PRELIMINARY REPORT ON INVESTIGATIONS OF OF COLUMBIUM AND AND OF CERTAIN ASSOCIATED MINERALS Rrcrrant W. HurcnrNSoN,* (Jniversityof Wisconsin, Mailison, Wisconsin.

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* Present address, _ American Co. of Canada, Ltd.,25 Adelaide St. W., Toronto, Ontario, Canada. I Numbers in parentheses refer to the bibliography at the end of the paper. AND TANTALUM 433 INVESTIGATIONSOF MINERALS OF COLUMBIUM proved to be par- Of the minerals studied, tapiolite and ilmenorutile in some detail in this re- ticularly interesting, u"a tnty are discussed of the relationships be- p"ri. Shay of tupiotit. Ied to an examination and the orthorhombic tween the tetragonal mossite-tapiolite series conclusions are pre- - series, and certain tentative to be an intergrowth of sented. Ilmenorutile in our specimensproved and its origin is dis- columbite and . This intergrowth is described cussed. that optical information During the investigation it became apparent associatedwith the was needed for certain minerals that are commonly minerals were made and columbium-tantalum minerals' Studies of these the results are also presentedin this report' minerals were Suitable samples of-the various coiumbium-tantalum of the major problems-of the difficult to obtain, and this factor was one checking by means oI.'x-ray i"t".tigutio". All available samplesrequired' determinations had to difiraction techniques,u"a i" 'o*t tu"'chemical be procured. Acr

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Rotalion Properties

(a) Apparent Angle of Roration in White Light

(6) Dispersion t INVESTIGATIONSOF MINERALS OF COLUMBIUM AND TANTALUM 439 anglesof rotation for monochromatic red and monochromatic blue light. The differencebetween the two values obtained is the amount of disper- sion. The amount of dispersion varies from .2 to .7 degree.This is a sig- nificant and measurabledifierence. Thus far, oil immersion methods have been attempted only on a limited scale, but immersion in oil of R.I. 1.515increases the range of the variation to.3 to 1.1 degrees. This variation is apparently not due to varying CbrOr:Ta2O5 ratios, for samples of columbite-tantalite with widely varying ratios have similar dispersions.Table 2 shows that the specimen(51-27-8) from the Van der Made main ,Southwest Africa has a CbrOr:TazOr, ratio oI 7:1, while tantalite from the Etta Mine at Keystone, South Dakota, has a ratio of 1:3.5. Table 1 showsthat both specimenshave very similar dis- persions. The analysesindicate that the amount of dispersion is related to the FeO content, or more specifically to the FeO:MnO ratio. When the ratio of FeO to MnO is high (greater than unity), the amount of dispersiontends to be high (.5 to.7 degree),and the color fringesin the polarization figure at the 45-degreeposition are strong. Columbite from Gunnison County, Colorado, and from Great Slave Lake in Canada are examples.Table 2 shows that both thesespecimens have high FeO: MnO ratios, and Table 1 shows that they have strong dispersions.One ap- parent exception is tantalite from the Old Mike pegmatite which has a high FeO:MnO ratio of 2.43:1 as shown in Table 2,but has only an intermediate dispersion. This discrepancy is explained, however, by the high content of tin which presumably substitutes for manganesein the sample. If the tin content is computed with , the ratio (of FeO:MnO*SnOz) becomes1.2:1, and this figure is in agreementwith the intermediate dispersion. When the ratio of FeO: MnO is low (lessthan unity), dispersionis cor- respondingly weak (.2 to.4 degree) and color fringes in the polarization figure are weak or absent. Specimen 5I-27-I from the Van der Made Claim, Southwest Africa, is an excellent example. Table 2 shows that it contains essentially no iron whatever but is rich in manganese, and Table 1 records that it has essentially no dispersion. Samples in which the FeO:MnO ratio is near unity have intermediatedispersions (.4 or.5 degree). The analyzed specimen from Tinton, South Dakota, has an FeO:MnO ratio near unity, and an intermediate dispersion of .4 degree is shown for the sample in Table 1. An independent test of this relationship was carried out on additional samplesfor which approximate analyseswere available. ft was possible to select, on the basis of strength of dispersion alone, which of these sampleswere FeO-rich and which were high in MnO. Additional samples of both iron and manganesevarieties are being RICHARD W. HUTCHINSON obtained and will be studied to substantiate this relationship further. It seemsprobable that with additional data (especiallyusing oil immersion) an empirical scale can be set up that will make it possible to determine the FeO: MnO ratio of a given sample from its amount of dispersion.

