Canadion Mineralogist Yol. 27, pp. 709-720 (1989) ABSTRACT The occurrence of the niobium-bearing mneral wiihlerite NaCa2(Zr,M)Si2O?(O,OH,F)2 has been extendedfrom agpaitic nepheline syenite to ijolite-pyroxenite, carbona- tite and silicocarbonatite. Wiihlerite generallycan be iden- tified by its diagnostic blue-green cathodoluminescence (CL), which allows it to be identified in rocks where it has escaped recognition by investigators using routine petro- graphic techniques. CL emission spectra for wcihlerite from different localities are essentially the same, consisting of aband Vt co.5@ nm, which is attribirted to Mn2+ activa- tion, and a broad sweeping band tlnt coversthe entire visi ble spectrumand is suspected to be intrinsic. Niocalite and hiortdahlite, two minerals that are sometimes compared chemically and crystallographically with wdhlerite, show distinctly different CL emission spectra because of triva- lent R.BEactivators. Results of electron-microprobe ana- lyses for wdhlerite from six localities show a range for Nb2O5 concqntration between 12al.d,16 wt.Vo. The ana- lytical data support the $tructural formula reported by Mel- lini & Merlino (1979). The inverse relationship of Nbs+ and Fr- suggests a coupled substitution with Ti4+ and d-, as described by Mellini & Merlino. The intensity of CL of wcihleriteis inversely related to the iron contetrt; wdhlerite containing more tian l9o FeO shows no CL. Wtihlerite from Prairie Lake, Ontario shows a zoned CL and a zon- ing ofFeO. In carbonatite exploration programs,Nb assays commonly are assumed to represent only pyrocblore miner- alization, but in the Prairie Lake complex, somecarbona- tites and otier alkaline rocks may derive their Nb content almost exclusively from wdhlerite. Keywords: wdhlerite, carbonatite, alkaline rocks, cathodoluminescence, chemicalanalyses, CL, emission spectra, niobium. Sonauenr La wdhlerite, NaCa2(Zr,Nb)gi2Oi(O,OH,F)2, mindral accessoire riche en niobium, estprdsent non seulement dans les sy6nites n€ph6tniques agpaitiques, m?is aussi dansles ijolites-pyrox6nites, carbonatites et silicocarbonatites. Sa couleur diagnostique bleu-vert en cathodoluminescence (CL) en assure I'identifiiation, m6me ori elle avait fuhapd i I'attention deschercheurs se fiant aux mdthodes p6tro- graphiques ordinaires. Les spebtres d!6rnission CL de la w6hlerite provenant de diff6rents niilielrx se ressemblent tous; ils contiennent une bandei environ 5.00 rim qui serait WOHLERITE: CHEMIGAL coMPosITIoN,cATHoDoLUMINESGENGE AND ENVIRONMENT.OF CRYSTALLIZATION ANTHONY N. MARIANO 48 Page Brook Road, Carlisle, Massachusetts 01741, U.S.A. PETER L. ROEDER Deryftment of Geologicol Sciences, Queen's University, Kingston, OntarioK7L 3N6 due i une activationpar le Mn2+, et une bandefloue qui couvrele dornaine visibledu spectre au complet, et qui serait intrinsbque. La niocalite et la hiortdahlite, qui sont sou- vent compar€es chimiquement et cristallogxaphiquement d la wdhlerite, possbdent un spectre d'€rrision distinct i cause d'une activation par desterres iares trivalentes. Les don- n€es obtenues i la microsonde dlectroniqrie pour des €chan- tillons de six complexes revblententre 12et 169o de M2O5. Ces donn6es confirment la formule structurale de Mellini et Merlino (1979). La relation inverse entre le Nd+ et le Fr- fait penser qu'il y a une substitution couplde impli- quant Ti4+ et O2-, telle que ddcrite par Mellini et Merlino. L'intensitd de la cathodoluminescence de la wdhlerite est inversement li6e i sateneur en fer.;les echantillonsqui con- tiennentplus de l9o de FeO ne montrent pas d'effet CL. La w6hlerite du lac Prairie, en Ontario, qui montre une zonation de I'effet CL, esteffectivement zon6danssateneur en fer. Dans les