 470 TTIE CANADIAN MINERALOGIST
images.Both showthe chemicalchanges occurring in image were provided by Scott Cornelius and Diane thesesamples. From Figure 1B, thereseems to be very Johnsonof The GeoAnalyticalLaboratory, Department little Ca in the natrolite,and the BSE imagesshow of Geology,Washinglon State University,Pullman, brighter areascorresponding to the higher atomicnum- Washington.MEG thankstheldaho Mining andMineral ber of mesolite. Resourcesand ResearchInstitute for partial supportof this project. X-Rey DFFnacnot REFERENcES As a structuralverification, precession X-ray-diffrac- tion photographswere obtainedon a compositecrystal ARnoLr,G., Snmr, J.V. & Kvrcr, A. 1tlA+;: Neurrondiffrac- mounted with c parallel to the dial axis and a as the tion studyof natrotte,Na2Al2 Si3O16.2H2O, at20K. Acta precessionaxis (Fig. 2A) in order to comparethe (O/cl) Cry stallo gr. C40, 1658-| 662. of the two parts.The natrolitetip of the crystalwas first placedin the X-ray beanr"followed by the mesolitetip. & PLUH, J.J. (1986): X-ray structurerefine- (The circles on the crystal drawing representthe ap- mentof mesolite.Acta Crystallogr.C42,937-942. proximate diameter of the X-ray beam). The results BsNcs, A.E. & Amr,s, A.L. (1968): Empirical confirm the optical and chemicaldata. Extra diffraction correction factorsforthe elecuonmicroanalysis of silicatesand oxides. spotsoccur in the mesolitephoto owing to the tripling J. Geo\.76,38243. ofits , axis.The position ofthe (022)natrolite reflection (Frg. 28) is occupiedby the (062) reflectionof mesolite Bloss, F.D. (1981): The Spinlle Sage: Principles and Prac- @ig. 2C), and a new reflectionfor (022) in mesolite rrce.Cambridge University Press,Cambridge, U.K. appearsat a smaller20 value(Fig. 2C). Go:rranor,G. & GALLI,E. (1985):NaturalTzolites. Springer- Suuveny Verlag, Berlin.
Gu|[rER, Bloss, (1988): Natrolite and mesolite are easily distinguishedby M.E., F.D. & Su, Ssu-CHw EXCA- LIBR revisited.A m. M ineral.73, 1481-1482. their optical properties.Routine optical microscopyled to the discoveryof compositenatrolite-mesolite crystals Hooprn,P.R., Wrnsrcn, G.D. & CA!F, V.E. (1985):Geologic in the Imnaha Basalt of the Columbia River Basalt map of the Clarkston 15 minute quadrangle,Washington Group.Thecrystals began growth as natrolite, andended and Idaho. Wash.Div. GeoI.Eanh Resources,GM-31. asmesolite. Natrolite andmesolite deviate only slightly from their ideal formulas(Ischernich l992,Fioss et al. Ross, M., Fr,oun,MJ.K & Ross,D.R. (1992):Crystalline 1992).Thus, a changein the petrogeneticconditions solution series and order-disorder within the natrolite occurredduring crystal growth, probably a changein mineral group.Am. Mineral. 77, 685-703. fluid chemistry to higher levels of Ca. Laboratory experiments would be required to verif this, but Su, Suu-CuuN,Bt,oss, F.D. & Guum, M.E. (1987):Proce- programs natrolite is difficult to synthesize,and mesolitehas not duresand computer to refine the doublevariation method.Az. Mineral. 72, 101,1-1,013, beensynthesized (Tschernich 1 992). TScHERMcH,R.W. (1992):Tzolites of the WorM. Geoscience ACKNOWLEDGETI4E},ITS Press,Phoenix, Arizona-
The authorsthank the two reviewersand R.F. Maftin Received April 21, 1992, revised manuscript accepted for improvingthis manuscript.The Ca scanand the BSE September9, 1992.