American Mineralogist, Volume 59, pages 166-171, 1974
On Studtiteand lts Composition
Kunr WarpNre Institut filr Mineralogie und Kristallchemie der Uniuersitiit Stuttgart
Abstract
The investigation of studtite from the uranium deposit of Menzenschwand in the Black Forest shows that this ill-defined species is not a uranyl carbonate as hitherto assumed but a hydrous uranium oxide of monoclinic symmetry. It is identical with a synthetic uranium com- pound which is regarded as peroxide hydrate UO.'4 H"o (Sato, 1961). If this interpretation is correct, studtite would be the first peroxide mineral found in nature. Unit-cell dimensions of the mineral from Menzenschwandare: a 11.85,D 6.80,c 4.25 AP93"5l',2 =2, spacegroup C2, Cm, or C2/m, density (calc) 3.64 g/cc. Strongestlines of the powder Patternare 5.93 (10), 3.40 (8), 2.96 (6). The X-ray data agree closely with those given by Debets (1963) for the synthetic compound. The structure of studtite probably consists of O-U-O-U-O chains in the direction of the c-axis as in caseof a-UOg. When heated to 60'C, studtite is transformed into a dihydrate, UO.'2 HrO with loss of 2 H,O. This dihydrate, which is alsg known as a synthetic compound, is orthorhombic with a 6.51, b 8.78,c 4.21 A, Z = 2, spacegroup probably Immm, density (calc) 4.67 g/cc. Strongestlines of the powder pattern: 5.24 (10) , 4.41 (7 ) , 3.54 ( 8 ) . A brief description is also given of a presumably new siliciferous mineral occurring at Menzenschwand associatedwith studtite and uranophane.
Introduction Mode of Occurrence The mineral studtite is an ill-defined secondary The mode of occurrenceof studtite at Menzen- uranium mineral. It was describedby Vaes (L947) schwand closely resemblesthat of the mineral at as a hydratedcarbonate of uranium,on the basisof Shinkolobwe.It forms aggregatesand crustsof min- qualitative tests. The mineral had been found as ute needle-likecrystals that frequently show a more aggregatesof thin fibers on uranophaneand ruther- or lessradial fibrousstructure (Fig. 1). The needles fordine at Shinkolobwe,Katanga. The optical prop- are elongated along the c-axis but lack observable ertiesgiven by Vaesled to the conclusionthat studtite crystal faces. Their maximum length approaches is orthorhombic (Frondel, 1958). The scantydata 0.5 mm; maximumthickness is about 10 pm. publishedby Vaes were later supplementedby Guil- The mineral is foun{ on barite, quartz, hematite, lemin (1958), who furnishedX-ray powder spac- and limonite associatedwith billietite, uranophane, ing data for the mineralfrom Katanga. rutherfordine,and other secondaryuranium minerals. The optical and X-ray data publishedby Vaes and In one caseit is intimately intergrownwith urano- Guillemin permitted the present author to identify phane and the unknown mineral describedbelow. In studtite amongthe secondaryminerals of the uranium another it occurs as younger crusts on uranophane. deposit at Menzenschwandin the Southern Black Forest (Walenta, 1960; 1963). The first specimen Chemical Composition of studtite found at this locality dates from 1959. A preliminary chemical investigation, however, Studtiteis slowly acted on by cold 1:1 HCI and showedthat the mineralis not a carbonate(Walenta, cold 1:1 HNOB, whereasit is more rapidly soluble 1967). Vaes evidentlyinvestigated an impure sam- in cold 1 :1 HzSOe.No effervescencewas observed ple. This may alsobe indicatedby the small amount during dissolution. Minute crystals of the lower of lead in his materialwhich he thoughtmight be due hydrate (p. 169) precipitatefrom the solution in to inclusions.Such impurities could alsohave caused HCI after partial evaporationof the solvent at room the evolution of carbon dioxide when the mineral temperature. was treatedwith acid. Electron microprobe and microchemicalanalyses r66 ON STUDTITE AND ITS COMPOSITION 167