programmes d'exploration. descomplexes de carbonatite, les teneursen Nb sont souvententibrement attribu€esau pyrochlore, mais danscertaines unit6s de car- bonatite et de rochesalcalines du massif du lac Prairie, le niobium serait presque exclusivement sous forme de w6h- lerite. (Traduit par la R€daction) Mots-cl6s: wdhlerite, carbonatite, rochesalcalines, catho- doluminescence,composition chimique, CL, specfie d'6mission,niobium. h.ITRODUCTION W6hlerite is generally considered to be a rare mineral and to occur in a restricted geological environment (Vlasov 1966). It is most often reported in nepheline syenite, both agpaitic.Fnd miaskitic, and their pegmatite derivatives. It is ffibo known to occur in fenites associated with alkaline mas$ifs. Vlasov (196O discussed occurrences of wdhlerite in basalt, trachyte and phonolite from Tasmania and Victoria, Australia, but no references or details ofthe occur- rences were given. According to Kapustin (1980), wdhlerite is not observed in carbonatites. In this study, we report the presence of w6hlerite in carbonatite, silicocarbonatite, melteigite, ijolite, urtite, pyroxenite and malignite, indicating that the mineral has a more widespread distribution than has 709
Transcript
Canadion MineralogistYol. 27, pp. 709-720 (1989)
ABSTRACT
The occurrence of the niobium-bearing mneral wiihleriteNaCa2(Zr,M)Si2O?(O,OH,F)2 has been extended fromagpaitic nepheline syenite to ijolite-pyroxenite, carbona-tite and silicocarbonatite. Wiihlerite generally can be iden-tified by its diagnostic blue-green cathodoluminescence(CL), which allows it to be identified in rocks where it hasescaped recognition by investigators using routine petro-graphic techniques. CL emission spectra for wcihlerite fromdifferent localities are essentially the same, consisting ofaband Vt co.5@ nm, which is attribirted to Mn2+ activa-tion, and a broad sweeping band tlnt covers the entire visible spectrum and is suspected to be intrinsic. Niocalite andhiortdahlite, two minerals that are sometimes comparedchemically and crystallographically with wdhlerite, showdistinctly different CL emission spectra because of triva-lent R.BE activators. Results of electron-microprobe ana-lyses for wdhlerite from six localities show a range forNb2O5 concqntration between 12al.d,16 wt.Vo. The ana-lytical data support the $tructural formula reported by Mel-lini & Merlino (1979). The inverse relationship of Nbs+and Fr- suggests a coupled substitution with Ti4+ and d-,as described by Mellini & Merlino. The intensity of CL ofwcihlerite is inversely related to the iron contetrt; wdhleritecontaining more tian l9o FeO shows no CL. Wtihleritefrom Prairie Lake, Ontario shows a zoned CL and a zon-ing ofFeO. In carbonatite exploration programs, Nb assayscommonly are assumed to represent only pyrocblore miner-alization, but in the Prairie Lake complex, some carbona-tites and otier alkaline rocks may derive their Nb contentalmost exclusively from wdhlerite.
Keywords: wdhlerite, carbonatite, alkaline rocks,cathodoluminescence, chemical analyses, CL, emissionspectra, niobium.
Sonauenr
La wdhlerite, NaCa2(Zr,Nb)gi2Oi(O,OH,F)2, mindralaccessoire riche en niobium, est prdsent non seulement dansles sy6nites n€ph6tniques agpaitiques, m?is aussi dans lesijolites-pyrox6nites, carbonatites et silicocarbonatites. Sacouleur diagnostique bleu-vert en cathodoluminescence(CL) en assure I'identifiiation, m6me ori elle avait fuhapdi I'attention des chercheurs se fiant aux mdthodes p6tro-graphiques ordinaires. Les spebtres d!6rnission CL de law6hlerite provenant de diff6rents niilielrx se ressemblenttous; ils contiennent une bande i environ 5.00 rim qui serait
ANTHONY N. MARIANO48 Page Brook Road, Carlisle, Massachusetts 01741, U.S.A.
