American Mineralogist, Volume 67, pages 1273-12E9,l9E2
Margaritasite: a new mineral of hydrothermal origin from the PefraBlanca Uranium District, Mexico
KnnBN J. weNnrcn, PETERJ. MoonBsxr, Rosenr A. Zrer-rNsrrAND Jeues L. Seerey U.S. GeologicalSurvey Box 25046,MS 916, Denver Federal Center Denver. Colorado 80225
Abstract
Margaritasite, a Cs-rich analogueof carnotite, is a newly discovered uranium mineral that is part of the ore at the Margaritas deposit in the Peffa Blanca uranium district near Chihuahua, Mexico. The margaritasite occurs as disseminated pore fillings and relict phenocryst linings within a rhyodacitic tuff breccia of the lower Escuadra Formation (Oligocene) and provides significant reserves of both uranium and cesium. It is a fine- grained yellow mineral and, with the possible exception of the index of refraction, is optically indistinguishablefrom carnotite. Margaritasiteis most easily recognizedby X-ray diffraction through a shift in the (001) reflection representing an increase, relative to carnotite, in the c dimension. This increaseis due to the large Cs atom in sites normally occupied by K in the carnotite structure. This cs-K uranyl vanadate, with cs:K about 5, is the natural equivalent of the compound Cs2(UO2)2V2O6synthesized by fusion (Barton, 1958).Chemical analysesof the mineral give the formula
(Cs1 3s(H3O)s.rsKo.zq)>z o2(UOt r.ee(V2Odr.o0. l.07H2O correspondingto the generalizedformula (Cs,K,HrO)z(UO2)2V2O6.nH2Owhere Cs > K,H3O and n : I Unitcellparametersarea:10.514,b:8.425,andc=7.254,p:106.01"(F4/a,Z:2). Microprobe analyses of margaritasite and Cs-enriched carnotites synthesized by ion exchangeof carnotite in aqueousCsCl solution at 2fi)"C suggesta solid solution between carnotite and margaritasite, but X-ray powder patterns reveal that two discrete c dimensions exist with no intermediate value between them. The margaritasitehas a (001) reflection at 12.7"(20) while that of carnotite lies at 13.8"(20); thesepeaks do not shift. It is likely that there are two distinct phases, perhaps as interlayered lamellae which are intergrown on a smaller scale than can be resolved by the electron microprobe beam. The discovery of Cs-rich carnotite in the Pefra Blanca uranium district provides important evidencefor local hydrothermal or pneumatolitic activity during or after uranium mineralization. Data from the geochemicalliterature indicate that the high Csltotal alkali elementratios required to produce Cs-rich minerals can be generatedand sustainedonly in high temperature environments. Synthesis experiments show that margaritasitecan form by reaction of Cs-rich solutions with natural carnotite at 2fi)'C, but the samereaction does not occur or is too slow to be observedin 61 days at 80"C. Nevertheless,it appearsunlikely that margaritasite is part of the ore in any Colorado Plateau-type uranium deposit. Reported "carnotite" occurrencesfrom uranium depositsofprobable hydrothermal origin are likely sites for new discoveries of margaritasite.
Introduction uranium district near Chihuahua. Mexico. The rec- ognition of margaritasite within these rocks is im- Margaritasite, a newly-recognized uranium min- portant for the characteization of this deposit and eral and a Cs-rich analogue of carnotite, is an ore is of interest in the exploration for other depositsof mineral at the Margaritas deposit in the PefraBlanca similar origin. The physical and chemical con- 0003-fixxE2l | | rLr27 3$02.00 1273 1274 WENRICH ET AL,: MARGARITASITE straints determined to be necessaryfor the forma- ably on Cretaceouslimestones. The volcanicshave tion of margaritasitecan contribute to knowledge of been datedby Alba and Chavez (1974)at 43.810.9 the genesisof ore depositshosting this mineral.The m.y. for the Nopal Formation,38.310.8 m.y. for mineral and name margaritasite were approved by the EscuadraFormation, and 37.3-r0.7 m.y.for the the Commission on New Minerals and mineral Mesa Formation (Fig. 1). The PenaBlanca volcani- names of the International Mineralogical Associa- clastic unit lies between the later two formations. tion. Margaritasite is named after the Margaritas Uranium mineralization occurs in stratiform and deposit where it was discovered. A specimenof fracture-controlled deposits within the Escuadra margaritasitehas been provided to the U.S. Nation- and Nopal Formations. al Museum of Natural History, Washington,D.C. Uranium depositsof PefraBlanca Cesium-carnotitewas first synthesizedby a fu- sion technique,using metavanadatefluxes, by Bar- The Pefla Blanca uranium district is unusual for ton (1958). This synthesis provided sufficiently North America becauseof its geologic setting in largecrystals that Applemanand Evans (1965)were silicic volcanic rocks. Over the past 20 years, able to determine the crystal structure of Cs-carno- numerous uranium occurrences at Pena Blanca tite. The X-ray diffraction pattern of the synthetic have been systematicallydrilled and developedby Cs-carnotitewas publishedby Barton (1958)and is underground mining (Goodell, 1981). In recent very similar to that of margaritasite(which contains years, severalof these have proven to be of com- about I wt.% KzO) except for a slightly larger c- mercial interest and are in active stagesof develop- dimension. ment. Presently the Nopal I deposit is partially exploited, an incline is being driven into the Puerto Geologicsetting III deposit and the Margaritas deposit is being The Pefla Blanca district is located within the prepared for exploitation by open-pit mining (Goo- Basinand RangeProvince of Mexico approximately dell, 1981).Based upon projected estimatesof U 70 km north of Chihuahua City in the north-central resources,the Pefla Blanca district could prove to portion of Sierra PefraBlanca (Fig. l). SierraPena be the most significant volcanic uranium district in Blancalies amongother fault-boundedranges con- North America. Reservesat the Margaritasdeposit tainingsimilar sequencesof Tertiary volcanicunits. aloneare believedto be 4.6 million poundsof U3Og A 300-m-thickTertiary sequenceof rhyolitic ash- with an averageore grade of 0.l03Vo(G6lvez and flow tuffs and volcaniclastic sediments caps the Y6lez,1974)along with molybdenumreserves at an bulk of this range. Three formations within this averagegrade of 0.3% (Goodell et al., 1978).In sequencecontain ignimbrites constituting outflow addition, the margaritasite provides a significant facies from an as yet unidentified