Concl,usions (o) The apparent angle of rotation does not vary significantly with composition in the columbite-tantalite series. (D) The amount of dispersionin the columbite-tantalite seriesappears to vary with the FeO:MnO ratio. Additional samples, analyses and measurementsare required to substantiate the relationship. (c) Comparison of chemical analyses and specific gravities clearly shows that specific gravity alone can be used only in a general way to place a given sample in the columbite-tantalite series, because both FeO:MnO and CbzOs:TazOrratios affect the specificgravity.

Yttrotantalite Two specimensof yttrotantalite were obtained and studied. Optically both are noticeably different from normal tantalite. They are isotropic or very weakly anisotropic, and neither sample showsthe degreeof aniso- tropism common to normal tantalite. Internal reflections are much stronger than those usually encountered in tantalite and they interfere drastically with the determination of rotation properties. It was suspectedthat both crystals were partially, or wholly, metamict in nature and perhaps radioactive. Tests with a simple Geiger counter showed that both samples are strongly radioactive. X-ray diffraction powder patterns of both specimens,in general,match that of yttrotanta- lite. In both cases,however, there are broad diffuse bands instead of sharp lines. This suggestsa partial breakdown of the crystal structure. An analysis of one of the samples was subsequently obtained and is given in Table 2. ft reveals appreciable amounts of , rare-earths and , thus accounting for the radioactivity and metamict prop- erties. In polished surface the specimen is notably impure and contains tiny inclusions of other unidentified minerals. Whether these impurities contain the radioactive elements disclosedby the analysis is not known. No general conclusionsare justified on the basis of the two specimens studied, but it appears that the yttrotantalite is partially metamict as indicated by Dana (9, p. 763). This could result normally from the pres- ence of the rare-earth elements and uranium that are present in the sample. The metamict condition would explain the isotropism of the specimensin reflected light. AND TANTALUM MI TNVESTIGATIONSOF MINERALS OF COLUMBIUM

B i smul ot ontalite and,S tibi otantalite was obtained and studied' A single sample of each of these minerals impure and contain vary- MicrosJopic eximination showedthat both are appears to replace them ing amounts of an unidentified mineral that semi-transparent and is along cracks and fissures' Stibiotantalite is softerthannormaltantalitewithstrongerinternalreflections,lower also is softer and anisotropism and stronger dispersion' Bismutotantalite reflections than tantalite. has Iower anisotropism-urd ,1rorrg., internal similar to each other in Thus stibiotantalite and, bismutotantalite are optical properties in reflected light, a bismutotantalite, "angles show simi Since the antimonY varietY is semi must be prepared for the determinati mitted light. of both samples were taken' X-ray diffraction powder photographs a pattern the The specimen of stibiotanialite gives llit. T"t:|es card'is availablefor bismuto- A.S.T.M.index card for that mineral' No pattern of stibiotantalite more tantalite, but its pattern resemblesthe tantalite' The similarity in closely than it d"., ;;;-putttrn of norma^l (9' p' 770)' For the two speci- r-ray patter.t, i, utrol"fi;;tby Dana powder patterns was es- *".ra .trrdi"d, the resemblancebetween ff-rav oi tt'" strong lines' Weaker pecially close with .";d to the positions closer in position to the cor- lines in the bismutotantalite pattern were respondinglinesofnormaltantalite.Thecrystalstructure.ofstibiotanta- from that of normal tanta- lite is known (10; 20, p' 227), andis difierent a trivalent cation lite (2; 22).InUir-oiot*'tuti'", ut in stibiotantalite' irt takes the Place of the bivalent normal tantalite. In view of this above, it seems likelY that bismut structure to stibiotantalite' It is evi< of the varieties containing the triv

M anganotantalite Two samPles designated as ma were secured and investigated' Tl as well as their r-ray diffraction pot of the ordinary columbite-tantalil that in these two samplesdispersic weak as shown in Table 1' Althor available, the weak disPersionis i content (or low FeO:MnO ratio), tion of thesespecimens as manganotantalites' 442 RICHARD W. HLITCHINSON

Mossnp-Tepror,rrE Method.sand. Scope oJ the Inaestigation rt was desired to investigate the opticar properties of the mossite- tapiolite seriesin a manner similar to that outlined for the corumbite- tantalite series. unfortunately very few specimens of the tetragonar serieswere availabre. only three prorr".r ,"-pr", were obtained, but o.re other, from the OId Mike pegmatite, has optical properties th"i *urr".rt its assignment to this group. The rotation prop.iti., of these specimens were determined and are summarized in TaLIe 3.

