THn AMERICAx MINERALoGIST JOURNAL OF THE MINERAI,OGICAL SOCIETY OF AMERICA

Vol 43 SEPTEN{BEROCTOBER. 1958 Nos 9 and 10

SYNTHESIS AND PI{OPEI{TIES OF CARNOTITE AND ITS ALKALI ANALOGUES*

Paur B. R.rnrorv,Jn., Ir'. S. GeologicalSuraey,IVashington 25, D. C.

AssrR,\cr

Carnotite and its Na, Rb, Tl, Cs, and H;e(?) analogues have been synthesized in several clifferent ways in attempts to olrtain crystals suitable for single crystal structural investigations. Aqueous methods failecl to procluce usable crystals, but a simple fusion technique using metavanatlate fluxes proved highly satisfactory. lvork in progress by D E. Appleman has shown that carnotite is composed of [(UOz):V:Os],,-2"layers boun

InrnotucrroN This report records the results of experimentsdealing with the syn- thesisof carnotite and its alkali analogues.The work was primarily con- cernedwith the preparation'ofsingle crystals suitable for detailedstruc- tural analysis,though some insight into the chemistry of the precipita- tion of carnotite was obtained.

PnBvrous Wonx oN SrrNrH-ESrs Carnotite is named for l,{. Adoiphe Carnot, French chemist and min- ing engineer. Carnot (1887) unknowingly synthesizedthe ammonium analogue of carnotite when he noted that uranyl salts quantitatively precipitate vanadate from aqueoussolution as ammonium uranyl vana- date. The mineral was not describeduntil 1899. Canneri and Pesteili (1924) synthesized anhydrous carnotite, Kr(UOr)rVzOa,&nd anhydrous sodium carnotite, Nar(UOr)rVrOr, by fusing a uranyl salt with an excessof the appropriate alkali metavana- * Publication authorized by the Director, U S. Geological Survey.

799 8OO ],AUL B. BARTON,JR. date. Their chemicalanaiyses sholv that the compound is equivalent to anhydrous carnotite. Canneri and Itestilli obtained well developed,dia- mond-shaped,platy crystals which they reported to be fluorescent.It was suggestedthat the crystaiswere orthorhombic,but no tr-ray or mor- phologicalwork was done. They proposedthat the crystals were salts of a uranovanadicanion, (UOzVOa)-l. Hillebrand (1924) formed carnotite by exchanging the calcium in tyuyamunite, Ca(UOz)zVzOenHzO, for potassium in a potassium-mer- curic iodide solution. Sundberg and Sill6n (1949) followed the vanadate fusion procedure of Canneri and Pestelli,but becauseSundberg and Siil6n were unable to obtain sufficiently large crystals by this method they fused a mixture of potassium carbonate,vanadium pentoxide and ammonium uranate (diuranate?)in an excessof potassiumcarbonate. After slow coolingthe resulting crystals were washed free of the flux with water and diiute nitric acid and were then used for the determination of the preliminary structure of KUOzVOa.Sundberg and Sill6n also recordedthe formation of at least one other potassium-uranium-vanadiumphase, but they did not define it. Nlurata, Cisney, Steifi, and Zworykin (1950) sludied base exchange amongthe potassium,barium, calcium, strontium and sodiumanalogues of carnotite and tyuyamunite. They mixed solutionsof alkali or alkaline earth pyrovanadateswith solutions of uranyl nitrate; the pH was ad- justed to 6 with varying amounts of the oxide or hydroxide of the alkali or alkaline earth used. Following the developmentof the precipitate the pH was lowered to 3 and the preparation was digested for two weeks on a steam bath. Platy crystals of the calcium preparation, that is, tyuyamunite, were visible under the optical microscope; the other prep- arations showed crystals only under the electron microscope. Morachevskii and Belyaeva (1956) revived the idea of canneri and Pestelli(1924) of a uranovanadicanion. By spectrophotometricmethods in the 250-400 mu region Morachevskii and Belyaeva noted that dilute solutions (less than 5x10-5.04 in uranovanadate)had a much higher optical density than either the uranyl or the vanadate solutionsalone, and from this they concluded the existence of some complex ion. They cited no evidence that the uranovanadate solution was not colloidal. Insoluble salts formed by the reaction of the uranovanadic anion with the divalent cations pl+2, Bat2, and UOz+2contained 6 to 8 per cent HzO (they also made a Mg+r compound but gave no analysis)' They assignedto thesesalts the structural formula Me[UOz(OH)rVOJz'nHzO' By analogy they give the same general formula to carnotite, tyuyam- unite, and sengierite. Morachevskii and Belyaeva give no x-ray data, .tI'IirZrESlS AND PROPERTIES OF CARNOT'ITE 801 but it seemsmuch more likely that they prepared the Pb+2,34+2, and UOz+2analogues of metatyuyamunite which wouid also contain about 6 to 8 per cent HzO but whose general structural formula would be I'Ie[(UOr)zVzO8]'3HrO. Recently R. F. l'Iarvin, U. S. GeologicalSurvey, (personalcommuni- cation, 1956) has synthesizedcarnotite and rauvite (a poorly defined hydrous uranyl vanadate having V:U greater than 1:1) in aqueous solution; the rauvite formed in the pH range from approximately 1.5 to 2.0.The productswere very fine grained. Becausecarnotite is very easily prepared, it is probable that many other investigatorshave also createdthis compound; but to my knowl- eclge,there is no proceduredescribed to form carnotite crystalssatisfac- tory for single-crystalr-ray rvork from aqueoussolution.