caldera source reserveof cesium. (Goodell, 1981).The uppermost(Mesa) and lower- The Margaritas deposit lies within a northwest- most (Nopal) formations are particularly volumi- trending syncline (Goodell, 1981)in the lower Es- nousand of regionalextent, being most widespread cuadra and underlying Nopal Formations (Fig. 2). in the Sierradel Nido volcanicprovince to the west This deposit lies at the intersection of two fracture and northwest (Fig. 1). This range is the major zonesalong which most uraniumoccurrences of the volcanic source area in the region, and contains PefraBlanca district occur: one trends northwest- rocks which are compositionally and temporally southeastand the other north-south(Calas,1977). more variable than rocks of the Pena Blanca range A geologiccross sectionof this depositwith inter- (Goodell,1981). Although the older rhyolitic tuffs of pretations by the Instituto Nacional de Energia Sierra Pena Blanca, Sierra Pastorias,and Majalca Nuclear shows the location of the analyzedsample Canyon are not peralkaline (Table 1), the younger of margaritasite(Fig.2). Occurrencesof margarita- Campanaand Cryptic Tuffs of the northern Sierra site are by no means minor, being scattered on the del Nido are (Dayvault 1980,Table 1; seealso Table surface toward the axis of the syncline. The other 1, this report). More information on the volcanic uranium ore minerals found in the Margaritas de- geology of the region can be found in Goodell and posit are uranophane, betauranophane, autunite Waters,1981. and weeksite (Goodell, 1981);all contain hexava- At the Pefla Blanca uranium district, the Tertiary lent uranium and are associatedwith anomalously volcanics overlie a regional continental molasse, high concentrations of Mo probably in powellite the Pozos conglomerate,which rests unconform- (Goodell, 1982)and V in metatyuyamunite(Goo- WENRICH ET AL,: MARGARITASITE 1275
106"30' 106000' 1050j0' J0000'
Sierra de la Tinaja Sierra Lisa de las Damas
rre€rmpana
la Mesa 37.0m.v. Pefia Blanca Chontes Escuadra 38.0 Nopal 44,0 Po2os Sierra Pefia Sierra Blanca de la Campana
29"00' CER.RODE RANCHERIA Capella Amarillo Tuff Quintas Tuff 44.7m.y. Acantilado Tuff Al dama
Sierra de 1a Sierra Gloria 1a rLana i huahua \ NORIH Eu[.a..LLa
oro Rhyolite I L2 ) D\l; 28-33m.y. x CHIHUAHUA''!a.i-'.r'r x13 Sierra Carreras 0 10 Miles Pastorias F--r -Y 0 10 Kilometers Sot
Fig. l. Location map for the Peia Blanca Uranium District, Mexico, (located in the center of the diagram). Sample locations are shown for analyseslisted in Table l. The Margaritas deposit is located at site 19. The stratigraphiccolumn for Sierra pefia Blanca is shown to the upper right of the area. The sampleof margaritasitecame from the Escuadraformation. (Modified from Goodell. l98l). dell, 1982)as well as in margaritasite.No tetrava- and a breccia of reworked tuff clasts. Margaritasite lent U minerals have been observed in the Margari- occurs as pore fillings and phenocryst casts within tas deposit, although uraninite occurs in the Nopal I the rhyodacitic breccia of reworked tuffclasts in the deposit (Table l). At the Margaritas deposit miner- lower Escuadra Formation (Fig. 2). The boundary alization is structurally controlled, and is in two of margaritasite mineralization is gradational with strata-bound environments, an altered vitrophyre hematitic, kaolinitic and silicic alteration scattered t276 WENRICH ET AL.: MARGARITASITE
Table 1. Chemical analysis of rocks from Sierra PefraBlanca and vicinity (oxides in percent, trace elementsin ppm except where noted otherwise).
l9-G80 2-G80 3-B-G80 9-A-G80 I0-A-G80 12-A-G80 l3-C-G80
si02 68.1% 41.7% 72.3% 69.4% 72.5 % 72.5% 73.2% 69.2 % 68.6 % 10.4 15.1 13.8 Al 203 12.0 6.01 LL.7 14.2 r 3.8 12.6 11.0 Fe203 5.48 0,39 2.19 1.83 1.69 1.18 2.83 4.63 2.55 I.65 MgO 0.1 <0.1 <0.1 0.2 0.4 0.3 <0.1 <0.1 0,4 0.4 CaO 0.77 0.34 0.35 0.65 1.1I 0.76 0.43 0.08 1.56 1.06 t Na20 <0.2 <0.2 <0.2 3.4 4.6 3.8 4.1 0.9 4.1 q.7 KzO 3.16 0.4l ?.38 5.87 3.31 3.92 3.66 8.87 4.47 2.99 Ti02 0.38 0.05 0.t8 0.29 0.34 0.23 0.20 0.17 0.51 0.34 Pz0s 0.3 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0,I <0.1 <0.t MnO <0.02 <0.02 <0.02 0.08 0.07 0.06 0.09 0.r? 0.06 0.08 Lot 4.62 6.94 0.99 4.64 3.56 3.95 0.56 1.51 4.75 U 1820 ppm 6.46q. 23.8 ppm 5.47ppm 19.6 ppm 2Q.7ppm 58.6 ppm 11.8 ppm 5.0 ppm * * * * * * * Th 30.3 n n 15 13 " 36 39 lll 12 13 F 0.06 0,0 9ppm 0. 1Sppm 0.07 0.05 0.03 0,13 0.10 0.06 0.05 s 0.33 0.18 0.01 0.03 <0.01 <0.01 <0.0I <0.01 0.04 0.01 As 1450 900 9?0 l0 <0.5 <0,5 <0,5 20 5 <0.5 Se 15 <0.I <0.l <0.1 <0.1 <0.I <0.1 <0.I <0.I Cs 2000 4 38 6 31 73 58 2L 3 61 Li 24 33 22 3? 15 26 90 154 38 L7 Rb 150 6 188 198 163 324 350 542 143 185 A9 0.92 <4.6 <4.6 <0.46 <0.46 <0.46 <0.46 <0.46 <0.46 <0.46 Au <4.6 <22 <46 <4.6 <4.6 <4.6 <4.6 <4.6 <4.6 <4.6 B 15 14 L2 5.8 34 38 r7 t2 7.r * * Ba 754 * 202 * 96* 1220 * 1030 * 123 * 55 26 1180 1720 Be 3.4 4.7 3.4 2q 2.0 2.6 6.5 r7 2,4 2.? Bi 110 <46 n <10 <10 <10 Table l. (continued) I 9-G80 2l -G80 o-b6u 9-A-G80 I 0-A-G80 I 2-A-G80 I 3-C-G80 sb 4.93* <46 <46 I.I * 0.4 * nQ* 0.9 * 8.6 * 0.4 * 0.4 * Sc 6.09* <6.5 <6.5 (o* 3.9 * 1r* 1.0 * 1.0* 4.7 * 3.8 * Sm 6.4 * 2.2* 26 * 8.0 * 6 q* 2L. * 22. * lI * 8.1* Sn 19 <4.6 <4.6 <4.6 (4.6 <4.6 <4.6 ?3 <4.6 <4.6 Sr 3060 * 57 ?40 72 225 * 64 ?? ?4 i56 * 146 * Telo* <500 <500 r.7* 1.6 * 2.5 * 5.0 * 16.0* 16* 16* Tb 0.58* 0.22* 2.3 * 1.1* r.2* <46 ?l * 7?* 16 * 12* T',l <1000 <4.6 <4.6 <4.6 <4.6 I3 <4.6 <4.6 <4.6 <4.6 Tm <9.3 <46 r.7 * <4.6 0.7 * <4.6 1.9 * 5.7 * 0.8 * 0.8 * V IIOO H68 ?R 8.0 <1.0 <1.0 24 6.0 Y 16 <40 47 35 35 26 90 240 44 31 Yb 0.98* 0.95* I0.3 * 4.0 * 4.6 * L2.