Tanrr 3, Rorerrolr pnorpnnBs ol Sour Taprorrrrs

Univ Wis. Mineral Ref. No. Red White Blue

i-ts.4 Tapiolite Best Bet Pegmatite, ess nil r )v, Drever Lake, N.W.T , strong Canada

Peg.M-3 Tapioli te Peg Tantalum Mines, ess.nil 2.0 f>a, RossLake, N.W.T., strong Canada

49-11-ts Tapiolite Old Mike Pegmatite,* ess nil t>!, Custer County, strong $outh Dakota

50-53-1 Tapiolite Peg Tantalum Mines,f ess,nrl f >o, RossLake,NWT., strong Canada

* This tapiolite ocors as inclusions in tantalite. t Indicates spcimen has been analyzed and is listed in Table 2

of the samples was _ 91" analyzed, and its composition as shown in Table 2 indicates that it is a tapiolite. Another of the samples studied consists of inclusions in the specimen of tantalite from the ord Nlike pegmatite.fnasmuch as the host tantalite has a low CbzOs:TazOsratio (Table 2) it seemslikely that the inclusions (if they .ouidlb" an-a\y"ed) would also prove to be'tantarum-rich. The composition of the other two specimensis not known. Thus it is evident thai the writer is in no posi- tion to discuss the relation between opticar properties and composition mossite-tapiorite series, i".tlr but the results of the rimited studies are briefly presented. The searchfor specimens of the mossite-tapiolite series,together with r-ray work on available samplesand a study of the literatu.i r.d, ho*- ever, to a critical examination of the rerationship between.the iet.asonat AND TANTALLIM M3 INVESTIGATIONS OF MINEKALS OF COLUMBIUM

columbite-tantalite series' mossite-tapiolite seriesand the orthorhombic and are discussedbelow' Certain aspects of this subject are of interest Rotation ProPerties (o) Distinction of Mossite-Tapiolite from Columbite-Tantalite quick and polarization f,gures and rotation properties offer a simple, tapiolite ard columbite- effective technique for distinguishing between similarity of the two min- tantalite in polished ,o,fucts]P'eviousty the of extreme difficulty' The erals has made their distinction a matter minerals are almost identical' pfrytf.tf and chemical properties of the two polished surfacesis difficult' and.x-ray work on ,,,t'ull'itp"'e grains in however, by their rota- The two minerals can be ,uiiaty distinguished, (Table 4)' tion properties and relative reflectivities (b) Angle of Rotation and Dispersion the apparent angle of rota- Significant and measurable d'ifierencesin of tapiolite. The values tion in air were roo.rJ to, the various specimens 1'7 to 2'9 degrees'Variations of the angle recorded in Table 3 range from described for columbite- in the strength of disfersiot' similar to those from Ross Lake, North- tantalite have also be"'r, e.r.orr.rtered.Tapiolite pegmatite hav-estrong blue and west Territories, and from the Old Mike at the 45-degreeposition' pink color fringes in their polarization figuies Territories has blue and Tapiolite from Drever Lake in the Northwest yellow color fringes. Althoughitisimpossibleatpresenttorelatethevariationsinrotation angles and dispersion to changing r series,it is interesting to note that tl tantalite have high dispersion' Strel the FeO:MnO ratio. Investigation photographs of tapiolite from Rost iho*tt that this material (specimen and has a rutile structure' Goldsc tapiolite is ordered, with tri-rutile s the tetragonal mineral tapio- seemsthat there are two distinct forms of properties may be related to Iite, and the observed variation in optical the degree of ordering' Ser'i'esand the Relationshi.pbetnpeen the OrthorhombicCotumbite-Tantalite T etragonal, M ossite-T apioQte S eries Old Nlike pegmatite and In the Ta-rich tantalites examined from the fromtheEttaMine,inclusionsoftetragonalmossite-tapiolitewere serieswere never dis- found: but inclusions belonging to the tetragonal 444 RICHARD W. HUTCHINSON