Pnocrprrarrox on C-tnNorrrE rRoM Aqurous SoruuoNs Carnotite usuaily occursin nature as a microcrystallineyellow powder impregnatingsandstones or coatingjoints and fractures.Crystals of suit- able size for single-crystalr-ray work (largest dimension greater than 0.1 mm.) are extremely rare.This is due, at least in part, to the very low solubility of carnotite, less than 1 pp- in pure water at pH 7 (R. F. X{arvin, personal communication), which causesit to precipitate in a very fine grained or colloidal state. As shown below, the reactionsin- volved in the precipitation of carnotite are very complicated,and this fact, also, may contribute to the very fine grain size in most natural carnotites. At room temperaturecarnotite is precipitatedfrom aqueoussolutions as the stablephase when stoichiometricproportions of potassium,uranyl, and vanadate ions are mixed in the pH range from 2 to 9.5. Above pH 9.5 another phaseof unknown compositionprecipitates at a more rapid rate than does carnotite, but the unknown compound apparently con- verts to carnotite on standing. In the pH range from 1.5 to 2 a brown phase,identified through its comparativelydifiuse *-ray powder pattern as rauvite, precipitates from solutions greater than about 0.01 M ln uranyl and vanaciateions. It is not known whether rauvite is metastable u'ith respectto carnotite. Below pH 1.5 the uranium is not precipitated, but VzOs'nHzOmay form if the vanadateconcentration exceeds 0.00I M- The pH rangesand concentrationsgiven above are onlv approximateas the attainment of equiiibrium is extremelysiow at 25oC. The uranyl ion, UOz+2,exists as the predominant uranium speciesin aqueoussolution below pH 3.5 (\Iiller, 1955). Above this pH various hydrolysis products becomepredominant, eventually leading to the pre- cipitation of uranyl hydroxides at about pH 4.5. In the presenceof 802 PAUL B. BARTON,.IR.

anions such as sul{ate,acetate, and especiallycarbonate, the degreeof hydrolysis is reduced through the Iormation of complex iols, and uranium-bearingsolutions are stableat higher pH values (up to i1 or 12 with the carbonate). These complexesmust be destroyedto precipitate carnotite, but even the very stable carbonate complexesseem to have little efiect in retarding precipitation (though they undoubtedly increase the solubility of carnotite to someextent). H. T. Evans, Jr., and It. lI. Garrels(U. S. GeologicaiSurvey, per- sonal communication, 1956) have summarized the available chemical and thermodynamicdata on vanadium in aqueoussolution. On acidifica- tion, an alkalineaqueous solution on pentavalentvanadium goesthrough a complex seriesof polymerization reactions which are shown in the lower two-thirds of Fig. 1. The lor.r.erpart of the diagram illustratesap- proximately how the concentrationsof lhe different vanadate species vary with pH, the central part showsthe predominant ion at any given pH. The formulas assignedto the various vanadate speciesare believed by Evans and Garrels to give the best agreementwith the diffusion, cryoscopicand titration studiesrecorded in the literature, but thesedata merely reflectthe averagedegree of polymerizationand other, undefined, ionic speciesmay be present in significant amounts. Likewise, the co- ordination of the vanadium by oxygen,a factor which is reflectedin the ratio of vanadium to oxygen in the various ions, may reasonablybe as- sumed to be 4, or 5, or possibly even some intermediate values in the casesof ions having vanadium in both degreesof coordination.Because the ionization constantsare not known, all of the anionic specieswith the exceptionof the polyvanadatesare written lvithout attached hydro- gen ions.Thus Fig. 1 presentsa grosssimplilication of the aqueouschem- istry of pentavalent vanadium. The chemical formula for carnotite, KUO2VO4 zH2O, suggeslsthat it is an orthovanadate,br"rt new tr-ray diffraction studiesclearly show that it is not. D. E. Appleman (U. S. GeologicalSurvey, personalcommunication, 1957) has done preliminary rvork on the structure of carnotit,e, using rnaterialsynthesized in the courseof the invesligationdescribed by this report, and has shown that the structure proposed by Sundberg and Sill6n (19a9) is in error so far as the coordination and position of the oxygenand vanadium atoms are concerned.The new work on the struc- ture of carnotite has shown that the vanadium occursin "divanadate" groups, VzOt6. The vanadium is in fir'e{old coordination with oxygen, so lhat the resulting polyhedra are distorted trigonal bipyramids; two such bipyramids sharean edgeto form the VzOs-6ion. This ion is similar in structure to trvo links from the five-coordinatedY,,Oe,,-,'chains found in the compound potassium metavanadate monohydrate, KVOe.HzO SI/A/T1IZ.S1SAND PROPERTIES OF CARNOTITE 803

PreciPilotion Time o \

.-- -4E

o E v2 o

PowderPottern ModeP

Predominont Vonodote Spec

& Voriotion ol v+5 Species

HV,^O^.-

; c o o-

o o o J

o 2 468lotz pH

fi rG. t. I{elation of rate oI precipitation of carnotite to concentration of various pentavalent vanadium species. 804 7AUL B. BARTON.JR.