*33* 4.5 * 4.6 * Zn 72 <140 <140 40 59 59 ltO 250 51 48 Zr 380 980 470 240 328 * 150 753 * 1930 * 375 * 351 Otherelements which were also determinedbut wereless than the detectionlimit are: Ho (10 ppm In (50 ppm Ir (46 ppm Re (46 ppm Rh irregularly throughout the deposit (P. C. Goodell, to averagecrustal abundancesfor rhyolites are: B, oral comm., 1981).The ore forming processesat Bi, Ce, Cr, Er, Ni, Pb, S and Sc. Margaritas are presently not well understood. Ba- High contents of uranium in ash flow tuffs are not zan Barron (1978)considers the depositsof Sierra unique to Sierra Pefra Blanca, but also occur at Pefia Blanca to be of Laramide age and hydrother- Sierra del Nido where the Campana Tuff contains mal origin. up to 59 ppm uranium (Table 1); this unit is also high in rare earth elementssuch as Dy and Er. Although Trace elementchemistry of volcanic rocks from and the low Na2O content suggests that this tuff has around Sierra Pefla Blanca been altered, the high Th content and a Th/U ratio The trace elementchemistry (Table 1, column l) of 2.6 suggest that the enrichment of uranium is of the tuff breccia, which hosts the margaritasite, mostly primary. Both the CampanaTuff and Cryp- showsseveral elements in addition to Cs, V, and U tic Tuff contain higher concentrations of F and Li to be in anomalouslyhigh concentrations(Table 1). than do the volcanics from adjacent ranges. High Molybdenum, As, Sr, and Ag are exceptionally concentrationsof Cs, in excessof 50 ppm, occur in high, and in fact Mo is considereda byproduct of the basal vitrophyres of the Cryptic Tuff (Sierra del uranium mining at the Margaritas deposit. Other Nido), the Toro Rhyolite (SierraPastorias) and the elementswhich are anomalously high as compared QuintasTuff (MajalcaCanyon). The QuintasTuff is t27E WENRICH ET AL.: MARGARITASITE believed to be correlative with the Nopal Formation occur in the porphyritic breccia clasts. The breccia to the east in Sierra Pefla Blanca (Goodell, 1981). clasts are fractured in places and are filled by Vitrophyres of the Nopal and Escuadra Formation interstitial kaolinite. What appears to have been likewise have high Cs concentrations(P. C. Goo- plagioclasehas been altered to clay. Accessory dell, written comm., 1981)particularly as compared minerals include apatite, zircon, and pyrite. The to the tuff units. Although some of these vitro- clasts are entirely volcanic, generally lapilli size, phyres have high U, there does not appear to be a indurated, and well sorted; the rock appears to be significant correlation between Cs and U. reworked. The matrix does not appearto be volcan- ic suggestingthat the brecciation was sedimentary Petrography of the tuff breccia in origin, although any transport could not have Margaritasite occurs as fine-grained yellow ag- been too great judging from the good preservation gregatesup to I mm in size within interstices of the of the tuff clasts. rhyodacitic tuff breccia. Some of the hematite- The margaritasite occurs in the interstices either stainedbreccia clasts are porphyritic tuffcontaining as (1) linings within relict phenocrysts (probably pumice fragments, and others are dominantly com- biotite), occasionally with kaolinite forming the posed of partly devitrified glass shards; the intersti- outer euhedralborder with an equally thick layer of tial material is neither hematite stained nor does it margaritasiteencasing a hollow core (Fig. 3a)' or (2) contain glass shards, but is comprised primarily of as amoeboid aggregates(Fig. 3b). The individual quartz and kaolinite with minor sanidine. Minor crystals of margaritasite are I to 3 pm in size and amounts of quartz, much lesser amounts of sani- are in some places intergrown with quartz (Fig. a)' dine, and sparse,highly alteredbiotite phenocrysts Becauseof its fine-grainednature, the margaritasite GEoLoGIc SECTION(E-W) THROUGHTHE I.4ARGARI TAS DEPOSI T, pefra sLnNcA uRANIUMDISTRICT, CHIHUAHUA, MEXICO tr45 | :6e 57 577 579579 ESCUADRAFM. Fig. 2. Section constructed from drill hole information. Location and identifying number of drill holes are shown. Distribution of uranium ore is indicated in btack, as interpreted by the Instituto National de Energia Nuclear from gamma-raylogging of drill holes (unpublisheddata). The margaritasitesample was collected in the vicinity of drill hole #33 (arrow). Margaritasite is present at the surfacefrom approximately drilt hole #46 to #64. (Diagram from Goodell, t981, with additions). WENRICH ET AL.: MARGARITASITE r279 purifying sufficient quantities of margaritasite com- bined with the small grain size and very high density have prevented measurementof the density; the calculated density of margaritasiteis 5.41 glcm3 while the calculated density of synthetic Csz(UOz)zV2O3is 5.52 g/.-'. X-ray crystallography The synthetic Cs analogueof carnotite is iso- structural with synthetic carnotite (Barton, 1958; Applemanand Evans, 1965).Both are monoclinic, space group P21la with unit cell content 2[M2(UOz)zVzOe].Margaritasite has a powder dif- fraction pattern very similar to the synthetic Cs carnotitereported by Barton (1958).The intensities and d-spacingsof the powder diffraction maxima of margaritasiteand those of the calculated pattern for Cs carnotite (Powder Diffraction File, card 25-1218) are remarkably similar, while distinct from those of natural (potassium)carnotite (Table 2). The d-spacingsand intensitiesobserved for mar- garitasite were obtained with CuKa (nickel filter) radiation,I : 1.54178A.The d-spacingswere aver- aged from multiple diffractometer recordings cali- brated with quartz as an internal standard. The sample of margaritasite on which the powder pat- tern was obtained also contained trace amounts of quarlz, which makes the true intensity of the Cs-- carnotite peaks observedat 3.3444 and 1.820A uncertain; the true intensitiesmay be lower than Fig. 3 (a) Photomicrograph of a relict phenocryst in the tuf thosereported here. breccia with an outer lining of kaolinite and an inner lining of Unit-cell parameters (Table 3) and calculated d- margaritasite (lighter toned material). Relict phenocryst outer spacings(Table 2) were obtained from a least- lengthequals |.38 mm (width of photo equalsl 46 mm); crossed (using a program written nicols. (b) Photomicrograph of an amoeboid margaritasite squares refinement by aggregatesurrounded by an alteration halo. Length ofaggregate equals0.87 mm (width of photo equals 1.37 mm); uncrossed nicols. appears grey and nonpleochroic in transmitted light, although, in an exceptionally thin section, yellow-brown absorptioncolors are present under crossednicols; reflected light is required to yield the characteristicyellow carnotitecolor. Margarita- site is optically indistinguishablefrom carnotite. The small crystal size, the biaxial nature of the mineral,and its very high indicesof refractionhave prevented measurementof the refractive indices. The synthetic anhydrousCs analogueof carnotite has a ( 1.83,B : 2.49,y : 2.70,2Y : 45'30' (Barton, 1958).The margaritasiteis not noticeably Fig. 4. Scanning Electron Microscope photograph of fluorescent in ultraviolet light. The difficulty in margaritasitecrystals. The distancebetween tick marks is I pm. 1280 WENRICH ET AL.: MARGARITASITE Table 2. X-ray powder diffraction data for margaritasite (Margaritas deposit, Pefra Blanca district, Chihuahua, Mexico). (diffractometer:CuKo radiation, I : 1.54178A,Ni filter; quartz internal standard.Last column showsselected d-spacings for natural carnotite (Frondel, 1958)for comparison.) 