Tentn 4. Ror:arrol,'pnopnnrms ol MrNnRu,s lr -

Mineral Cryst. ReBectivity relative System and listed in descendingorder

Hematite / >?, weak 2.4 highest

Rutile B tet. />?, weak to 32 2.9 2.4 r)s, weak or nearly equal to distinct masked hematite

Ilmeni:e r)r, distinct 2.2 2 9 3.1 ,>/, distDct definitely lower than rutile

Tapiolite 2.9 28 t7 / >?, strong nearly equal to

Columbite- r.5 1.4 tantalite r )r, weak to slightly lower than strong tapiolite F

Euxenite none none definitely lower than columbite

Cassiterite 1.9 nearly equal to to euxenite

Ferberite ess nil 26 r>r, distinct very close to cassiterite Monazite all dispersionsand rotation properties masked by inter_ low, definitely lower nal reflections than ferberite, cassiterite

Zircon all-dispersions and rotation properties masked by inter_ lowest nal reflections

covered in any of the numerous Cb_rich columbites. Columbium is a much more abundant elementthan tantalum (19, pp. 604_605)and this is reflected in the fact that columbite is much more abundant than tantalite. By the samestandard mossiteshourd be much commonerthan tapiolite' This is not the case. Dana (9, p.77s) lists onry two occur- rencesof mossite. The first of these was describedby Brdgler (3) and is the original material from nrloss,near obstford, Norway, from which the mineral takes its name. The analysis of this specimen given in Dana,s table (9, p. 776) shows only combined columbium_tantalum content. A M5 COLUMBIUM AND TANTALUM INVESTIGATIONSOF MINERALS OF

OrrnN Assocrarso Wrfir Cb-Ta MrNar''lr's --_--_ Separation of in 45-degreePosi- Rotation ol Description of Figure in Description ol Figure upon Internal Isogyres /6 Polarizer and Analyzer Crossed Mineral in CrossedPosition tion, Reflections ol Field Analyzer, Diameter fringes on con- lringes on black isogyres, violet red- black isogyres, weak violet rare, weak & cave side, blue-green lringe on convex concave side, deeP green on convex dish if present side, center of field Yellow side, center of field Yellow isogyres (if internal narrow pink black to reddish strong & gray to reddish isogyres, sides pale numerous, blue reflections present), concave frioge ot concave sides, narrow reddish blue, convex sides PinL (or nasLed) Iringe on convex sides pink {ringe on fringe on black isogyres, distinct black isogYres, weak blue con- concavesides, green-blue lringe on coneave side, Pink fringe on convex vex

blue black or dark green-blue isogyres' black i$gyres, no yellow green fringe on concave side' Iringes convex

concave side pale blue' indistinct black isogyres, red- black isogyres, Iringes weak to rare, weak & convex sides Pink, {rings dish stlong

no separation oI isogyres no color fringes black cross, none or very rare black isogYres,

very weak often no color black isogyres, fringes varied black isogyres, essentially numerous & masked by internal reflections-appar fringes (or pink concave, blue convex white, graY, brown, blue concave,pink convexbut thisweak and very weak) red and possibly due to internal reflections

black isogyres, Iringes distinct-con- 3070 black isogYres, no color lringes rare cave side blue, convex orangey-pink

reflections reflec- figure unobtainable, internal cannot tell figure unobtainable, internal numerous & strong, flood field tions flood field always Present, white, brown, red, patchy gure unobtainable, internal refl ections gure unobtainable, internal refl ec- fi strong & alwaYs cannot tell fi f,eld flood fieLd present, reddish tions flood

Iater analysis of this material in 19( 52/6 and'a CbzOscontent oI 3170" rich and should properly be called ta that the mineral name mossiteProJ be dropped becausethe sampl should ('mangano-n samplelisted by Dana is a possibly a columbite' it is described(9, p. 776) asdoubtful, and existenceof mossiteand of a AII thesefeatures lead one to question the The suggestion is ad- complete tetragonal mossite tapiolite series' M6 RICHARD W. HUTCHINSON