PETER L. ROEDERDeryftment of Geologicol Sciences, Queen's University, Kingston, Ontario K7L 3N6
due i une activation par le Mn2+, et une bande floue quicouvre le dornaine visible du spectre au complet, et qui seraitintrinsbque. La niocalite et la hiortdahlite, qui sont sou-vent compar€es chimiquement et cristallogxaphiquement dla wdhlerite, possbdent un spectre d'€rrision distinct i caused'une activation par des terres iares trivalentes. Les don-n€es obtenues i la microsonde dlectroniqrie pour des €chan-tillons de six complexes revblent entre 12 et 169o de M2O5.Ces donn6es confirment la formule structurale de Melliniet Merlino (1979). La relation inverse entre le Nd+ et leFr- fait penser qu'il y a une substitution couplde impli-quant Ti4+ et O2-, telle que ddcrite par Mellini et Merlino.L'intensitd de la cathodoluminescence de la wdhlerite estinversement li6e i sa teneur en fer.; les echantillons qui con-tiennent plus de l9o de FeO ne montrent pas d'effet CL.La w6hlerite du lac Prairie, en Ontario, qui montre unezonation de I'effet CL, est effectivement zon6 dans sa teneuren fer. Dans les programmes d'exploration. des complexesde carbonatite, les teneurs en Nb sont souvent entibrementattribu€es au pyrochlore, mais dans certaines unit6s de car-bonatite et de roches alcalines du massif du lac Prairie, leniobium serait presque exclusivement sous forme de w6h-lerite.
W6hlerite is generally considered to be a raremineral and to occur in a restricted geologicalenvironment (Vlasov 1966). It is most often reportedin nepheline syenite, both agpaitic.Fnd miaskitic, andtheir pegmatite derivatives. It is ffibo known to occurin fenites associated with alkaline mas$ifs. Vlasov(196O discussed occurrences of wdhlerite in basalt,trachyte and phonolite from Tasmania and Victoria,Australia, but no references or details ofthe occur-rences were given. According to Kapustin (1980),wdhlerite is not observed in carbonatites.
In this study, we report the presence of w6hleritein carbonatite, silicocarbonatite, melteigite, ijolite,urtite, pyroxenite and malignite, indicating that themineral has a more widespread distribution than has
709
THE CANADIAN MINERALOGIST
WOHLERITE: CHEMICAL COMPOSITION, CATHODOLUMINESCENCE 7t l
been previously reported. W6hlerite commonly isfound with other niobium-bearing minerals includ-ing pyrochlore, betafite, latrappite, niobian perov-skite, and niobian zirconolite. The identification ofw6hlerite in carbonatite and associated alkaline rocksis of importance if niobium partition coefficients arebeing evaluated for specific rocks. Furthermore, inexploration programs that focus on carbonatite, nio-bium assays are generally assumed to representpyrochlore mineralization, but at least in the PrairieLake carbonatite in Ontario, the malignites andijolite-pyroxenite rock types may derive their nio-bium content almost exclusively from w6hleritemineralization.
Wdhlerite occurs as elongate prisms that may beseveral centimeters long; it can be recognized withrelative ease at the type locality at Langesundfjord,Brevik, Norway. Most commonly, w6hlerite assumesmicroscopic dimensions and is intimately associatedwith other minerals, obscuring its identification. Inmany cases, however, wdhlerite can readily bedifferentiated from other minerals by itscathodoluminescence (CL), which consists of a diag-nostic blue-green coloration.
Wdhlerite has been found in several carbonatitecomplexes including: Prairie Lake, Ontario; Oka,Quebec; Tchivira Mt., Quilengues, Angola; MuriMountains, Guyana; In Imanal and Anezrouf, WestAfrican Craton, Mali, and Magnet Cove, Arkansas.In this study, we include a description of the petro-graphy of samples from the new w6hlerite-bearinglocalities. Electron-microprobe data are provided forw6hlerite from five ofthe above localities and fromBrevik, Norway. A correlation of CL in w6hleritewith the electron-microprobe data indicates that theintensity of CL in w6hlerite is dependent on the Fecontent.
CATHoDoLuMINESCENCE INSTRUMENTATIoN
Cathodoluminescence is a valuable tool for theexamination of carbonatites and alkaline rocks. Itcan be used to identify the mineral species in a
complex assemblage and to reveal subtle aspects ofgeochemistry and petrology (Mariano 1978, 1988,Roeder et al.1987).
A Nuclide Luminoscope was employed as the elec-tron source, and a Gamma Scientific spectroradiom-eter was used to measure the CL emission spectra.The experimental conditions are the same as thosedescribed by Mariano & Ring (1975). An electron-beam diameter of approximately 2 mm was used forthe generation of the CL emission spectra. Since theluminescence of wdhlerite has a moderately weakintensity, voltages of about 15 kV were usuallyemployed, with a beam aurrent of about 0.6 mA.
In a sample in which wdhlerite shows CL, the colo-ration is a moderate light greenish blue that can bereadily differentiated from associated blue-luminescing apatite and feldspar (Fig. l).