20 observed Natural Synthetlc+ hkl d(calc.) d(obs.) I( obs,) CuKo carnotlte Cs-carnoclte 001* 6.971 6.965 100 t2.71 6.56 7.033 I l0* 6.47l 6.454 10 L3.72 6.481 ItI^ 5,225 5.215 4 r7.00 5.254 200* 5.053 5.048 t2 t7.57 5.12 5.049 lll* 4.373 4.362 14 20.36 4.25 4.393 020* 4.2t3 4.208 7 21.11 4.225 2t l* 4.t46 4,t42 14 2t.45 4.158 021* 3.605 3.600 70 24.73 3.53 002* 3.485 3.489 60 25.53 3.5t7 2tr 3.343 3.25 3.35r 202 3.330 3.344\ 7ot 26.66 220* 3.236 3.23t 50 27.61 3.240 3.174 3rr 3.164 50 28.20 3.r77 22t 3,155 3.165 3I0* 3.128 3.129 35 28,53 3.r2 22r 2.754 2.752 6 32,54 2.762 t 22 2.744 130 2.706 2.702 t7 33.16 2.709 3r2 2.696 3lt* 2.6t5 2.616 l0 34.28 2.594 2.6r8 400* 2.527 2.527 3 231* 2,4L9 2.421 4 37.14 2.426 003 2.324 2,336 4 38.54 2.344 42I* 2.224 2.224 5 40.56 331* 2.t72 2,172 3 4t.57 2.t76 330* 2.157 2.157 14 41.88 2.160 040* 2.to6 2. t05 6 42.96 2. l12 223* 2.068 2.069 6 2.068 5l I* 2.040 2,040 9 44.40 403,l4l 2.008 2,007 4 45.l8 510,33r 1.965 r.967 6 46.t5 5r2 1.950 l4l | .948 t.947 t2 46.66 240 L.944 24t* r.926 1.928 4 47.14 24r r.823 520 1.822 l.82ox 7x 50.tI 402 l,82r 432 1.817 602t t.7 t3 t.712 3 531* I.683 1.684 5 54.48 a = 10.514 std. error = .003 b = 8.425 std, error = .003 c = 7.252 std. error = .005 ?-= ro6.oro v = 617.48 A' std. error = 0.44 Ar Values of d(calc) obtalned fron refi.nenent baaed on cell parahetera glven = above, uslng 23 reflectLons; space group !l/L {1th non-extlnctlon condltlons qCL) tt:2t, (OtO) t=2g. * = refleatlons used in cell refinenent. xThe lntenslties reported here for the 3.344 A and 1.820 A peake nay be slightly high because of contrlbutlon fron the (l0I) (3.34 A) and (I12) (1.817 A) peaks of a trace of admixed quartz, However, a powder photograph obtal.ned wtth a Gandalfl caDera on a quartz-free sanple of Mrgarltaslte stlll shoved these sane peaka with conparable lntenslty. +Weak llnes not observed ln the mrgarttaslte pattern are omltted froil the calculated C6-carnotlte data fr@ Powder Dlffractlon F1le card 25-I218. Higher angle lldes of Cs-carnoctte are omltted because the effect of the snall difference 1n unlt cell dinenslons becomes lncreaalng nagnlfted wlth hlgher angLes, and lt ls thus dlfflculc to correlate IIne6 between mrgarltaslte and synthetic Cs-carnotlte. WENRICH ET AL.: MARGARITASITE IzEI Table 3. Comparison of properties-K and Cs carnotites humidity at 27"C) for a total of 68 hours at succes- sively increasingtemperatures between 100'C and K carnotite K carnotite Cs carnotite l4argaritasite synthetic, natural, ttah synthetic, (present work) 500'C decreasedthe c dimension of margaritasiteto anhydrous (Donnay and anhydrous (Barton, 1958) Donnay, 1955) (Barton,1958) 7.17 A. The initial X-ray powder patterns were Space group P21/d P2,/a P21/a Pz,/a obtained on material that was stored in air at about z 2 2 22 25"C and 30Vorelative humidity. Natural carnotite g (in A) to.47 t0.47 10.51 10.s14(3) b (in A) 8.41 8.41 8.45 8.425(3) from the Colorado Plateau showed no significant g (in A) 6. 9l 7.32 7.2s2(5) changein the c dimensionof 6.63A upon exposure B 103.80 103.70 106.lo 106.010 to 100%humidity and 25'C, and a decreasein c to v (in A3) 563.5 590.8 624.3 617.5(4) n(") 1,71 <1.83 6.524 upon heatingto 500"C.Donnay and Donnay n(B) ?,01 2.49 (1955) report that carnotite can undergo partial n(Y) 2.09 2,70 dehydration upon grinding; after 4 minutes of grind- 2V 6i 60 45.50 d (cal c. ) 4.99 4.91 5,52 5.40 ing in an agatemortar, no changewas observedin c q(obs. ) 4.95 4,70 5.48 of margaritasite,and a small, non-significantin- crease(6.63 to 6.644) was observedin c of carno- tite. VanTrump and Hauff, 1978, and based on the algorithm of Appleman and Evans, 1973)of the Chemical analysis powder data using 23 reflections, with the non- extinctionconditions h : 2n for (ft01)and k : 2nfor Chemical analysis of margaritasite was done by (0k0),appropriate for the space gronpP21la. Agree- both standard laboratory methods and by election ment between observedand calculatedd-spacings microprobe. The microprobe analyseswere made is very good, with computed standard errors of on an ARl-snlrreautomated electron probe, at 15kV 0.003for a, 0.003for b, and 0.005for c, and 0.4443 accelerating potential and 10 nA sample current. for the unit-cell volume; assignmentof observed The microprobedata were correctedusing a version reflections in the cell refinement was aided by of the rraecrcIV program(Colby, 1968).A counting comparison with a calculated powder pattern for time of 40 secondson each peak and 4 secondson margaritasite supplied by Howard Evans (written backgroundwas used. Standardsused were KCI for comm., 1981)in addition to the calculatedpattern K, pollucite for Cs, Rb, Si, and Al, albite for Na, from the Powder Difiraction File (card 25-1218). calcite for Ca, uranium metal for U, vanadium The unit-cell parameters(Table 3) agreewell with metal for V, orpiment for As, barite for Ba, and those of synthetic Cs-carnotite (Powder Diffraction celestitefor Sr. The KB X-ray fluorescencelines of File card 25-1218or Barton, 1958).The presenceof potassiumand vanadiumwere usedinstead of Ka to Cs is most strongly manifestedin the c dimension, avoid interferenceby overlappinglines of U and Cs. which expandsin responseto the substitutionof Cs Multiple analyses(14) were made on individual as ions for the smaller K ions (ionic radii for 12- well as different aggregatesof margaritasite. The coordinatedCs* is 1.96,{and that for l2-coordinat- results showed the aggregatesto be relatively ho- ed K+ is 1.684, estimatedby Whittaker and Mun- mogeneous(Table 4). The margaritasite totals for tus, 1970, p. 952) that reside between layers of the four major componentsonly sum to 92 wt.Vo; [(UO2)2V2Oa].