complete tetragonal seriesof comoosi_ but only an orthorhombic series.In rorhombic form may revert to retrag_ iolite. This would explain the anoma_ inc olu mbi te an d theover-au r"r,rrl'"1tJl:*T:rffj il:T:[ dance of columbiunr. ;ffiTl: Dana stated (9, p. 176) that it has not been establishedthat the natural tetragonal seriesextends to a high columbium member. Brandt (2, p' 6) reported that if a minerar unutyli. gives more than 2satom per r.r"t g-f columbium, a columbite phase must exist. she further stated that if Iarger amounts of tantarum are found in an iron-murrrur".. mineral either orthorhombic or tetragonal compounds can be formed. Apparently synthesis of the high ta;talum members can yield either tetragonal tapiorite or orthorhombic tantalite, but at trr. rrigr,'."r"mbium end only an orthorhombic member port to our hypothesis which is als< currence of columbite, tantalite anr pegmatites while tantalite and tapiolite may be explained by the greater reli which, in the course of difierentiation, causes the earry crystallization of columbium-rich compounds. Tantalum becomesenriched of crystallization io the point at a rater time and ,e.utt, in the formation or (because of tantalite of the enrichment in tantalum relative tapiolite. to corumbium)

Polymorph'ism

cause of porymorphism Th.: in the proposed columbite-tantarite- tapiolite series is not now understood. brder-disorder cannot be the solution as it would meatr that tapiolite (the higher symmetry and sup_ posedly disordered form) would be early while tantalite (the low sym- metry and ordered form) would be late. This is the reverse logic of the geo_ occurrenceof these minerals in p"g-;tit"s. rt is also known that a tlite (24, p. tI; 13, p. 17) and this red structure and finally rules out phism. llent columbium and tantalum ions ne between two different co_ordina_ e of the columbium and tantalum try in the Ta_rich range. fn actual uinquevalent columbium and tan_ ally in the octahedral range. co-ordination TANTALUM M7 1NVESTIGATIONSOF MINERALS OF COLUMBIUMAND

structure It may be significant that tapiolite has a tri-rutile or rutile based on the based on the mineral rutile while columbite-tantalite is in tapio- structure (2, p'4; t3; 17)The causeof polymorphism the case of lite-tantalite may be similar, or comparable' to the causein and cooling rutile-brookite but this has not been determined. Heating be con- experiments to d.eterminewhether orthorhombic tantalite can versa' ,r.it"d to tetragonal tapiolite at high temperatures, and vice Addi- would be of interest in studying the polymorphic relationships' would tional syntheses of the columLite-tantalite-tapiolite minerals also be of value.

Summary aniJ Conclusi ons (o) Tapiolite can be distinguished quickly and readily from the colum- figures and bite-tantalites in polished suifacesby the use of polarization rotation proPerties. (b) The apparent angle of rotation varies significantly in,the different composi- ,p".i-"n, oi iapiolite siudied. This variation may be related to tional difierencesor to the degreeof ordering' (c) Strength of dispersionalso varies in the different tapiolites studied. the speci- This variation may be related to varying FeO:X4nO ratios in mensinvestigated. (d) The existenceof the mineral mossite, and of a complete tetragonal that a single -orrit"-t"piolite series is questioned. It is proposed -only nature and seriesof composition (Fe,nin)CbrOo-(Fe,Mn)TazOooccurs in proposal includes the minerals'coiumbite, tantalite and tapiolite' This the light of is an hypothesis only, and should be critically examined in further information. (e) The causeof polymorphism in tantalite-tapiolite is not understood' experi- Additional work, inciuding heating and cooling and synthesis. ments, is needed to determine the cause of the dimorphic relationship' study' (/) A second distinct form of tapiolite was encountered in this form with tri- Goldschmidt previously described (13, p' 17) an ordered from Ross rutile structure and a superlattice' The analyzed specimen The relation- Lake, however, is a disoriered. form with rutile structure. further in- ship between degree of ordering and optical properties needs vestigation. h.uBNonuur-B GeneralStatement (16, pp' The name ilmenorutile was orginally proposed by Koksharov 352-355) for what he called "a new variety of rutile'" Later-analyses apparently showed that this material contained appreciable columbium andminortantalum.Dana(9,p.558)describedilmenorutileascolum- M8 RICEARD W. HUTCEINSON

bian rutile, and this is the accepteddefinition at the present time. A_.number of specimensberieved to be ilmenorutile were obtainedand studied. The first two samplesreceived were cry..italsfrom lt[,ba, French cameroons and from the Tonkolili District, sierra Leone. oriented polished surfaces of these,crystaiswere prepared. Subsequentlyrwenry additional sand samples from pits alongside streams, arso from sierra Leone, were obtained. These were berieved to contain ilmenorutile and polished surfaces of them were prepared. The resurts of the studies of these materials to date are presented below.