(christ, ciiark, and Evans, i9.54). The lirear uranyl ior-ris coorciinated in a fairly regular rvay by five oxygensfrom three \rz(J,6groups, {orming [(UOr)rVrO8],-2"sheets, with the axes of the uranyl ions nearly normal to the sheets.The alkali ions fit betweentrre sheetsas in the caseof the micas. As will be discussedlater, water moleculesmay also fit between the sheets,but they are not essentialto the structure nor do they greatly affect the cell dimensionsexcept for the interlayer spacing.The struc- tural formula is Kz(UOr)rVzOspHzO with 2 formuia units per unit cell. The rate of precipitation of carnotite as a function of pH was deter- mined, and the resultsare plotted in the upper part of Fig. 1. rn the rate erperiments 20 ml. aliquots of 0.01 M Navor were mixed with 20 ml. portions of a solution 0.009 M in NaaUOz(COa)sand O.SM in KzSOr. Beforemixing, the pH of eachportion was adjusted to the desiredvalue with KoH or HNos. The solutions were r,r.ellmixed and the pH re- corded. The time for precipitation was definedas the time required to obtain the first noticeablecloudiness in the solution. obviously, refine- ments could be made in the experimental techniques,but the present results are satis{actoryfor this study. The scatter of points in tle acid range (Fig. 1) is due in part to difficulty in detecting the beginning of precipitation; however, disequilibrium is a more important factor. Be- causeadjusting the acidity upsets the equilibrium between the various condensedvanadate speciesand becausethe depolymerizationreactions are sluggishin the pH range4 to 8, the measuredpH valuesdo not neces- sarily reflect the concentrations of the various vanadate species.Due to the presenceof considerableamounts of Na+, it is probable that the carnotite contained at least a small amount of sodium although the powder patterns of the precipitates have spacings very similar to those of the pure potassium carnotite. The rapid increasein rate of precipitation up to pH 7 showsthat pre- cipitation involves reaction with a speciesof ion that increasesin con- centration with pH. The decreasein rate from pH 9 to 9.5 suggeststhat the reaction involves the ionic speciespredominant in the pH range 7 to 9, that is the metavanadates.The return to instantaneoui precipita- tion at the highest pH values suggeststhat another .o-porr.,d may be forming instead of carnotite above pH 9.5. powder patteins of the low pH precipitates (below 9.5) show difiuse but definite carnotite lines, whereasprecipitates from the high pH region (above 9.5) show weaker carnotite linesplus a faint, fuzzyline at about 11 A. one precipitate that formed atpIJ7.4 also had the line at r1 A, but this line was weak com- pared to the carnotite lines. The difierencesbetween the powder oat- ternsare not pronount'ed,and the highpH phasecannot be.hara.terired with certainty from the powder patterns; but the:u-ray diffraction work SI'1I/?'11ES1SAND PROPERT'IES OF CARNOTITL, cloeslend permissivesupport to the interl)retationthat a poorly crystal- lized uranyl vanadateo{ uncertaincomposition precipitates more rapidly than cloescarnotite at pH above 9.5 and that this irhaseconverts to carnotite on standing. At lower pH carnotite precipitatesdirectly i'om solution. There are several possiblereactions by which carnotite can be pre- cipitated while fulfilling the restrictions placed upon the precipitation by the rate study. The upper part of Fig. 1 showsthat the rate-determin- ing reaction is controlled by a speciesthat has its maximum abundance in the pH range 7.0 to 9.5. The work of Evans and Garrelsshows that metavanadate,perhaps VnOtr-', is predominant in the pH range 7.0 to 9.5; thus there is a strong possibility that the precipitation reactionis of the sort: 4K+ + 4UO2+':* V.Or:-a+ 6HrO: 2K:(UOi):VzOs.HzO-l 8H*.

However, this is not the only possiblereaction. Simpie calculationsshow that any ion of the type (H2nV^Os*a)-*, which includes HzVOa, HzYzOz2, I{aVzOs-2,and others, will have its maximum concentration in exactly the same region as the metavanadate plateau becausethe formula (H2nY^O3^1n)--is merely the general formula for hydrated metavanadateion, (Hr,VnOrz+,)-a.There are no experimental data to indicate which of the possibleionic speciesdetermines the rate of reac- tion. Whatever the formula of the metavanadate,there will be other possibleions correspondingto the hydrated metavanadatesdiscussed above so that we cannot be certain of the identity of the ionic species that reacts to precipitate carnotite. l'he rapidity of the precipitation in the metavanadateregion suggests, but certainly doesnot prove, that the metavanadateion containsat leastsome S-coordinated vanadium atoms. The availabledata are too poor to permit evaluation of the order of the rate-determiningreaction in terms of metavanadateconcentratiot'r. It may be noted that the metavanadate(V+Orz a) plateauin the lower part of Fig. 1 is slightly broader than the region of rapid precipitation of carnotite shown at the top of this diagram. Although the metavana- date plateau is plotted as if it representsonly one species,it probably represents several species such as VnOrza, HV+Orz 3, H2VaO12-2,and HaV+Orz-1;and possibly several VrOe-t, VrOo-,, or other species.This situation is parallel to that in the polyvanadates,except that some of the ionization constantsare known Ior the polyvanadatesand therefore they may be plotted as separateions. It seemslikely that the rate-de- termining reactionfor the formation of carnotite doesnot involve all of the possiblemetavanadate species;thus the composite metavanadate pH rangeis wider than the pH range of rapid precipitation. 806 ]'AUI, B. BARTON,JR.

If the reaction does involve mclavanaclateion, there must be some sort of interaction at the crystal-solutioninterface by which metavana- date is depolymerizedto divanadate,VzOs 6, becausethe crystal struc- ture containsVrO3G ions. Under thesecircumstances the problem arises of how the initial carnotite nuclei originate.It is possiblethat the initial carnotite nuclei may form from a small amount of aqueousdivanadate ion (if it existsin appreciableconcentration in aqueoussolution), or per- haps some other vanadate speciesmay provide a transient seedphase on which the carnotite crystallizes.The line at 11 A in the powder pat- tern of the precipitate obtained at pH 7.4 suggeststhat this transient phase may be the same one that precipitatesat a high pH, but there are few data at presentto justify such a conclusion.