-2"(Appleman and Evans, 1965). water probably accountsfor no more than another 3 Carnotite can have a variable water content of wt.Vo and the remainder of the determined trace between zero and three molecules of H2O per elementssum to an insignificanttotal. This discrep- [M2(UO2)2V2O3]formula unit, and hydration or ancy in the total may be a matrix effect due to a lack dehydration of carnotite is reported to affect the c of appropriate standards for the various elements, cell dimension(Donnay and Donnay, 1955).They but is more likely a result of the imperfect surface reported that the c length of a natural carnotite polish attainable on the fine-grained soft margarita- increasedfrom 6.594 to 6.63Awhen subjectedto an site aggregates.The mean CszOiKzO ratio by atmosphereof 100%humidity for one day. Margari- weight of margaritasite aggregatesand individual tasite that was placed in a l00Vo humidity atmo- grains equals ll.3-r4, so that the Cs2O is always sphere at 25"C for 88 hours showed an analytically significantly greater than the K2O content. In the insignificantincrease in c from 7.254 to 7.264. natural margaritasitefrom the Pefla Blanca district, Heating the margaritasitein air (30Voto55Vo relative there is minimal heterogeneityin the Cs2O/K2O 1282 WENRICH ET AL.: MARGARITASITE concentration and no evidence for a complete solid spectrometer with an air-acetylene flame. Match- solution to the carnotite end member. ing-matrix synthetic standardswere prepared from Standardlaboratory analyseswere made on puri- standard solutions of high-purity materials, with fied samplesof margaritasitewhich were separated each sample and standard solution containing 2000 from the bulk rock by methylene iodide heavy ppm Li to further buffer potential matrix differ- mineral separations,Franz isodynamic separations, ences.Replicate analyseswere made on three sepa- and hand picking. Determinations of U and V were rate splits of the margaritasite, each weighing ap- done by inductively coupled argon plasma atomic proximately 10 mg, with resultant coefficients of emission spectrometry (tcee-aes) using a Jarrell- variation of 0.81% for K, 0.65% for Cs, 0.30Vofor Ash ModeVl160 direct-readingpolychromator; K U, and 1.58%for V. Sampledissolution was accom- and Cs were determinedby atomic absorption spec- plished by HF, HNO3, HCIO4 digestion. Water trophotometry using a Perkin-Elmer Model 603 determinationswere made on a Perkin-Elmer 2408 Table 4. Chemicalanalyses of margaritasite(in wt.Vol Standardlaboratory methods ElectronMicroorobe (AtomicAbsorpti on spectrometry unlessothervrise indicated) Recalcul ated Rangeof 14 Recalculated Rangeof Ave. to 100% detenminati ons Ave. to l00i 3 determinations (assume2.5X water^) Cs20 17.6% 18.7 15.6 - 19.0 17.0% 10 7 16.9 - L7.L KzO 1.63 1.73 1.07 - 2.14 r.20 1.39 l. 19 - 1.20 uog 55.4 Rao 5r.7 - 58.4 49.6 57.5 49.5 - 49.8 * vz0s 17.0 18.1 16.1 - 18.9 15.9 18.4 15.6 - 16.0* Hr0 (2.5) 2.5 2.9 -_*** Total 91.63 99.9 aR2 99.9 Na <.06% <.06? <.0011 Rb <.12 <.r2 .08 Ca ?? .27 - .37 .3 Sr .07 <.0008- .24 .03 Ba lo <.0009- .32 .16 si .74 .05 - .25 AI .02 <.0005- .06 <.001 As <.06 <.06 Mo <.08 Mn .0ll Fe .0004 Zn '04 Mg .02 Li <.001 P <.01 Ti E** Cr .015** .01 ** 7r .015** Ga .002** * Determinationsmade by ICAP-AES ** Semiquantitativeemission spectroscopy *** Carbon-hydrogen-nitrogenanalyzer Not determined WENRICH ET AL,: MARGARITASITE t2E3 carbon-hydrogen-nitrogen analyzer. Determina- and the ideal formula is tions of trace elements were done both by atomic (Cs,HsO, K) O absorption spectrophotometryand by semiquantita- ?(U ) 2YzOs'nH2O tive emission spectroscopy;the method used for where Cs > H3O,K and n - 1 each element is indicated on Table 4. Unfortunately The empirical formula based on the microprobe the fine-grain size (approximately 1-3 pm) of the data, ignoringwater, is: margaritasitemade it impossible to totally separate (Cs1.31Ke.36) O)z.or(VzOe)o.st from the qtartz with which it appears to be inti- >r.oilU mately intergrown. Thus, the sum of Cs2O+ K2O The apparent presence of oxonium ions in mar- + UO3 * V2O5 + H2O only equals 86.2%; the garitasite may be compared to other oxonium- remainder is probably quartz, as the entire sample containing layer structures such as boltwoodite, went into solution when treated with hydrofluoric K(H3O)UO2(SiO+)'nHzO(Stohl and Smith, 1973), acid. Nevertheless,as can be seenfrom Table4, the torbernite group minerals (Ross and Evans, 1965) average element determinations for margaritasite hydrous uranyl oxides (Sobry, 1971), illite, and separatedfrom 5 pounds ofhost rock are in propor- hydromicas.An oxonium end-memberin the car- tion to thosedetermined by the electronmicroprobe notite-metatyuyamunite group vanuranylite, on individual grains. In fact, when both analysesare (H3O,Ba,Ca,K,Pb)r.6(UO)2(VO4)z'4H2O(?) has recalculated to l00Vo (assuming 2.5VoH2O for the been reported by Buryanova et al. (1965), and an microprobe analysis) the results are in excellent apparentoxonium end-memberanalogous to carno- agreement(Table 4). tite was synthesizedby Barton (1958).Published The ideal formula for margaritasite is analyseshave shownminor amountsof Ca in carno- (Cs,K)z(UO)2Y2Os.nH2Owhere Cs > K and n tite (Donnay and Donnay, 1955;Frondel, 1958). varies from 0 to 3. The empirical formula basedon 2 vanadium atoms and the standardlaboratorv analv- Cs-carnotitesynthesis and results sis shown in Table 4 is An attempt was made to synthesizecesium-rich carnotite by reaction of natural carnotite from the (Csq.rgKs.2e)>r.oz(UOz) 1.62H2O. r.ss(VzOa)r.oo' ColoradoPlateau with 0.1 M CsCl solution (pH of 5.6) at 80'C and 200'C Oable 5). The natural Unfortunately, this results in an alkali charge defi- carnotite contained less than 0.01 Cs atoms per ciency. Both the microprobeand atomic absorption formula unit but was contaminated by quartz, with analysesshowed a consistentrange in total alkalies minor amounts of calcite, potassiumfeldspar and rather than the expected 2.00. This appearsto be a albite. Reactantswere encapsulatedin pinch-sealed real deficiency and cannot be accounted for by or welded Pt capsules which were loaded into other alkalies such as Rb, Na, Li or by alkaline reaction vessels made of screw-cappolyethylene earths such as Mg, Ca, Sr, or Ba; all of these bottles (80"C) or lengths of stainlesssteel tubing elements summed to less than 0.1 formula unit with compression fittings at each end. Approxi- although Ca approaches0.1 (Table 4). The total mately 10ml of CsCl solutionwas alsoloaded in the formula unit contributedby Ca (0.086),Ba (0.013), reaction vesselsto provide additional reagentin the Rb (0.011),Mg (0.009),Zn (0.007),Sr (0.004),and Mn (0.002)is 0.132.These remaining cation vacan- ciesare believedto be filled by HgO" ions; because of the uncertainty in which sites the trace elements Table 5. Experimental conditions and products (Table 