CHEMICAL ANALYSIS
The electron-microprobe analyses were performedat Queen's Universiry using an ARL SEMQ electronmicroprobe. The analyses were conducted at 15 kVusing energy-dispersion analysis for Si, Ti, Zr, Nb,Fe, Mn, Ca and Na, and at 25 kV for wavelength-dispersion analysis of the otler elements usingmethods similar to those described by Roeder (1985)and Roeder el al. (1987). The energy-dispersion spec-tra for the w6hlerite samples do not show peaks forany other elements; thus the low analytical sum mayreflect some water or excess oxygen in the structure.
The electron-microprobe data of wdhlerite fromsix samples are shown as columns 3 to 8 in Tablel. Analysis 3 pertains to a wdhlerite from the typelocality in Brevik, Norway (Queen's UniversityMuseum collection), which was first described byScheerer (1843). The analytical data given byScheerer are shown in column I in Table 1. The tau-talum oxide reported by Scheerer in 1843 was, in allprobability, mainly niobium oxide (shown as suchin Table 1) since most chemists, until 1844, thoughtthat M and Ta were identical elements (Weast 1984).
Analysis 2, from Brdgger & Cleve (1890), pertains
Ftc. 1. Cathodoluminescence micrographs ofw6hlerite-bearing rocks. a. Silicocarbonatite from Prairie Lake, Onrarioshowing light blue CL wdhlerite grains and orange calcite with associated nonluminescing pyroxene and biotite. Scalebar 0'2 mm. b. Pyroxenite from Prairie Lake, Ontario with zoned idiomorphic wdhlerite. The weak, or dark blueCL zones correlate with relatively high Fe content, as opposed to the strong, or light blue CL zones, which are lowin Fe content. The zones :ue too narow to determine whether or not the spectral response is different for the twozones. Scale bar 0.07 mm. c. Nepheline syenite from the Anezrouf carbonatite, Mali. The light blue-green grainin the center is a wdhlerite prism cutting into a nonluminescing nepheline grain. The equant blue grains that com-prise most of the micrograph are Tia+-activated orthoclase, and the light violet gxains arc LfuEE-activated apatite.The orange grains are calcite, and the red bands are veins ofFe3+-aitivated feldspar that have crystallized fromlate fenitizing alkaline solutions (Mariano 1988). Scale as in a and d.d. Wcihlerite iir s6vite from St. Lawrence Pir,Oka, Quebec. At least 3 light-blueJuminescing wdhlerite grains appear in this micrograph. Two are clearly discerni-ble idiomorphic prisms. The dark blue grains are ovoids and clusters of ZR.EE-activated apatite (Roeder et a/. l98Z).The matrix is Mn2+-activated calcite with orange CL. The nonluminescing grains include diopside, pyrochlore andlatrappite. Scale bar 0.2 mm.
to w6hlerite from the same Brevik locality. Here, thetantalum oxide is probably combined with niobiumoxide. The electron-microprobe analy$is 3 has a sig-nificantly higher TiO2 and lower ZrO2 content thananalysis 2, which may be due to an artifact of theclassical wet-chemical analytical technique used byBrdgger & Cleve. The classical wet-chemical tech-nique often involved separation of a filtrate withTiO2 + ZrO2 + HfO, after precipitation of SiO2.This filtrate was used to precipitate ZrOr, but TiO2+ HfO2 could also precipitate, thus giving a highZrO2 and low TiO2 content (J.R. Foster, pers.comm.). The three compositions (l-3) of Brevik
w6hlerite are remarkably similar, considering theyare of different samples analyzed over a period of145 years using quite different techniques.
The number of calculated cations for thirty-sixanions for the chemical data are shown in Table l.The cations have been summed for the structuralpositions in wdhlerite and are shown in Table 2together with the theoretical structural formula. Thecalculated number of cations fit the theoretical struc-tural formula quite well, although the number ofcations in the silicon position is consistently above8 (8.15-8.26) and the number of cations in the nio-bium position is quite variable (1.57-2.03). Mellini
& Merlino (1979) suggested from their structuralstudies that fluorine can substitute for oxygen by acoupled.substitution of Tia+ for Nbs+:
T i a * + F 1 - - N b 5 + + 0 2 -
The inverse relationship between the Nb and Fcontent (Fig. 2) supports the coupled substitutionsuggested by Mellini & Merlino. The number of Tications is shown beside each point on Figure 2; theTi content decreases as expected with increasing Nbcontent. However, the coupled substitution is morecomplex than the one-for-one substitution impliedin the above equation since the Ti increase is onlyhalf the decrease in the Nb content. The number ofcations in excess of 8 in the Ca position (Table 2)is inversely correlated with the deficiency in the Nbposition, and the total of cations in these two positions is close to 10. Thus it looks as though someof the elements assigned in Table 2 to the Ca posi-tion may compensate for the deficiency in the Nbposition.