Cnvsr.q.r Cun'ursrnv The compositionof carnotite reflectsthe unique crystal structure of the [(UOz)zVrOr)n-t"layers. Because of its coordinationin the dumbbell- shapeduranyl ion, UO2+2,the uranium is not likely to be replacedby any other ion. The asymmetric,fivefold coordinationof the vanadium is nol similar to that found in any other ion; therefore,elements such as phos- phorous and arsenic, which sometimesform isomorphousseries with tetrahedrallycoordinated vanadate compounds (pyromorphite-mimetite- vanadinite), will not proxy for vanadium in the carnotite structure. The tendency for vanadium to occur in peculiar coordination may also ex- plain the absenceof naturally occurringvanadium analoguesto torber- nite, autunite, zeunerite, and abernathyite. Isomorphous substitution in the carnotite-lype structure is very likely limited to the interlayer sites, that are occupiedby potassium in carnotite and by calcium in tyuyamunrte. Natural carnotite contains from 1 to 3 moles of water per formula weight (Palache,Berman, and Frondel, 1951). The synthetic material on which the present structural work was done was originally entirely anhydrous (seedescription of synthesisbelow), yet this synthetic ma- terial, when kept anhydrousby immersion in mineral oil, gave a\ )c-ray powder pattern practically identical to that of natural carnotite.Luede- mann and Bates (1957)show that the curve for weight lossversus tem- peraturefor natural carnotite is smooth,with no sharp breaks,and their differential therrnal analysis curve shows no pronounced endothermic or exothermicpe--ks. If carnotite is heated to 1100oto 1200" C. it will melt and recrystallizeas carnotite (unlessheld at the high temperature for severaldays, in which caseit decomposesthrough loss o{ oxygen to UrOs and other as yet unidentified compounds).Therefore, none of the water in natural carnotite is essentialto the structure. Under normal ,SIT/TIIESISAND PROPERTIESOF CARNOTITE 807

Iaboratory humidity the amount of water in carnotite is sufficientlylow so that there is little variation in the cell dimensions,although Donnay and Donnay (1955) have shown that some natural material (Table 2) may show a considerablerange in the c dimension,presumably due to variation in water content. Carnotite doesnot seemto hydrate in a step- wisefashion as do autunite and tyuyamunite. X-ray diffractionexamina- tion showsthat the natural carnotite crystalsare noticeably disordered, especiallyin the c direction, as would be expectedif there is disorderin stacking of the layers or variation in water content.

SvNrunsrs In this work severaltechniques were usedin attempts to obtain well- crystallized carnotite. Aqueous systems from room temperature to 200" C. were tried without success,and thereforeit rvasnecessary to re- sort to fusion methods.

Aqweowsmethods In aqueoussystems at low temperaturesit is difficult, if not impossible, to avoid the precipitation of carnotite from a solution containing ap- preciableamounts of potassium,hexavalent uranium, and pentavalent vanadium from pH 2 to pH 9.5. Beyond these pH limits other phases precipitate. (See discussion of precipitation from aqueous solution above.) The rapid precipitation from cold aqueoussolution producesa carnotite rvhich is very poorly crystallized,as is indicated by its difiuse r-ray powder pattern. Sharp powder patterns, but no crystals distin- guishableat 500X magnification,are obtained by prolonged digestion of the precipitate on the steambathor by placing it for a few days in a closedtube with rvater at temperaturesas high as 200oC. Variations in pH, concentration,and relative proportionsof reagentsseemed to make little improvement in the crystallizationof the carnotite.A singleexperi- ment in which fine-grainedcarnotite was agedin an approximatelyneu- tral solution in an autoclaveat 150' C. and about 35 atmospheresCOz pressurefailed to improve the crystallinity of the carnotite measureably. It was thought that perhapsthe very slow mixing of uranyl and vana- date solutionsmight permit the growth ol singlecrystals. Excellent crys- tals of many insolublecompounds can be grown by slow mixing of react- ants by diffusionthrough silicajelly. When uranyl (or uranyl carbonate) and vanadate solutions containing potassium are introduced into the opposite sides of a U-tube partly filled with silica jelly a precipitate forms at the contact of the difiusing ions. The precipitatesdevelop in 2 to 20 days and often show rhythmic Liesegangbanding but, unfor- tunately, no visible crystals.When the solutionshave a lorv pH (about 808 PAUL B. BlltTON, lR.