4) belong or if they occur in admixed impuri- ties, all vacant alkali sites have been assignedto l{t. l,lt. carnotite 0.lM CsCl Duration H3O* (oxonium).This reducesthe total H2O,which Experiment Seal (mg) (ms) Toc (days) could be as high as 1.5 assumingall 0.132 trace p'i >400 80 14 element formula units went into this site. The t nch 10 empiricalformula basedon the wet chemicaldata is pi nch l0 >400 80 6l therefore 4 wel ded l0 200 200 14 5 wel ded 10 200 200 61 (Csr.re(HrO)o.rsKo.zs)>2.o2(UOtr.ss (v2o8)r.0o.1.07H?), 6 wel ded IO 200 200 209 t2E4 WENRICH ET AL.: MARGARITASITE event of capsule leakage and to act as a pressure Figure 5 shows the changes in Cs/(K + Cs) transmittingagent. Loaded, sealedcapsules which between the 5 experimental runs, the natural carno- were used in the 200"C experiments were weighed tite, and margaritasite. The two experimental runs before and after the experiment; no leaks were at 80"C (14 and 6l day durations) produced carno- detected. Constant temperature environments tites with small,less than 0.05mole fraction Cs, but (*5'C) were provided by an air-circulating drying measurablyincreased Cs contents. The carnotite oven (80'C) and a mufle furnace (200'C). Experi- starting material had a significantly smaller but ment pressurewas ambient vapor pressureat the measurableCs content (0.02mole fraction Cs). The experimentaltemperatures. In all experimentsthe 200"C products showed increasingly higher con- ratio of Cs/K in the system(solid + liquid) exceed- tents of Cs with increasing duration of the experi- ed 3.0. At the end of an experiment the reaction ment (Fig. 5). After 209 days at 200'C some of the vesselwas cooled with water to room temperature experimental products had a Cs/(K + Cs) ratio of in approximately 5 minutes. The recovered solid greater than 0.7 mole fraction Cs which is equiva- phases were washed with H2O, dried at room lent to the lowest Cs mole fraction of the Pefla temperature and submitted for X-ray diffraction Blanca margaritasite (Fig. 5). (XRD) and election microprobeanalysis. Microprobe traverses across several carnotite Microprobe analyses and atomic proportions of grains in the 200'C,209 day experimentat 0.5 pm the natural carnotite starting material and the 5 ion steps, using an electron beam of approximately I exchangeproducts are shown in Table 6. In addi- pcmin diameter, showed an irregular zonation of tion to the elementslisted, Na, Rb, and As were CszO/KzO,with the numberof K atomsperformula also determined but are not shown because their unit ranging from about 0.2 to 1.2 (equivalentto concentrations were below the limit of confident about I to 5.5 wt.VoKzO). Therewas no evidenceof determination (0.01-0.2 wt.Vo oxide). The apparent Cs-rich marginson K-rich cores. deficiency of alkalies in all of these analyses may The potassium and sodium feldspars present as indicatethe presenceof oxonium ions in the alkali an impurity with the natural carnotite starting mate- site. rial do not appear to have incorporated a significant Table 6. Microprobe analysesof synthesizedCs-carnotites. Calculated atomic proportions (on the basis of U + V : 4) shown in parentheses Numberof analyses * in average Cs20 Kzo uog vzos Ca0 Ba0 Si0Z A1203 TotaI Natural 0.12 8.27 69.29 21.89 0.33 0. 14 0.05 0.02 0.r2 100.23 Carnot i te (0.007)(1.4s4) (2.006)(r. ee4) Experiment#l 800c 0.75 8.36 67.25 21.t8 0. 18 0.01 0.03 0.03 0.06 97.85 14 days (0.045)(1.517) (2.00e)(l. ee1) Experimentf2 800c 0.85 8.51 67.t0 20.88 0.2t 0.06 0.05 0.04 0.05 97.73 61 days (0.052)(1.s57) (2.022)(r.e78) Experiment#4 2000c 2.45 8.68 65.90 20.22 0.16 0.01 0.04 0.03 0.r2 97.62 14 days (0.1s4)(1.628) (2.03s)(1. e65) Experiment#5 2000c 4.13 7.90 65.18 20.86 0.34 0.00 0.03 0.04 0.04 98.52 6l days (0.256)(1.468) (r.ee3)(2.007) Experiment#6 2oooc 12.83 4.80 59.75 18.91 0.16 0.04 0.07 0.17 0.05 96.77 209 days (0.873)(0.e7e) (2.004)(1.ee6) *!'latencontent not determined WENRICH ET AL.: MARGARITASITE 1285 2 209 CoC.o . - .Co 200 I TE I I4ATERI AL o A CARNOT START NG z . MARGARI TAS I TE tr RUNSFRoM 8oo EXPERIMENT,RUNS 1,2 F z U .RUNS FROM2OO"C EXPERI14ENT, RUNS I+,5,6 E 5 d /,\NUI.4BER BY CLUSTER U OF SAI"IPLESREPRESENTED L OF DATA POINTS U J : d u I F o I L o z o 5 46 5 F Al IItr orlir rC oor r ro d l a -- 1.0 I'1OLt FRACT I ON :, Nf L 5 Fig. 5. Variation of the mole fraction of Cs/(K + Cs) with duration of the Cs-+arnotite synthesisexperiments. amount of Cs during the experimental run; micro- product which precipitated during the quenchingof probe analysis of spots near the edge of feldspar the run. Alternatively the pollucite may have grown grains in run 6 (200"C and 209 days) showed less during the run as a stable reaction product of the than 0.03 wt.VoCs2O. The Ca and Si presentin the CsCl solution + quartz * feldspar. The presenceof runs (from the calcite, quartz, and/or feldspar impu- several strong pollucite diffraction lines in the pow- rities in the starting material) did react with some of der pattern from experiment 6 (pollucite is most the carnotite in experiment 6 to produce a Ca uranyl abundant in this run) confirm the identification vanadate phase as well as a Ca uranyl silicate basedon chemistry. Pollucite has been found to be phase; the vanadate was probably tyuyamunite or a common phase precipitated from heated Cs-bear- meta-tyuyamunite,but due to its paucity its identity ing aqueous solutions in contact with aluminosili- could not be confirmed on the X-ray diffraction cates(McCarthy et al.,1978). Despitethe observed pattern. In addition, globules and glassy-appearing formation of new phasesfrom the apparent break- coatings (on carnotite, qvartz, and feldspar) of a down of some carnotite, a significant proportion of Cs-Al-silicate of pollucite composition (CsAlSi2O6) the carnotite obviously underwent ion exchangeas were observed on the microprobe in all three of the can be observed in Table 6 by the reasonable 200'C runs; this material has the appearanceof a formula proportions. 