Obseraalions ond, Di scussion The crystals from M'ba, French cameroons and from the Tonkolili District of Sierra Leone proved to be comprex, intricate intergrowths of columbite and rutile. rdentification of the constituent mineralswas first accomplished using rotation properties and polarization figures on polishedsurfaces. The identificationwas rater sulstantiated byLeans of x-ray diffraction powder photographs. The r_ray patterns obtained showedstrong rutile lines and weak columbite iines. There was no aD_ parent shift in position of the rutile lines (from the pattern of ,,normai,, rutile) of the kind often resurting from substitutional solid solution. Examination of a number of the concentrates in porished surfaces revealed that the non-magnetic portion of each ,u-pr.t also contained some mineral grains that are intergrowths of corumbite and rutile. These intergrowths, therefore,account for at least part of the columbium con- tent of these specimens. rn addition to the intergrowths, each non- magnetic fraction contains numerous grains of homogeneousrutile. From one of the non-magnetic samplesof concentrates,several grains were selected, and individual *-ray diffraction powder photographs of these grains were taken. These photographs showed that there was no visible shift of the rutiie rines betwee.r u gruin of pure rutile and a grain containing the intergrowth of columbite and rutile. rn both cases the rutile lines agreed with the A.s.T.M. index card for normal rutile and with the pattern of a difierent check sample of our own rutiie. This is, at best, a rough experiment but it indicates that littie columbium is contained in solid solution in either the homogeneousrutile grain, or in the grain of rutile that has the corumbite intergrowth. rf Jolumbium is present in the rutile, it is in insufficient amounts to cause a shift in position of the lines of the rutile powder pattern. Data at hand, therefore, suggest that ilmenorutile as a distinct min-

of the concentrate specimens had been divided into magnetic and non_magnetic Iractlons_,_^^ll^::O TANTALUM M9 TNVESTIGATIONSOF MINERALS OF COLUMBIUMAND

only intergrowths of eral is not present in the material studied, but doubt about the columbite with ordinary rutile. This fact raises some and Ied the writer existenceof ilmenorutile as a.t independent mineral' to an examination of the literature in this regard' was an independent Prior (18, pp' 88-39) concluded that striiverite striiverite from mineral and was related.to ilmenorutile. He considered Italytobeasolidsolutionofthetapiolitemoleculewiththerutile and Russia molecule. He also believed that ilmenoiutiles from Norway His conclusions were solid solutions of the mossite molecule with rutile. and thus do not and formulae, however, were based on chemical analyses take into account the possibility of mineral intergrowths' a distinct mineral Schaller (21, p.38i stated ihat ilmenorutile was in isomorphous subspeciesof rutile in which Fe, Cb and Ta were present analyses' and like- mixtures. This conclusionalso was based on chemical intergrowths' wise does not allow for the presenceof fine mineral of Lond'on A more recent article in a bulletin ol the Imperiol Institute of Sierra Leone (15) describedilmenorutile from the Tonkolili District optical proper- as a distinct columbium-bearing variety of rutile. The but the sample ties of the specimenin transmitled light are discussed' r-r.ay difiraction was not apparently examined in reflected light or by intergrowth of methods. All our specimensfrom this area contain the columbite and rutile. X-rayinformationconcerningthemineralilmenorutileisscarce. in ilmenorutile Wyckoii (24, p.17) indicated thit columbium is present the results in solid solution and is disordered.And,o and Nitta published their article was (1) of some r-ray studies of Japaneseilmenorutile but are not not available to the writer. Hence the results of their investigation consideredhere. is-of interest' The nature of the intergrowth in the samples studied features In the two crystallin" sp.Ji-e,ts of ilmenorutile the following were observed. of the inter- (1). Rutile is invariably the more abund'ant component the intergrowth growth and constitutes thl host mineral' For this reason may be describedas columbite in rutile' and have (Z). Crains and blebs of columbite are of irregular shape variousorientations,butareratherevenlydistributedthroughoutthe host crystals of rutile. fnthesamplesofconcentrates,thefollowingfeatureshavebee.nnoted' of the inter- (1). Rutile is invariably the more abundant component growth and constitutes the host mineral' in a single (2). The orientation of intergrown columbite grains varies grain of rutile, so far as can be determined' 450 RICHARD W. H\]TCHINSON