2) two distinctly different phasesresult. On the uranium side is a bright yellow precipitate of carnotite which gives a sharp r-ray pattern, while on the vanadium side there is a brown cryptocrystallinematerial identi- fied as rauvite through its relatively difiuse pattern. At the higher pH (about 8) investigatedby this method only carnotite appears.Because of the buffering action of the silica jelly (preparedby mixing 5 volumes of 2 per cent HzSO+with 8 volumes of 1:6 sodium silicate) the exact hydrogen ion concentrationat the point of precipitation is unknown. At the lower pH the predominantions are UO2+2,VroOzs-6, K+, Na+, H+, arndSO;2; at the higher pH they are UOr(COe)a-a,VaOrz-a, K+, Na+, and SOn-2. llost natural carnotite was apparently precipitated from solutions derived from sourcescontaining tetravalent uranium and tri- and tetra- valent vanadium compounds;this suggestsanother method of precipita- tion in which vanadate and uranyl solutions could be combined very slowly. Tetravalent vanadium and hexavalenturanium do not precipi- Lateeach olher from solution. Tetravalent vanadium is amphoteric; the vanadyl ion, VO+z,is stable in solutionsup to about pH 3 where V2Oa hydrate precipitates;above a pH of about 8.2 the VzOaredissolves as the vanadite ion, possiblyVrOg-t. Trvo types of experiment were set up in an attempt to duplicate 1he natural method of crystal growth. In the first, sulfur dioxidewas bubbled ihrough a solution of potassiumsulfate and sodium vanadateso that the vanadium was reduced to the tetravalent state. Hexavalent uranium in molecular excessof the vanadium was then added to the vanadite solution and the mixture rvaspermitted to drip at the rate of about one drop per hour on a sloping pieceof filter paper where the sulfur dioxide either escapedas a gas or was convertedto sulfateby air oxidation. The vanadium slowly oxidizedto the pentavalent state. The pH of the solu- tion beforeloss of sulfur dioxide was about 3. A yellow film of carnotite C-evelopedin a few days but even after two months there were no crys- tals visible under 80 power magnification. The secondexperiment involved diffusion of ions through U-tubes. A silica jelly containing sodium, potassium, and hexavalent uranium was prepared.Vanadyl sulfate was introduced into one limb of the tube and potassiumperoxydisulfate (a strong oxidizing agent) was placed in the other. Small, well formed, yellow crystals in the shape of roughly equant plates developedafter a few days, especiallyalong the glass- jelly conlact. Thesewere at first thought to be carnotite which had had its tendency to form diamond-shapedplates suppressedby the inter{er- ence of the jelly. But the appearanceof penetration twinning and the fact that the crystalslater redissolvedproved that the crystalswere not SI/1I/7IIBS.TSAND PROPERTIESOF CAIINOTITE 8Og carnotite. X-ray data suggestthat the crystals are a potassiumuranyl sulfate.Later, the usual bandsof cryptocrystallinerauvite and carnotite appeared. There is another mechanismthrough which the well crystallizednat- ural carnotitesmay have beenformed, but this has not been tried in the iaboratory becauseof the successof the fusion techniqueswhich are de- scribed later. Tyuyamunite (or metatyuyamunite), being considerably more soluble than carnotite, occurs in nature as visible crystals much more frequently than does carnotite. It is also possibleto synthesize suitable crystals of tyuyamunite (l'{urata et al., 1950). Such crystals could then be convertedto carnotite by treatment with a solution con- taining a high concentrationof K+ relative to Ca++ (Hiliebrand, 1924)' A possibledrawback to this method is that the base exchangeprocess might well result in random stacking of the layers.As is discussedlater, the natural carnotite crystals commonly exhibit stacking disorder.

Fwsion methods By following the proceduredeveloped by Canneri and Pestelli (192-t) carnotite crystals up to a centimeter acrosswere obtained. The same techniquealso provided the Na, Rb, T1, and Cs analogueso{ carnotite as crystalsrn,hich were suitable for singlecrystal *-ray work. The crystals of the K and Cs compoundsrnere also sufficiently well developed{or goniometricwork (D. E. Appleman, personalcommunication). The procedureused to crystallizecarnotite and its alkali analoguesis very simple,although the sizeto which the individual crystalsgrow is, as yet, controlled more by luck than science'Ammonium metavanadate' NHIVO3, and an alkali hydroxide, chloride, carbonate or nitrate were fused together in a platinum crucible to give the aikali metavanadate. The reactionswere quite rapid, especiallywith the hydroxides.Usually one or two per cent NH+VO3in excessof the stoichiometricamount was useclin order to prevent the possibility of excesshydroxide which might react with the crucible.Then a small amount of uranyl nitrate or acetate, or uranium trioxide hydrate was added to the aikali metavanadateand the mixLure was fused. On cooling, beautiful, yellow, rhombic plates coulclbe seento grow rapidly at the surfaceof the melt just before the main mass of the metavanadate crystallized. The crystals were then separatedby handpicking,or the flux was dissolvedarvay with rvater or dilute nitric acid. If the acid used was too strong, VrOshydrate precipi- tated and lvas then redissolvedin a solution of the appropriate alkali. In an attempt to perform the synthesisin the manner of Sundberg and Si116n(1949), stoichiometricamounts of uranyl nitrate and potas- Siummetavanadate rvere lUsed in a great excessof potassiumcarbonate. 810 pALrL B BARTON,JR.

This technique gave orange, tabular, pseudotetragonar(orthorhombic) crystals with three mutually perpendicularcleavages. The (001) cleav- age is micaceous,the other two are good. The unit-cell dimensions,de- terminedby Joan R. Clark, U. S. GeologicalSurvey, are4.33,4.33, and rc.2r A, or possiblysome multiples thereof.T'he intensity distribution on r-ray patterns of the compound shows that the two short axes are not equivalent.The crystalsbreak down rapidly in contact with atmospheric moisture, but they can be preservedunder mineral oil or toluene. The optical character of the crystals changeswith hydration from sensibly uniaxial to biaxial with 2v of 10" to 15", and the crystals soon become disordered.The powder patterns of the hydrated material show some carnotite lines; the other lines present cannot be indexed on either the carnotite or the above-mentionedpseudotetragonal cell. A more detailed study of this very interesting phasehas thus far bee'prevented by its instability. Similar sodium and rubidium compounds have been pre- pared using melts of the appropriatealkali carbonate,but no :r-ray work has been done on these.Attempts to make the thailium member have been unsuccessful,presumably due to the difficulty of preventing the thermal decompositionof the thallous carbonate. Besidesthe carbonateand vanadate fluxes some other types of flux were tried. As describedabove, uranyl salt was added to the vanadate fluxes; to the other fluxes (Table 1) a small amount of the appropriate alkali analogueof previously prepared synthetic carnotite was added. The mixtures were fused and allowed to cool slowry. rn general,there were only two types of uranium-bearingcrystals formed: carnotite, and the previously describedorange, orthorhombic compouncl.Table 1 sum- marizes the results of the fluxing experiments. rn the courseof the fusion synthesisexperiments the preparation of the lithium analoguewas tried. An insolublecompound with a greenish- yellow sheen,too fine grained to be resolvedunder the microscope,was retained after the leaching away with water of the lithium metavana- date flux. The compound gave a powder pattern that was practically identicalto that of carnotite.This was tentatively explainedas due to the baseexchange of Li+ by H3o during the washing,so as to yield a hydroni- um carnotite practically identical to the natural carnotite in cell dimen-

rt was not possibleto make the ammonium analogueof carnotite by lhe fusion technique becausethe ammonia was rost on heatins. Fine- ^'I'N7'I1ES1SAND PROPERTIE.SOF CARNOTITE 811