zone of uranium depositsas a member of a normal secondaryparagenesis is astonishing.It would be the first natural occurrenceof a peroxide compound. Becausethe existenceo,f such a compoundin nature seemed unlikely, it was thought possible that its interpretation as peroxide might be wrong and an- other formula, UO2(OH)2.4 }J2O = UOs.5 HzO, interpreting it as hydroxide, was taken into con- sideration. However, all chemical tests carried out speakin favor of the peroxidenature of the synthetic compoundand the mineral. If for instanceMnO2 is addedto a solutionof the compoundin dilute HsSOa, gas is given off with effervescence.With KI the solu- tion yields a precipitate of iodine, and a solution of FIc. 1. Studtite forming aggregatesof needlelike crystals KMnOa is decolorizedby it. on limonite, Menzenschwand,Southern Black Forest. Mag- If studtite is indeed a peroxide, the question re- nification about 60 X. tnains of how it formed under natural conditions. Although this problem is not yet solved,possibly the small amountsof hydrogenperoxide known to occur revealedonly uranium as a major constituent.Tests in rain water and snow are responsible.Another for other cations and anionswere negative. possibility is an organic source of the hydrogen A spectrographicanalysis performed by H. Wilk, peroxide. ChemischesUntersuchungsamt der Stadt Stuttgart, yielded similar results, though a number of other Physical and Optical Properties elementswere detectedin minor amounts(Mg, Cu, The needle-likeyellow crystals exhibit no distinct Ni, Zn, Al, Si) and in traces(Ca, Co, Fe, Mn, Bi, cleavage,are soft, and translucent to transparent. Sb, Sn). In part their presencemay be ascribedto Studite does not fluorescein ultraviolet light. The impurities. calculateddensity is 3.64 g/cc. The attempt to de- The qualitative chemical analysesreveal that termine the density of the natural mineral by means studtite is not a carbonateand that no cations other of heavy liquids was not successfulbecause the than uranium play an essentialrole in its structure. crystals were too small. The density of the artificial Its true nature was recognized when the powder compoundas measuredby tfe pycnometermethod pattern of pure studtite proved identical with that is 3.58 g/cc, in reasonableagreement with the cal- of a synthetic hydrous uranium oxide, the formula culatedvalue. of whichis thoughtto be UO+.4 HzO (Sato,1961; The optical propertiesare summarizedin Table 1. Debets, 1963). Likewise, the powder diagrams of Becausethe small crystals were not suitable for ob- UO4-2 HzO as obtainedby heatingstudtite to 60'C taining polarization figures,the optical characterand are identical to those of the similarly dehydrated the optical axial angle 2V c,ouldnot be determined synthetic compound. directly. Yaes (1947) states2V to be large; how- Chemicallythe compoundUOa.4 H2O generally is regarded as uranium peroxide hydrate (Katz and Tenre 1. Optical Properties of Studtite Rabinowitch,I95I; Sato, 1961). Thus its formula 'H2O2.3 is alsowritten UOB H2O.It can be obtained Studtite as a light yellow precipitate by addition of hydrogen Shinkolobwe Menzenschwand peroxide to a concentratedsolution of uranyl nitrate (Vaes, 1947) (Present study) or uranyl acetate.Peroxo-uranates are formed by a 1.545 a L.537 t 0.003 treating the compoundsimultaneously with hydrogen B r..sss peroxide and alkalies. Heating of the uranium per- y l-. 68 y 1.680! 0.003 oxide leadsfirst to a lower hydrate and then to UOg Nearly colorless with evolution of oxygen and water. 2V Taxge The presenceof this compound in the oxidized Z paraLleL to elongatlon 168 KURT WALENTA ever, this is not consistentwith the indicesof refrac- Tesrs 2. X-ray Powder Data for Natural and Synthetic tion he determined.The indicesof refraction c and p Studtite do not differ greatly, as confirmed by the present Studtlte Synthetic study, so the mineral must be optically positive and Menzensctred sbLDkolobre Uo4 ' 4 H2o (Piesent study) (qri11*in' 195E) (Debets, 1953)* have a small2V. a