(3)' The size of the intergrown corumbite grains varies in a single grain of the host from tiny to large (relative to the size of the host grain). (4)' The shape of intergrown columbite grains is extremely irregular. (5). The distribution of columbite intergrowths varies within a single rutile grain. Portions of one rutile grain may contain more colum- bite than other portions. (6). columbite intergrowths generally occur in the more cracked and pitted grains of perfect rutile. unpittedand unfractured grains of rutile tend to be homogeneous and lack corumbite inte.gro*-ths. This is a generalization and exceptions were encountered. whether the imper_ fections are earlier or later than the intergrowth is not known. The origin of the intergrowth is yet another problem. No information is available to the writer concerning the geologic occurrence of the ilmenorutile studied from r{'ba, French cameroons. The specimensfrom the Tonkolili District of sierra Leone are reported to occur in alruviar stream gravels (15). The folrowing information concerning the possible derivation of the ilmenorutile has been summarized from additionar references(a; fl; I2). The streams in which the aluvium is found drain an area of crystal- line rocks, including various and a bert of schistsand migmatites. rlmenorutile is thought to occur in sugary quartzveins, in different kinds of medium- and coarse-grainedg.u.rit", near migmatitic fringes of the schist belt. rt may also occur in . unfoitunately thiJ informa- tion is not of direct aid in understanding the origin of the intergrowth. The laboratory findings are, likewise, inconclusive. The iriegular physical characteristics of the intergrowth are unlike those of many other intergrowths of unmixed origin. rf 'nmixing is the answer, then we would expect to find some content of columbium still in solid sorution in rutile from grains showing present the intergrowth. c-ray experiments do not indicate that this is the case,but more detailed *-iuy ,t,rdi.s are essen- tial' Thus the observations to date do not seem to support an unmixing origin but it may be argued that they do not excludell entirely. other possible explanations . would be that the intergrowth is of re- placement origin, or that it is the result of simurtaieous (eutectic) crystallization. observation (1) above would satisfy either of these pos- sibilities. Replacement of rutile by minor amounts of corumbite courd result in this feature, but so could eutectic crystallization. The fact that some grains of rutile contain the intergrowth, whereas others are homo- geneouscould also be due either to replacementor to eutectic crystalliza_ tion where rutile was the initiar pure phase to crystallize. observation (6) above would support a replacemenl origin o"ty ii we could be cer_ tain that it is a causativefeature. INVESTIGATIONSOF MINERALS OF COLUMBIUMAND TANTALUM 451

Conclusions (o) In the specimens studied, ilmenorutile is not an independent mineral but is an intergrowth of two distinct mineral phases, namely columbite and rutile. (6) The origin of the observed intergrowth is not known. Unmixing seemsunlikely to be the answer but is not excluded. Either replacement or eutectic crystallization could also account for the observed features. (c) Further work is needed to determine whether ilmenorutile occurs as an independent mineral in nature, and to determine the origin of the intergrowth.

Oprrcar, PnopBnrros ol MTNERALSAssocrATED wrrH CoruMsruM- TeNr-q.r,uu MrNnnars Generaliliscussion During the courseof the optical investigations of columbium-tantalum minerals it became apparent that the distinction of these minerals from certain of their common associateswas a problem. It was decided to in- vestigate the optic properies of these minerals, and especially their rotation properties, in the hope that the problem could be simplified. The minerals investigated, in addition to columbite, tantalite and tapio- lite were hematite, ilmenite, rutile, cassiterite,euxenite, monazite, zircon and ferberite. Some of the optical properties of these minerals, including their ap- parent angles of rotation in air, dispersions and relative reflectivities harre been determined and are summarized in Table 4. Since most of these minerals are known to vary in composition, the data given apply only to the specific material studied. Additional data for other composi- tional members may later be found to modify or extend the range of properties given. Nevertheless, it was found that the properties so far measured and recorded are extremely valuable in the distinction and identification of the various oxides. They permit simple and rapid iden- tification in polished surfaces,and are of special value where work with mixtures of the minerals is involved.