'IABLE 1. Sulttntrtltv or FusloN [-']n'l;ntltrr'I'rs

' Melting point C Nature of uranium-bearing FIux nf n,,rp ll nv Phase

KVOs 650 (estimated) yellou. carnotite crystals KzCOr 891 orange orthorhombic crystals KzCOs:KVO;2:1 600 (estimated) orange orthorhombic crYstals K,CO3KF 5:1 750 (estimated) orange orthorhombic crystals KzSO+ r076 orange, poor crystals KCI 776 orange pseudomorphs of original yeLlow carno- tite crystals KNOs 334 yellow, no recrystallization UO:(NO,): 6HrO 60 1'ellow,no recrystallization VrOa 690 no micaceous uranium-bearing phase (carnotite requires alkali) KHSOr 210 orange melt turns yellow on coolingl no crystals, too acid NaVOr 630 yellow Na carnotite crystals Na:COr 851 orange orthorhombic(?) crYstals TIVOs 424 yellow Tl carnotite crystals TlzSOq 632 yellow Tl carnotite crystals T],C03 273 unsuccessful due to oxidation of T1; requires further work RbVOr 500 (estimated) yeliow Rb carnotite crystais Rb:COI 837 orange orthorhombic(?) crystals CsVOr 500 (estimated) yellou' Cs carnotite crystals LiVO: 600 (estimated) very fine grained insoluble greenish yellolv ma- terial that gives carnotite-like pattern after n'ashing with rvater grained ammonium carnotite prepared by K' J. \{urata by aqueous methodsgave a powder pattern very similar to that of natural carnotite (r*rr**: 1.43 A) . 'Ihe synthesisof the Na carnotite from the NaVOa melts could not be repeatedat will as someas yet undeterminedfactor o{ten preventedthe formation of visible crystals.\{urata and others (1950)record a peculiar formula, in which there is an excessof V2Os,for their Na "carnotite," and they suggestthat further investigation on the nature of this com- pound be carriedout. They rePort that the Na compoundis also readily baseexchanged in K+ bearing solutionswith the formation of K carno- tite. In addition, the present study showedthat Na carnotite dissolves in acid much more rapidly, and at a slightiy higher pH than the other carnotites. These observationsindicate that Na carnotite is relatively unstable comparedto the carnotiteswhich contain larger alkali cations. Il seemsunlikely that pure Na carnotite would occur in nature because 812 PAUL B, BARTON,JR

'l'anlri 2. UNrr cr:.r,r,l)ar,t r,

Slrecific gravity Matcrial a l) (in A) (in.{) ('aIcu- uDServed lateo

Nar(UO)rVrOr 10 3910 03 8 3910.03 6 1410.10 100'10,+30, 516 Kr(UOr)rVzOe to +7!0 02 8 4110 01 6 s9+0.0r 103'50'+05', 4 99 4 9.510 05 Tlr(UOr)rVrO8 10 481 0.02 8 4110 02 6 8210 01 105'15'+10' 6 76 6.5810 05 Rbz(UOr)zVrOr 10 491 0 02 8 4310 02 6.93!0 02 105"25'i 10' 5 29 5.1,1100.5 Cs:(UOr)rVrOe 10 5110.02 8.4510 01 7 32].0 01 106"05/t05, 5 52 5 48+0 05

Carnotiter 10 4810 03 8 3710 02 6 7510.05 t04"20,+ 1" Carnotite2 10.471003 8.3910 02 6 63t0 0.1 103'50',t15' Carnotitea 10 47 8.,11 691 103"40',110, 4 91. 4 70+0 05 KI(UOr)zVrO6'zHrOr 10 46 8. ,{0 659 r03'40'+20' s 00 4 8010 05 Kr(UOr)rVzOss 10.43 840 659 101"12', .s.03 Sengieriteo to 62 810 10 11 103'36',110', 3.99 4 41 Metatyuyamunite? 10 63 8.36 t6 96 orthorhombic ,l 04 3 9210.05 Metatyuyamunites 10 54 849 t7 31 orthorhombic 3 61t0 05 Ca(UOr)rVrO8 3 -sHrO, r0 38 846 17 A2 orthorhombic

1 Carnotite lrom Cane Springs pass, Utah , Ca.notite from Congress Junction, Arizona 3 Carnotite lrom Cane Springs Pass, Utah, fully hydrated (Donnay and Donnay, 195.5). a Synthetic hydrated carnotite prepared by K J Murata (Donnay and Donnay, 19.i5) 5 Synthetic anhydrous carnotite (Sundberg and Sill6n, 1949). o Sengierite from Haut-Katanga, Belgian Congo (Donnay and Donnay, 1955). 7 Metatyuyamunite from May Day mine, Mesa County, Cotorado (Donnay and Donnry, 19.5.5) s Metatyuyamunite from Small Spot mine, Mesa County, Colorado (Donnay antl Donnay, 1955) c Sl'nthetic metatyuyamunite prepared by K .J Murata (Donnay and Donnay, 1955) sufficientamounts of K+ are present in most natural solutions to base exchangethe Na+ by K+. Apparently the decreasein stability of Na carnotite relative to K carnotite is related to the decreasein ionic sizeof Na+ relative to K+ (rry,+: 0.97 A; /K+: 1.33 A) .