Tlsrs 3. Unit-Cell Dimensions of Natural TesLB 4. X-ray Powder Data for the Dihydrate and Synthetic Studtite
Dihydrate* synthetlc1JOL'2H,2o Studtite, Synthetic (Present study) (Debets,1963)** Menzenschwand uo4 4H2o d (i) r*** d (i) r hkl (Present study) (Debets,1963) 5,24 10 5.22 100 10!. 4.47 7 4.39 33 002 3.80 7 1 70 33 011 a 11.85 t O.O2E 11.85A 3,54 8 J.)J 34 110 b 6.80 t 0.01 b 6.785 3.26 6 3.25 15 200 t 2,75 5 2.75 c 4.25 0.01 4.245 2,67 5 2,665 2L r03 | 2.61 3 2.6LI 9 202 B 93'51'r 20', R 93037 2.41 s 2.470 L9 zLL 2.403 L5 013 = 2.40 4 a:b:e L.743zL:0.625 2.),9 2 2.r94 004 2,LO 5 2.r04 12 020,301 V34I.7 t1L3 v34L!L&3 1,955 4 1. 953 L2L 1,931 213 5 Z = 2Uo4 4HZO L,927 1.926 8 310 1.897 022 Space group C2, Cm or C2/m '!,.452 3 1 ,863 13 LL4 r,819 I 1.817 4 204 1,768 4 L.766 220 1.761 8 3L2 lines could be indexed in this way, but some re- r,745 2 r.742 5 303 1.695 0.5 dlf, r.693 2 105 mained unindexable.Thus the chosencell was only I.654 2 v. dif, 1.551 pseudocell 1.639 222 a and not the true one. L628 2 v. dif. r.625 400 The c-axis of the pseudohexagonalcell is identical 1.618 015 7.527 I 402 with the c-axisof the monocliniccell. Debets (1963) 024 also statesthat the I.49s 3 |.494 4tL strongerreflections of his pattern 1,489 32L indicated hexagonalsymmetry at first sight. Closer r.463 006 r.450 3 1 .450 zr) inspection,however, revealed that they are doublets r.448 3L4 or triplets. The true symmetry of the unit cell is C- r,376 1 dlf. r.376 1,372 130 face-centeredmonoclinic. I .351 I.349 3 dif. L.346 4r3,323
1.310 I 1.310 !32 Thermal Behavior r.305 404 When studtiteis heatedto 60'C, it is L292 0.5 v. dif. |.286 5OL,420 converted 1.277 0.5 v. dif. 1.275 237 into a new phase which yields the same powder t.266 033 L,24O I v. dif. pattern as the synthetic compound UOr.2 FI2O 1.199 I v. dif. (Table 4). The powderpattern given in Table 4 was 1.180 0.5 L,166 0.5 obtainedafter heatingstudtite for 2 hoursto 100"C. 1,r49 0.5 I.t29 1 However, as mentioned above, dehydration takes 1.112 0.5 placeat temperaturesas low as 60oC.The dehydra- 1.098 0.5 1.085 I v. dif. tion phasemay be calledmetastudtite in accordance L053 0.5 v. dlf. 1.040 1 v. dlf. with the nomenclatureusually applied to dehydrated 1.000 I v. d1f.
uranium minerals. The dehydration processis irre- *0btained by heatLng naturaL studtite, *4AS?M versible, and natural occurrenco the file No. 76-207. of dihydrate *4*dif. = diffuee, u. &tf. = uerg diffuse. seemspossible. Sato (1961) mentionsthat at temperaturesbelow 50oC the compound UOr.4 HzO is formed but 5 ) differ only slightly from the values given by above70oC, UO4.2 HzO. This is more or lesscon- Debets for the synthetic compound. The cell is body- sistent with the results of the investigation of the centered as only general reflections with h + k + I natural mineral, though in one case the lower hy- even occur. drate was obtained from a solution in HCI at room The dihydrate has higher indices of refraction than temperature,as mentionedearlier. At higher tem- studtite.The value of a risesto 1.758 * 0.003. The peratureso-UOs is formed (Cordfunke,1961.). birefringence of the crystals remains about the same The dihydrateis orthorhombic.The unit-cell di- as before heating. The calculated density is 4.66 mensionscalculated from the powderpattern (Table g/cc. KURT WALENTA