Rnrnrumcns

1. ANoo, R., ervo Nrtta, I., X-ray study of some Japanese minerals, II, ilmenorutile at Tesirogi, Hukusima Prefecture: Jour. Chem.Soc' f apon,621978 (1941)' 2. Bnauor, Ke.mN, X-ray studies on ABOr comPounds of rutile type and ABOs com- (1943)' pounds of columbite type: Arkitt. for Kemi,., Min. och GeoL, Bd' 17A, N:o 15 3. Bnciccrn, w. c., uber den Mossit und iiber das Krystallsystem des Tantalit (Skog- bolite aus Finnland): Vidensh. Akatl. OsIo, Mat.-Nat. Kl' (1897). 4. Colonial Geology and Mineral Resources,Vol.4, No. 1,p.72. 452 RICHARD W. HUTCHINSON

5. Caurnon, E. N., eNn GnnrN, L.H,,Polatization figures and rotation properties in re- flected light and their application to the identification of the minerals: Ec. GeoI., 4s, 7r9-7 54 (1950). 6. CeurnoN,E. N., HurcnrNsoN,R. W., awnGnrnx, L. H., Sourcesof errorin the measurement of rotation properties with the ore microscope: Ec. Geol., 48, 574-Sg0 (1es3). 7. Celrcnow, E. N., JauNs, R. H., McN.rrn, A. H., .ll,to pecn, L. R., The internal struc- ture of granitic pegmatites: Ec. GeoI.,Mono.2, (1949). 8. Dana's Textbook of Mineralogy 4th ed., Edited by W. E. Ford, John Wiley and Sons, New York (1932). 9. Dana's System of Mineralogy 7th ed., Vol. 1, Edited by Palache, Berman and Frondel, John Wiley and Sons, New York (19rt4). 10.Dtrtsrnou, K., Uber den Bau des wahren Antimontetroxyds und des damit iso- morphen Stibiotantalit SbTaOa: Zeits. A norg, Chem., C h. 239, 57-65 (1938). 11.Geological Survey of Sierra Leone, Annual Report (1952) pp. 1a-18. t2.Geological Survey of Sierra Leone, Annual Report (f9a9) pp. 4-5. lJ. Gor.nscruror, V. M., Geochemische Verteilungsgesetze der Elemente, VI, Uber die Krystallstrukturen vom Rutiltypus mit bemerken zur Geochemie Zweiwertiger und Vierwertiger Elemente: Vid.ensk.Ahad.. Oslo, Mat.-Nat. Kt.,Skr.(1926),yol. l, paper I. 14. Golnscnuror, V. M., Geochemische Verteilungsgesetze der Elemente, VII, Die Ge- setze der Krystallochemie: Vidensk. Ahad. Osto, Mat.-Nat. Kl., Star.(1926),yol. l, Paper IL 15. rlmenorutilefromSierraLeone, Imperiallnstituteof LondonBullerin,42,4s47 (1944). 16. Korsnenov, N ., M oter'iali en zur M i,neralo gie Russl and.s, 2, (1854) . 17. Pnron, G. T., Note on a connexion between the molecular volume and chemical com- position of some crystallographically similar minerals: Mineral. Mog., 13, 217-223 (1903). 18. Pnron, G. T., axn Zeuooxrnr, F., on struverite and its relation to ilmenotttile: Min- eral. Mag., 15, No. 68, 78-89 (1908). 19. Rnrq alra, K., em Sananre, TH. G., Geochemistry, Univ. of Chicago press (1950). 20. Rosnx, O., aNn WrsrcnnN, A., Minerals of the Varutrask pegmatite, XII, On the structure and composition of minerals belonging to the pyrochlore-atopite group and on rc-ray analysis of stibio-microlite: Geol. Fdr. Fi)rh., Mar.-April (f938). 21. Scrnr.r,un,W.T.,MineralogicalNote,Series2,Astudyof therutilegroup:U.S.G.S. Bu.lletin SO9, l-39 (1912). 22. Srunorvexr, J. H., The crystal structure of columbite: Zeits. Kryst ,Bd.7S,l/Z (lg3}). 23. TtrosnN, analysis in Bndccon, w. c-, Die Mineralien der sudnorwegischen Granit- permatitgange, I, Niobate, Tantalate, Titanate und Titanoniobate: Vid.ensk. Akad., OsIo,Mat.-Nat. K\.6,52 (1906). 24. Wvcrolr', R. W. G , Crystal Structures, Vol. II, Chap. IX, text 1951. il[anuscri,pt recei.led May 20, 1954.