IlrenpnnrarroN oF CL EurssroN SpscrRA
As shown in the lower three spectra in Figure 3,the CL emission spectra for wdhlerite from differentlocations and rock types are essentially the same.They consist of a very broad emission that spans theentire visible spectrum and peaks in the blue-greenat approximately 475 nm. Superimposed on thispattern is a narrower band that peaks at about 500nm and is attributed to Mn2* activation. The broadsweeping band is responsible for the whitish color
713
of the luminescence; the more intense and narrowband gives rise to the greenish blue color.
Whereas no definitive conclusions can be madeconcerning the cause of CL in w6hlerite withoutresorting to synthesis and the controlled introduc-tion of impurities, some speculation can be madebased on chemistry and correlation with lumines-cence behavior in other silicates. The broad sweep-ing band is assumed to be intrinsic, probablyassociated in some way with impurities affecting theSi-O bonds; it may be enhanced by the presence ofTi. In many silicates, Ti acts to intensify an other-wise weak blue luminescence (Leverenz 1968, p. 192).
The narrow band that peaks at about 500 nm isassigned to Mn2* impurity activation, based onshape and the chemical data. Iron in sufficient quan-tity commonly acts as a quencher of CL in minerals@ierson 1981, Fairchild 1983). W6hlerite from fourlocalities shows CL, and all four contain less thanl9o iron oxide; the two wdhlerite samples that donot show CL contain more than l9o iron oxide(Table l).
The spectra for wdhlerite reported in this study,and for w6hlerite from Mont St. Hilaire, Quebec,the Red Hill syenite complex of New Hampshire, andfrom Magnet Cove, Arkansas, are very similar, withonly small shifts in the peak emissions, which maybe due to crystal-field effects. Wdhlerite, niocaliteand hiortdahlite are somewhat similar in compositionand structure (Nickel et al. 1958,Liet a|.1966, Mer-lino & Perchiazzi 1987); however, their CL emissionspectra are quite different and can be used as a diag-nostic tool for their identification. The CL spectrumfor hiortdahlite from the Kipawa nepheline syenite
Frc. 2. Number of fluorine ions versas number of niobium ions on the basis of 36anions in wdhlerile. The number of titanium cations is shown beside each point.
1 . 5
714 THE CANADIAN MINERALOGIST
400 500 600 700nm
FIc. 3. Cathodoluminescence emission spectra for w6h-lerite from Prairie Lake, Ontario (c), Oka, Quebec (d),and Muri Mountains, Guyana (e). The wdhlerite spec-tra, which are essentially identical, are compared tohiortdahlite (a) from Kipawa, Quebec and niocalite (b)
Oka, Quebec.
complex, Quebec and niocalite from Oka, Quebec(Bond Zone) are shown in Figure 3. The niocalitespectrum shows line emissions from Sm3+ at 600nm, 650 nm and a small unlabeled peak at 700 nm.The presence of Sm3+ activation in the niocalitereflects the ZREE dominance of caxbonatite environ-ments. In contrast, for hiortdahlite the major acti-vator is Dy3+, with line emissions at 485 and 525nm. In addition, the strongest emission of Tb3+also is resolved at 550 nm. The broad bands at 600and 650 nm probably are due to multiple unresolvedemissions from Sm3* and Mn2+, or they may be aresult of REE peak overlap. The hiortdahlite spec-
trum, with peaks due to Dy3+ and Tb3+, reflectsflREE-dominant activation, which also is manifestedin other minerals from the Kipawa Complex.
The identification of the trivalent lanthanide acti-vators in mineral hosts is facilitated by the presenceof multiple peaks for each activator, which consis-tently give specific wavelengths of emission regard-less of the crystal-field influence of the host.Although wdhlerite crystallizes in rocks that are rela-tively enriched in REE, tbe kEE have not beenobserved as activators in wdhlerite from anylocality.