PnopBnrms oF CARNorrrEAND Rorernn CoupouNos 'fable 2 presentsthe unit-cell data for natural and synthetic carnotite, its alkali analogues,and somerelated natural compounds.The new data were obtained using filtered molybdenum radiation (IKa:0.7107 A) with a precessioncamera; corrections were made for film shrinkage. The cell dimensionsof the synthetic compoundsshow systematic variation with the ionic radius of the alkali cation. The similarity of the a alnd.b dimensionsof carnotite,sengierite, and metatyuyamunite (and tyuyam- unite) suggeststhat all have essentially the same [(UOr)zVrOs1,," layer structure with only the interlayer cationsand the degreeof hydra- tion accountingfor the difierences.Tyuyamunite and metatyuyamunite contain 2[(UOr)r%Oa],-2' Iayers per unit cell and are orthorhombic; whereascarnotite and sengieritehave only one such iayer and are mono- clinic. Further evidencefor the similarity of the layers in theseminerals is furnished by the similarily in physical properties (palache, Rerman, SYNTIIESISAND PROPERTIESO]I CARNOTITE 813 and Frondel, 1951)and by the ability of thesecompouncls Lo convert to one another through base exchange(Hillebrand, 1921; llurata et al., 1950;and Donnay and Donnay, 1955). The indexed powder patterns for the synthetic carnotites are shown in Table 3. Thesedata were calculatedfrom measurementstaken from powder patterns obtained using CuKo radiation (I:1.5418 A). Co.- rectionsfor fiim shrinkagehave beenmade. Becauseof the platy nature of the carnotite, it was almost impossibleto avoid alignment of crystals in the spindle, thus the relative intensities of the various reflections were influenced by this preferred orientation. rntensities were visually esti- mated. The reflectionswere indexed on the basisof the unit cellsof the synthetic crystals.The NHa and H:O(?) carnotiteswere indexedon the basisof the K carnotite cell. Lines with d valuesof lessthan 2.0 A *".. not indexed. Single crystals of the naturai carnotite commonly exhibit very diffuse, streaky spots in the h\l and }kl precessionphotographs, although the the hko patterns have fairly well defined spots. This indicates that the crystals are relatively disorderedin the c direction, presumably due to dis- order in the stacking of the layers or random variation in the water con- tent so that the 001spacings are nonuniform. The carnotite crystals are brilliant canary yellow to greenish yellow. Although most vanadatesdo not fluoresce,the Cs carnotite and a K carnotite that had beenheld at 1000oc. for three hours fluorescedyellow- green under ultraviolet light. Twinning by reticular pseudomerohedry, as described (and errone- ously recordedas reticular merohedry) by Donnay and Donnay (19S5), was noted in the precessionpatterns from the K, Cs, Rb, and Tl carno- tites. The micaceous(001) cleavageof carnotite is well developedin all of the synthetic and natural carnotitesexamined. A good cleavageparallel to (100) is seen in the larger crystals of the natural carnotite and in most of the synthetic crystals.Less frequently (010) and (110) cleavages appear. The diamond-shapedplaty crystals are commonly bounded by {110} facesin addition to the (001) cleavageplanes. The Cs carnotite also shows [0i0] and {100} faces.In the natural carnotite from Con- gressJunction, Arizona, the [110] facesare so well developedthat the crystals are almost equant rather than tabular; however, there is much disorder in the c* direction in these crystals. The optical data for various synthetic carnotites are presented in Tabie 4. The crystalsare biaxial (-), and the micaceouscleavage flakes give centered optic axis figures. The optic angles were measuredon a universal stage using optic axis figures; the values are reproducibleto within 0.5oor less.The optic angle of the thallium carnotite is too large 814 PAUT, B. BARTON, JR.

'f,rnr,r: 3 -- ,Y-I{av Powurtt D,q.r,r -r'on C,ltNort'lri r\ND Irs Ar,t<.lr,r AN,rr,ocols (d in angstronrs).Cu1(a; I:1'5418 A

Na Carnotite K Carnotite

hkt d(calc.) d(obs.) hkl d(calc.) d(obs.) 110 649 647 w 110 6.48 6.46 W 001 6.04 6.04 VS 001 6.40 6.36 \/q

200 5.11 5.12 NI 200 5.08 5.06 W

+.24) 201 4.30 4 .29 VW 111 +'Lz, n. M 020 4.201 111 4.19 4.16 S 201 ? <1,O 201 3.603 3.579 VW 02l 3 -sr4j

02r 3447 3437 M 220 3.240\ 1 1^1 002 3. 199.r 220 3.243 3.222 w 310 3.r43 3.140 310 3 159 3.140 M 311 I 191\ 3 1oo 002 3022 301.5 M 221 J . U6O,J

221 3001 3000 M 112 2 709 2 705 212 2.681 2.680 W 311 2.602 2.600 VIV

311 2.628 2.612 M T31 2..54e1 312 2.5421 2.54+ 410 2.445 2.437 M 400 2.s42) 312 2.38i 2.387 VW 202 7 +s-9\, z.4ss 230 Z.+JJ: 402 2.148 2.150 w 330 2.160] n 1r/- 222 2.097 2.104 VW 203 2.1601

NHr Carnotite 003 2 t33 2 140

d(calc ) d(obs ) 312 2 028 2.031 VW 110 6.48.| Tl Carnotite 001 6.40l 6.61 VS d(calc.) d(obs.) 111 ,1.24\ o20 4.2Oi 419 001 6.58 6.56 M

201 J.J6.\l 200 .5.06 .5.05 M 021 3. s141 3.534 M 210 433 4 33 X,I 211 3.297 3.314 111 4.29 M 220 3.240] 002 3 reel 3.220 020 4.21 4.23 W

VS very strong W weak S strong VW very weak M moclerate 5'Y1TTIIES15'AND PROPERTIES OF CARNOTITE 815

Tarr,B 3 (continued)

NHq Carnotite Tl Carnotite hhI d(calc.) d(obs.) hht d(calc ) d(obs )