1286 WENRICH ET AL.: MARGARITASITE The X-ray powder diffraction data for the natural run at 80'C yield cell dimensions which are not carnotite and the experimental products were ob- analytically distinguishablefrom those ofthe start- tained from repeated scans between 10oand 50"2e ing carnotite. Powder patterns (Fig. 6) of the 14 and with CuKa radiation scannedat ll2" (20)/min.The 61 day, 80'C experimentsshow a very smallpeak at X-ray patterns are shown in Figure 6 where they are about 12.36"(20); this peak doesnot appearto be compared with the pattern for margaritasite. The the (001) reflection of a margaritasite phase. The sample of natural carnotite and consequently its same weak peak was observed at 12.40" (20) in experimental products have a strongly enhanced powder patterns of the natural, Cs-free carnotite, (001) peak caused by preferred orientation of the and it appears to be due to some trace inpurity carnotiteplatelets on the mounting medium. phase.In contrast, the X-ray difffaction data on the Unit cell parameters for the experimental prod- 200'C products show two distinct (001)peaks, one ucts were calculatedfrom the powder X-ray difrac- for carnotite at 13.8" (20) and one for margaritasite tion data (Table 7). The X-ray data for the products at 12.7" (20). I'IARGARITASITE CARNoTITE+ 0.1 M CsCl (001 >-l ) Fl Hl 200oc,14 DAYS al ZT Hl 2l >l Fl ;l zl tLJ I =l =l CARNOTITE 001) CARNOTITE+O.lMCSCI (001) 80oc, il+ DAYS (002 ll -l LI (tl (002 zl r.uI FI -l zl Fl 01) cnl zl trlI zll-l Hl 8ooc,61 DAYS 30 28 26 24 22 20 18 16 14 t2 10 DEG2"0 (CuK0) tll 30 28 26 24 22 20 18 16 14 12 10 Fig. 6. X-ray powder diffraction patterns for margaritasite,Colorado Plateaucarnotite and synthesizedCs-+arnotite. WENRICH ET AL,: MARGARITASITE 1287 Table 7. Unit cell dimensions of natural and synthetic unknown temperatureof formation. However, only margaritasiteand carnotite G- = not determined) two (001)reflections are observed;that is a discrete increasein the c lattice dimensionoccurs from 6.64 a(A) e(o) to 7.3A with no intermediatevalues. This may be Iilargaritasite phase due either to (1) a sudden c-axis increasewhen a critical number of the large Cs atoms are stuffed natural marg. 10.514 8,425 7.250 106.01 betweenthe layersof (this structural Experiment6 f0.6I 8.42 7.3I 106.1 [(UOz)zVzOs]" Experiment5 10.54 8.41 7.30 105.6 change occurs somewhere between Car&6Marg2s Experiment4 -- 7.30* and CarnalMarg5e)or (2) an extremely fine inter- growth of discrete phasesof margaritasite and car- Carnotite phase notite which is below the resolutionof the electron Experiment6 -- 6.62* microprobebeam. Nevertheless,a K mole fraction Experinent 5 10.43 8.42 6.59 103.5 of at least 0.2 is acceptable into the margaritasite Experiment4 10.41 8.43 6.62 103.8 structure; analysesof the natural margaritasiteyield Experiment 2 10.52 8.41 6.65 104.7 K contents up to this concentrationwith no evi- ExperimentI 10.52 8.42 6.65 104.8 denceof the carnotite(001) reflection in the margar- natural carn. 10.45 8.41 6.63 104.I itasitepowder diffraction pattern. t calculated from the (001) and (002) reflections only Geneticimplications for the Pefla Blanca deposit For the series of 200'C experiments the diffrac- The discovery of Cs-rich carnotite in the Pena tion lines for the Cs-rich phase (of which the (001) Blanca uranium district provides important evi- and (002) reflections are the most pronounced) are dence for local hydrothermal or pneumatolitic ac- barely discernablein the 14-dayrun and increasing- tivity during or after uranium mineralization. Data ly strong in the 60- and 209-dayruns. The products from the geochemical literature (summarized be- of all three experiments at 200"C (4, 5, 6) are low) indicate that the high ratios of Cs:total alkali mixturesof a high-Csand a low-Cs carnotite,each elements which are required to produce Cs-rich with distinct d-spacingswhich do not shift with minerals can be generated and sustained only in varying Cs content.These data suggestthat there is high temperature environments. an abrupt structural transition from carnotite to The mobility of Cs in low temperature ground margaritasite which may be triggered by the incor- water is severely restricted by adsorptive and ion poration of a threshold amount of Cs. exchangeuptake by clays and zeolites(Ames, 1960; Jenneand Wahlberg, 1968;Chelischev, 1976). Re- Discussion ported concentrations of Cs in ground and surface According to the chemical data shown in Figure 5 water are generallyless than 0.001ppm (Wedepohl, the synthetic Cs-rich carnotites are not as homoge- 1970).These observationsexplain why Cs enrich- nous as margaritasitealthough no discrete zoning is ment has never been reported in carnotites from present. Even the 200oC, 209-day experiment in- low temperature Colorado Plateau uranium depos- cludes two measured products with anomalously its. In contrast, thermal waters in young rhyolitic low Cs2O/K2Oratios. Only low Cs-carnotitegrains rocks of New Zealand contain <0.1 to 2.6 ppm were detected by the microprobe in the material dissolvedCs (Ellis and Mahon, 1964).Experiments from experiment4 (200"C,14 days) (Fig. 5), but the designedto model the generation of such Cs con- presenceof weak (001)reflections at 6.97A (Fig. 6) centrations showed that leaching of rhyolite glass indicatesthe presenceof a small, incipient quantity by aqueoussolutions at 400-600'C produces nearly of Cs-enrichedcarnotite in this experiment also. quantitative removal of Cs in two weeks time (Ellis The electron microprobe analyses suggestthat an and Mahon, 1964). Additional experimental mea- asymmetric,limited solid solution may exist between surementsof Cs partitioning between feldspar and K2(UOz)2V2O3.nH2Oand Cs2(UO)2V2Og.nH2O(see an aqueous phase at 400-800"C showed that Cs Fig. 5). At 200"Cthis solid solution appearsto be stronglyfavors the aqueousphase (Eugster, 1955). between CarnlsoMarg6 and Carn27Marg73and at Synthesisexperiments in this study indicatethat in 80"C only between Carnl66Margqand Carne6Marga. 