CL in w6hlerite was first observed in rocks fromthe Prairie Lake carbonatite during a niobium explo-ration study (Mariano 1979).The Prairie Lake com-plex is a quasicircular plug, with an approximate sur-face area of l0 km2. The complex has been datedat I Ga (Gittins et al. 1967, Bell & Blenkinsop 1980).The geology of the complex, based on outcrop andexamination of drill core, consists of an outer annu-lus of sdvitic rocks that grade into silicocarbonatite,ijolite and pyroxenite. The central area of the com-plex contains a pseudoleucite-bearing malignite thatgrades into ijolite and sdvite. In addition to feniti-zation (alkali metasomatism) in parts of the complex,the malignite and ijolitic rocks contain some areasof intense Ca-metasomatism, including the forma-tion of secondary wollastonite, pectolite and vesu-vianite, whose identification is enhanced under CLexamination.
Wdhlerite is a primary mineral in Prairie Lake(Fig. 4), where it occurs sporadically in the s6vite,silicocarbonatite, pyroxenite, ijolite, urtite,melteigite, un6 slalignite. The w6hlerite contentreaches 7 vol.vlo in some of the silicocarbonatite andmalignite- It usually occurs as idiomorphic to subi-diomorphic prisms, and some of the crystals exhibitcomplex lamellar twinning.
Other niobium-bearing minerals that may occurin the same rocks with wdhlerite include pyrochlore,betafite, niobian perovskite and niobian zirconolite.The wdhlerite crystals usually are isolated from theother niobium-bearing minerals, and there is no indi-cation of replacement. In some pyroxenite, wdhleritecontains central inclusions of niobian zirconolite andbaddeleyite (Fig. a). The wdhlerite content of thepyroxenite shown in Figures lb and 5 is approxi-mately 5 vol.9o; it occurs in clusters and as idio-morphic prisms that exhibit color zonation whenviewed in CL @g. 1b).
Some crystals show an inner and outer zone withdecreased intensity of CL. A back-scattered electronimage of the large zoned crystal at the center ofFigure lb is shown in Figure 5a. Figure 5b is an iron
( b) n iocol i te
(c) wdhler i te
(d) wiihlerite
(e) wbhlerite
X-ray image of this same crystal. The iron contentseems to be higher in the central and outer zones ofthe crystal, which show a lower intensity of CL.Table I contains results of pafiial analyses for theless luminescent zone (analysis 9) and the moreluminescent zone (analysis l0). Each compositionquoted represents an average of 5 and 6 analyses,respectively, at different sites. The standard devia-tion from the average of each analysis is shown; theonly significant compositional difference (more thantwo standard deviations) between sample 9 and 10is the iron content. Iron commonly acts as a quencherfor cathodoluminescence. In this sample, a differ-ence between 0.4 and 0.9 wt.Vo FeO changes theintensity for the two zones, as shown in Figure la.The CL speclrum for the Prairie Lake wdhlerite isshown in Figure 3c. We do not know how the spec-tral response would change for the two zones, butassume that the color does not change, only theintensity. These crystals that show CL zoning showno zoning using normal petrographic techniques.
It is apparent from the texture that at Prairie Lake,w6hlerite is a primary mineral that crystallized duringthe precipitation of the more alkali components, asindicated by its.concomitant formation with aegirine-augite that rims diopside cores @ig. 4c). The zon-ing of the wohlerite crystals suggests that there wereat least two discrete periods of wdhlerite grofih.
The Prairie Lake carbonatite is a well-mineralizedniobium resource with near-surface niobium gradesthat approach economic values. The major niobiumcarrier in the complex is pyrochlore but, in someareas, wdhlerile is the dominant niobium mineral,whose recognition is best accomplished by means ofCL examination.
Oka, Quebec
Although numerous studies have been conductedin various parts of the Oka carbonatite, wdhleritehad not been detected until the present study.W6hlerite occurs as a trace mineral in sdvite fromthe St. Lawrence Pit in Oka, where it is associatedwith or attached to pyrochlore and latrappite. It hasnot been found in the Bond Zone, where the nio-bium silicate niocsliteis an accessory in soivite fromat least three trenches (Mariano 1985). The pres6nceof niocalite and absence of wdhlerite in the BondZone are probably partly due to low Zr in the sys-tem, as indicated by the absence of zircon and bad-deleyite. Both of these minerals, if present, wouldalso be easily detected by CL examination
In the CL micrograph of Figure ld, three wdhleritecrystals in s6vite can be recognized by their lightgreenish blue luminescence, in contrast.to the darkblue CL of associated apatite. Another micrographof this sdvite taken in normal polarized light alsoincludes two wdhlerite crystals that are obscuredwithin a cluster of apatite prisms (Fig. 6).