310 3.140 3.132 M 211 1.06 4.06 M

311 3.107 3.115 n{ 021 3.543 3 53.5 S

002 3.289 3.285 VS HrO(?) Carnotite 311 3 136t 110 648 648 \{ 310 3.128i 3.128 N{ ,)1 3.116) 001 6.40 6.37 VS 012 3.064 3.054 VrV 200 508 .505 VW 2r2 2.961 2.956 \rW 111 t ?t\ 4.20 M 020 4.20( 130 2 741 2 -705 W

021 3.51; 3.507 N{ 031 2 579 2s79 W 121 3 464 3.4.53 w 401 2.176 2.179 VW 220 3 .240\ 330 2.155 2.154 W 002 3.199 J. Zr.) 222 2 133\ 2.130 VW 121 3.194 3.193 w 032 2.133 | 310 3.143 3.126 M 232 2 098 2 098 w 311 3.1071 312 2.032 2.032 W 221 3.086/ 3 .095

1t2 2 709 2.698 VW Cs Carnotite

311 2 602 2.589 \I/ d(calc ) 1(obs.) 312 2.542\ 001 7.03 7.O2 S 400 2.s42 2.543 110 6.48 6.50 VW Rb Carnotite 200 5 .0.5 5.04 W hkt d(calc.) d(obs.) 111 4 39\ +.37 001 668 670 VS 210 +.JJi 110 o.+.) 6.46 w 2rl 4.16 4.r3 M

200 5.06 5.08 M 121 3 . 57.5 3.591 M 201 4.67 4.68 w 002 3 517 3.482 VS

111 4.30 4.30 M 202 3 .351) 21r 3.3s1i 3.339 020 4.21 4.21 M t12 3 3511

2tl 4.09 M 220 3.240 3.220 N,T t20 w 816 PAUL B. BA]

Tarra 3 (conlinued)

Rb Carnotite Cs Carnotite

d(calc.) d(obs.) d(calc.) d(obs) 02l 3.s6.5 3.562 S 311 1 !?.6"\ r 170 221 J. IO.l l 002 3 339 S 212 3.118 3 r02 M 220 J.LJI 3.231 M 112 ?..881 2.867 w 310 3.130 3 r37 S 312 2 708 2.706 212 2.997 2 .988 W 031 2 6t1 2.61+ M 221 2.736 2.726 VW 230 2.460\ r L

?1) 2.631 2.625 M 003 2.314\ 2.331 T13 2 3421 401 2 .605 2.(fi3 M 4t2 2.298 2.294 VW 222 2.552 2.543 VW 013 2.268 2.262 VW 111 2.489 2.488 VW 222 2.196] 2.193 230 2.4s71 401 2 1911 2.+52 M 131 2 .44eJ 232 2.157 2 t53 2.314\ 2 230 x.[ JL I 7,.292i 422 132 ili?\ 2 077 013 z. l.).t 2.154 M 023 2 037 040 2 107 2.108 w 041 i},],\ 140 2.06+ 2.067 w

fJz 2.03+ 2 036 W

T,rnm 4. Oprrcer D,q.r'a pon SvNrimrrc ColrpouNls l{er,ernl ro CenNotrtnr

Material a2 2V (Na light)

Na:(uor)zvuos 221 +001 2.2t +0.o1 13' + 1' Kr(UOr)'VrO' 1.77 +0 .04 2 010+ 0 005 2.090+0.005 53"30',+1" Tlz(UO:)rVrOs >2 70 >2.70 > 64"40' Rbz(UOr)zVzOs 1.86+0.04 2.125+0005 2.205+0005 51'30',+10 CS,(UO,,V,O3 < 1.83 2.49+0.01 >2 70 45'30',+1'

1 The inclices of refraction rvere cletermined by C S Ross, U. S. Geological Survel', using sulfur selenium mixtures 2 Calculated values. 817 SiTliI7'I1ESlIi AND PROPNRTIES OF CARNOTITF'

eyepiece to obtain a centeredligure, but it was measuredusing a ruled 'fhe to b which rvas calibrat.edut lo*e. angles' optic plane is parallei plane' and, so far as can be determinecl(within a degree),lies in the bc*

Acr

of the orthorhombic phase.c. S. Rossmeasured the indicesof refraction Evelyn synthetic maferials. Mary \{rose, Daphne Riska Ross, and pat- ii.n.y identified fine-grained materials by means of r-tay powder terns. This work was done by the U. S. GeologicalSurvey on behalf of the Division of Raw }rlaterialsof the U' S. Atomic Energy Commission'

I{nlrnnNcns clim italiana,54, c.q.NNnrr,G., ,rNo I'risrolr.r. v., (1924), La sintesi clellacarnotite: Gozz.

Comm., Tech. Inf. Service Extension. Oak Ridge, Tenn' in Colorado ancl Hrr.lnen'Lt'to, w' F', (1924), Carnotite and tyul'amunite and their ores IJtah: Am. Jou'r. Sci.,5th ser., 8' 207-216. of several h)'drous Lruonu.LN, Lors, aNn Bems, T. F., (1957), Mineralogical study vanadates:PennsylvaniaStateUniv.,CollegeofMinerallndustries,Tech.reportno. Library of con- 8 to Office of Naval Research. Available as microfrlm or photocopy, gress,Washington, D. C. 30, 1954 to April 1' 1955' Part 1"fhe Mnln*, Lao J., (1955), Annuai report for June Comm' I{MII-3110 chemical environment of pitchblende: U S' Atomic Energy Extension' (Pt. 1),49 p., issued try U. S. Atomic Energy Comm' Tech' Inf Service Oak Ridge, Tenn. the Monacnnvsr

IIanuscripl receittedJanuary 21, 1958