0.1 M CsCl solutions margaritasite can form from The composition of margaritasite suggeststhat the carnotite precursors within 6 months at 200"C. solid solution exists up to CarnlaMargs6at some Crystalline rhyolites (felsites) are typically de- 1288 WENRICH ET AL,: MARGARITASITE pleted in Cs relative to juxtaposed comagmatic instruction during the electron microprobe analyses. James G. vitrophyres, (Lebedeva and Shatkova, 1975;Zie- Crock performed the atomic absorption determinationsfor trace elementsin the margaritasiteand F. W. Brown linski, et al. 1977).Consistently large cesiumdeple- determined the water content. EugeneE. Foord provided valuableadvice on the tions in felsites show no positive correlation with crystallographic work as well as some of the standardsfor the sampleage, and thus suggestthat Cs is liberated electron microprobe analysis. We thank Deane K. Smith, How- during or shortly after high temperature devitrifica- ard Evans, Edward Dwornik, and Philip C. Goodell for useful tion. High Cs concentrations(over 50 ppm) occur in discussions.Analysts who contributed to the data in Table I were H. T. Millard, Jr., R. B. Vaughn, S. W. Lasater, vitrophyres but not in the associatedfelsites, of B. A. Keaten, Z. Hamlin, D. Kobilis, J. S. Wahlberg,J. W. Baker, volcanic rocks from Sierra Pena Blanca and the J. E. Taggart,J. G. Crock, C. A. Gent, H. G. Neiman, and R. adjacent regions of Sierra del Nido and Sierra Anderson. Thanks also goes to Paul B. Barton, Jr., Howard T. Pastorias(Table 1). The availability of Cs in a high Evans, Jr., Daniel E. Appleman, and Malcolm Ross for time temperaturegaseous or liquid phaseis evidencedby spent reviewing the manuscript and providing useful additions. the formation of Cs minerals pollucite The Colorado Plateaucarnotite used in the Cs-+arnotite synthe- such as sis experiments was (Cs,NaXAISi2O6).H2O supplied by Harry C. Granger. A special in pegmatites. Gaseous thanks to Howard Evans for sharingwith us his calculatedX-ray transport of Cs as a complex with fluoride is sug- powder pattern for margaritasite. gestedby the formation of (K,Cs)BF+and CsBFain volcanic sublimates (Wedepohl, 1970; Shatkov, References r97r). Alba, L. A. and Chavez, Rafael (1974) K-Ar ages of volcanic High Cs abundance in the vicinity of uranium rocks from the central Sierra Pefla Blanca, Chihuahua,Mexi- co. Isochron/West.10. 2l-23. depositsis not unique to the PenaBlanca uranium Ames, L. L. (1960)The cation sieve properties of clinoptilolite. district. Uranium and Cs are similarly incompatible American Mineralogist 45, 689-700. elementsin magmaticcrystallization processes and Appleman, D. E. and Evans, H. T., Jr. (1965) The crystal tend to be enriched in the more silicic magmas. structuresof synthetic anhydrouscarnotite, K2(UO2)2V2O8, Walker (1981)has demonstrateda significantcorre- and its cesium analogue,Csz(UOz)zVzO.. American Mineral- ogist 50, 825-842. lation between U and Cs in Tertiary rhyolites of the Appleman,D. E. and Evans, H. T. (1973)Indexing and Least- northern basin and range. This correlation also Squares Refinement of Powder Ditrraction Data. National occurs within other ore deposits located in late TechnicalInformation Service, Washington, D. C.,9214. stagemagmatic rocks. Rytuba and Conrad (1981) Barton, P. B., Jr. (1958) Synthesis and properties of carnotite have reported that Cs values are in excessof 100 and its alkali analogues.American Mineralogist 43,799-817. Bazan Barron, (1978) y ppm within rhyolitesfrom the McDermitt, Nevada, Sergio Genesis depositacion de los Yacimientos de molibdeno y uranio, en el Distrito de Villa uraniumdistrict, and that rhyoliteshosting uranium Aldama, Chihuahua. Boletin Sociedad Geologica Mexicana, depositsshow an excellentpositive correlationwith 39.2s-33. Cs. Although the backgroundCs levels are lower Buryanova, E. 2., Strokova, G. S., and Shitov, V. A. (1965) (lessthan 20 ppm) in the Marysvale,Utah, areathe Vanuranylite, a new mineral: Zapiski VsesoyuznogoMinera- logicheskogoObshchestva, 94, 437-443. Cs content is elevated in rocks which occur near Calas, Georges (1977) Les phenomenesd'alteration hydrother- uranium deposits, such as the uranium ore at the male et leur relation avec les mineralisations uraniferes en ProspectorIV mine (Wenrich, unpublisheddata, milieu volcanique: le cas des ignimbrites tertiaires de la Siena 1980).This U and Cs correlation within late stage de Pefra Blanca, Chihuahua (Mexique). Scientific Geologists magmaticproducts suggeststhat ore depositsrich Bulletin. 30. 3-18. Chelischev,N. F. (1976)Ion exchangeproperties in U and Cs may well have a magmaticsource. ofnatural high silica zeolites, (abs). Zeolite 76, An International Conference Basedon the above discussionit is unlikely that on the Occurrence, Properties and Utilization of Natural margaritasitewill be found in low-temperature ura- Zeolites, Tucson, Arizona, 22-23. nium depositssuch as those of the Colorado pla- Colby, J. W. (1968) Quantitative microprobe analysis of thin teau.Conversely "carnotite" occurrencesin urani- insulatingfilms. Advancesin X-Ray Analysis, 11,287-305. Dayvault, R. D. (1980)Peralkaline um deposits of hydrothermal origin are probable ash-flow tuffs in Santa Clara Canyon, north of Chihuahua City, Mexico, a possible source margaritasitelocalities. rock for uranium. American Association of Petroleum Geolo- gistsAbstract, Southwestsection, 24. Acknowledgments Donnay, Gabrielle and Donnay, J. D. H. (1955)Contributions to the crystallography of uranium minerals. United States Geo- The authors wish to thank JosephF. Mascarenaswho assisted logical Survey Trace Element InvestigationsReport 507, l-42. with the X-ray ditrraction work, mineral separations,and data Ellis, A. J. and Mahon, W. A. J. (1964) Natural hydrothermal tabulation, and spent many hours hand picking impurities from systems and experimental hot-water/rock interactions. Geo- the margaritasite sample. Ralph P. Christian provided valuable chimica et CosmochimicaActa. 28. 1323-1f57. WENRICH ET AL.: MARGARITASITE t289 Eugster, H. P. (1955) Distribution of trace elements. Camegie potassium,and ammonium ions, and water molecules.Ameri- Institute of Washington Year Book Geophysics Laboratory can Mineralogist, 50, l-12. Reports,54, ll2-114. Rytuba, J. J. and Conrad, W. K. (1981)Petrochemical charac- Frondel, Clitrord (1958)Systematic mineralogy of uranium and teristics of volcanic rocks associatedwith uranium depositsin thorium. United States Geological Survey Bulletin 1064. the McDermitt caldera complex. In P. C. Goodell and A. C. Gdlvez, L. and Vdlez, C. (1974)Uranium and its projecteduse in Waters, Eds., Uranium in Volcanic and VolcanoclasticRocks, nucleargeneration of electricity in Mexico-Summary. Ameri- p. 63J2. American Associationof PetroleumGeologists Stud- can Association of PetroleumGeologists Memoir, 25,522-524. ies in Geology#13. Goodell, P. C. (1981)Geology of the Pena Blanca uranium Shatkov, G. A. (1971)The mode of occurrenceof cesium in deposits,Chihuahua, Mexico. In P. C. Goodell and A. C. felsic volcanic glasses. Geochemistry International, 8, 553- Waters, Eds., Uranium in Volcanic and VolcanoclasticRocks, 556. p. 275-291. American Association of Petroleum Geologists Sobry, R. 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