715
The fine-grained nature and scarcity of wdhleritein the Oka srivite probably are the reasons that ithas escaped recognition until now. Although themajor niobium carriers at Oka are pyrochlore andperovskite, there could be an isolated area in thecomplex in which w6hlerite is the dominant niobiummineral, as is the case for niocalite in selected partsof the Bond Zone.
Carbonatites in Mali
During an exploration program sponsored by theUnited Nations Revolving Fund for NaturalResources Exploration in the eastern margin of theWest African Craton, w6hlerite was discovered asan accessory mineral in nepheline syenite from twocarbonatite complexes tirouglr the use of CL exami-nation (Mariano 1983). Of four distinct but closelyassoci'ated carbonatite bodies dgscribed by Sauvage& Savard (1985), the In Imanal and Anezrouf car-bonatites contain accessory wdhlerite associated withpyrochlore in nepheline syenite. Both complexesinclude ijolite, nepheline syenite, rauhaugite, feniteand late transgressive rodbergile dikes, but wdhleritewas found only in nepheline syenite.
In the Anezrouf nepheline syenite, wdhlerite is atrace mineral occurring as disseminated blebs orhypidiomorphic crystals with an average long dimen-sion of 50 pm. It is found attached to, or includedin, nepheline, orthoclase, aegirine-augite, andradite,apatite and calcite, which suggests a lack of specificmineral association during crystallization. Late crys-tallizing fluorite, wollastonite and pyrite show noclose association with wdhlerite crystallization. A CLmicrograph of w6hlerite in nepheline syenite fromthe Anezrouf carbonatite is shown in Figure lc,together with abundant violet-luminescing apatite.
In the nepheline syenite from the In Imanal car-bonatite, wdhlerite was found only as an inclusionin calcite (Fig. 7), which demonstrates the close tem-poral relationship of crystallizalion between the twominerals. The common app*uance of round outlinesin the wdhlerite from Anezrouf and In Imanal, andtheir attachment or inclusion in calcite, may bea ves-tige texture of crystallization of immiscible carbonateand silicate liquids.
Muri Mountains, southeastern Guyana
W0hlerite has been reported in syenite andmicrosyenite from the Muri Mountains, Guyana byBarron & Belshaw (1978). The average of fiveelectron-microprobe analyses for wdhlerite from theBarron & Belshaw report is given in column 1l Clable1), whereas the results of the microprobe analysisperformed in the present study are shown in column4. In a separate study, wdhlerite was reported innepheline syenite from the same locality, where it wasfound as an accessory with pyrochlore (Mariano1981).
WOHLERITE: CHEMICAL COMPOSITION, CATHODOLUMINESCENCE
WOHLERITE: CHEMICAL COMPOSITION. CATHODOLUMINESCENCE 7t7
FIc. 5. A back-scattered electron image (a) and an iron X-ray image (b) of the same w<ihlerite crystal from Prairie Lakeas shown in the center of Fig. lb. The X-ray image was exposed for 15 minutes to accentuate the small differencein iron content of the wdhlerite zones; as a result, the biotite (Bt) image is overexposed because of its high iron content.
Although unaltered carbonatite has not beenobserved either exposed in this area or from drill coretaken from shallow depths, laterite mineralogy andgeochemistry, and the occrurence of fenite, suggestthe presence of buried carbonatite in the area(Mariano 1981). The Muri Mountain wOhlerite showsCL characteristics similar to those of the other occur-rences described in this studv.
In southern Angola near the town of Quilengues,two impressive mountains of Cretaceous age,
approximately 13 km apart, rise abruptly above thesurrounding plains underlain by Precambrian gra-nitic gneiss. One of the mountains, Bonga, consistspredominantly of carbonatite, whereas the neighbor-ing Tchivira is composed mostly of alkaline rocksand only subordinate carbonatite. The alkaline rocksof Tchivira include syenite, nepheline syenite,ijolite-urtite, essexite, and dike rocks including tin-guaite, monchiquite and other lamprophyres.
W<ihlerite was fust reported from a nephelinesyenite body exposed at'the highest altitude ofTchivira (2385 m) by Matos Alves (1968). Hereported results of a semiquantitative X-ray-
Frc. 4. Samples from Prairie Lake, Ontario photographed in plane-polarized light and a scale bar equal to 0.14 mm,except as noted. (a) Apatite-biotite pyroxenite, with a dark central cluster of niobian zirconolite (Nbz) surroundedby polycrystalline wdhlerite (Woh) and three grains of baddeleyite (Bd) on the right. Scale bar 0.055 mn. Crossedpolars. (b) Apatite-biotite pyroxenite, with a central polycrystalline cluster of wdhlerite (Woh) with baddeleyite (Bd)and opaque pyrrhotite (Po) in diopside (Di). (c) Pyroxenite with a white diopside core surrounded by polycrystallinewiihlerite that crystallized contemporaneously with aegirine-augite (Act). (d) Silicocarbonatite that has three wdh-lerite grains that appear in the color CL micrograph of Fig. la. Note the concentric radiation halos around the inclu-sions in biotite in the upper right corner.
7t8 THE CANADIAN MINERALOGIST
,!
FIG. 6. Apatite s6vite from St. Lawrence Pit, Oka, Quebec. The dark rectangulargrain in the upper left corner of the micrograph is latrappite (La|. The roundgrain with the irregular surface attached to the latrappite is pyrochlore (pyc).Another pyrochlore grain with some translucency is isolated in the upper center.Just to the lower left of this pyrochlore is a prism of wdhlerite (Woh) that is slightlycurved and points downward. The two opaque grains in the lotver center are pyr-rhotite (Po). Another wdhlerire gnin surrounded by apatite (Ap) grains occurson the upper right side of the largest opaque pyrrhotite erain. Photographed inplane-polarized light; the scale bar represents 0.055 mm.
Frc. 7. Wdhlerite (Woh) grain as a central inclusion in calcite (Cal) within a nephe-Iine syenite from the In Imanal carbonatite of Mali. The dark grains are alterednepheline (Ne), and the light grains are orthoclase (Or). The dark outline on thelower edge of the calcite is biotite @t). Photographed in plane-polarized light;the scale bar represents 0.14 mm.
fluorescence analysis for wdhlerite. During an explo-ration program for the Kennecott Copper Corp.,wOhlerite from the Tchivira complex also was foundin nepheline syenite from the base of the mountain(unpubl. data, Mariano 1969). The mineral occursas idiomorphic prisms randomly disseminatedthroughout the rock, and in some cases intimatelycrystallized with niobian zirconolite. Unlike wdhleritefrom the other carbonatite occurrences included inthis study, the Tchivira wdhlerite contains greaterthan I wt.9o FeO (analysis 5), and does not exhibitCL.
CoNcLUSIoNS
This study exemplifies the value of CL for theidentification of obscure minerals that most oftenescape recognition in conventional petrographicexamination. Through the use of CL, the environ-ment of wdhlerite has been extended to carbonatite-bearing complexes, and in some cases to carbona-tite and silicocarbonatite rock units within these com-plexes. Wdhlerite commonly is found cocrystallizedwith pyrochlore, latrappite, and niobian zirconolite,and in some rocks it may constitute the major sourceof niobium.
CL emission spectra for wdhlerite from differentlocalities are essentially the same and consist of abroad band of luminescence that sweeps across thevisible spectrum and is assumed to be intrinsic. Itis modified by a narrower band that peaks at ap-proximately 500 nm and is suspected to be fromMn2* impurity activation. The CL of w6hlerite isradically different from that of niocalite andhiortdahlite, two minerals that. ure often comparedcrystallographically and chemically with wdhlerite.Niocalite and hiortdahlite both exhibit REE activa-tion; the nature of the REE activator distribution(LREE or HREE dominance) reflects the geochemis-try of their envkonment.
The chemical composition of six wOhlerite sam-ples has been determined by electron-microprobeanalysis. The inverse relationship between niobiumand fluorine fits the structural interpretation ofMellini & Merlino (1979), who suggested a coupledsubstitution of Nb5+ + 02- for Tia+ + Fl-. Thevariable inlensity of CL among different wdhleritesamples is mainly a function of the ability of ironto quench CL. This is demonstrated by microprobeanalyses of different zones in a crystal that showsvariable CL intensity.
ACKNoWLEDGEMENTS
We appreciate the receipt of wOhlerite samplesfrom S.W. and J.W. Cares, from G. Robinson andR.A. Gault of the National Museum of Canada(sample #368W) and from M. Badham, Queen's
719
University Geological Museum. We also gratefullyacknowledge the assistance of Mabel Corlett, AlanGrant, David Kempson and Sheila McPherson.Financial support was provided by Natural Sciencesand Engineering Research Council of Canada. Wethank G.R. Lumpkin, R.F. Martin and an anony-mous reviewer for many helpful suggestions.
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Received February 21, 1989, revised manuscrtptaccepted August 4, 1989.
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