Tenr,n 5. Unit-Cell Dimensions of the Natural birefringenceon the (100) plane should be rather and Synthetic Dihydrate small [as observedby Christ and Clark (1960) for (001) groupl. Dihydrate* rJo4 2 ll2o sectionsparallel to in the schoepite (Present study) (Debets,1953) This is not the case. On the contrary, the birefrin- genceof studtite is remarkably high in this orienta- a 6.5L 1 O.O1^E o o.sozE b+ 8.78 t 0.02 b 4.2L6 tion, and the crystals moreover are elongatedalong ct 4.21 I 0.01 e 8.778 the axis with the 4.25 A period, whereasthe mem- a:b:c = O.74I:120.479 bers of the schoepitegroup usually are tabular on v 240.6 t t Et v zto i' (001) and show pseudohexagonalforms. Z=2UO4 2H2o These properties of studtite, on the other hand, Space group probably fntwn could be explainedby assumingsome kind of chain- like structure along the c-axis. Indeed, substantial *1btained by heating nattpal studtite. lln order to cornply vLth the IM argumentsfavor such an assumption.Another uran- reeornnendation c1a1b, b and c as ium compound is known whose cell constantsre- chosen by Debets haz.tebeen eechanged. semblethose of studtite. For example,the hexagonal uranium trioxide ,*-UOs, which has a 3.97'1.and c Discussion 4.168 A (Zachariasen,1948), agreeswith c for The comparison of the unit-cell dimensions of studtite. The a-UOgstructure contains O-U-O-U-O studtite and the dihydrate shows that the c-period chains along the c-axis,with a U-O distanceof 2.08 of the monoclinic cell is not much affectedby de- A. Studtitemay possesssimilar chainsalong c. The hydration. Nor is cell-edgeb (a of the orthorhombic additionaloxygen and water contentof studtite,how- cell). However,a (becomingD) is reducedas much ever, make the cell larger and the structure more as 3 A by dehydration.This behaviorcan be inter- complicatedin comparisonwith that of o-UOs and preted by assumingthat the four additional water produce monoclinic, or in caseof the dihydrate, or- moleculesper unit cell which make the difierence thorhombic symmetry. betweenstudtite and the dihydrate are arrangedin The thermal behavior of studtite lends credence layersparallel to (100) in the studtitestructure. to the correctnessof these assumptions.Dehydra- Similaritiesexist betweenthe unit-cell dimensions tion of studtite leads to *UOg. Schoepitedoes not of studtite and those of members of the schoepite form as an intermediatedehydration phase, as might group. The schoepitegroup comprisesuranyl oxide be assumedif very close structural relations existed hydrates of different compositions with structures between studtite and schoepite. characterizedby UOr(OH)p layers of pseudohex- agonal symmetry. Schoepiteitself has the formula Unknown Siliciferous Mineral UO2(OH)r.HzO. The UOr(OH)2 layersin mem- As already mentioned,studtite is intergrown with bers of the schoepitegroup are stackedparallel to an unknown speciesand with uranophaneon one of (001) with the distancebetween consecutive layers the specimens from Menzenschwand.2The three varyingfrom 7.1 to 7.6 A (Christ and Clark, 1960). intergrown minerals form yellow fibrous aggregates This distance corresponds to the c-period of a within a small vug and are macroscopicallyindis- pseudo-orthohexagonalcell which can be derived tinguishable.At first it was assumedthat the un- from the true orthorhombic unit cell of these min- known speciesmight be a decompositionproduct erals. The constants of the pseudo-cell within the of studtite, but this could not be confirmed. (001) planerange from 6.93 to 7.16 A for a and Small fibrous crystalsof the unknown mineral are from 4.03 to 4.21A for b. Theseresemble those for colorless under the microscope and show parallel the (100) planeof studtite(b 6.80;c 4.25A). extinction with positive elongation.Indices of refrac- -+ -+ Despite these similarities it is unlikely that close tion: a 1.570 0.002, ^/ 1.584 0.002. The relations exist betweenthe structure of studtite and crystals are too small to yield good polarization that of the schoepite group. If one assumesthat figures. Possibly they are biaxial and have ortho- UO2(OH), layers alternate with layers of water rhombic symmetry. The crystals are somewhatflat- moleculesparallel to (100) in studtite, as was * The specimen was found by E. Killius in 1961 during one thought possibleif the mineral had the composition of the excursions to Menzenschwand undertaken jointly UO:(OH)2.4 H2O insteadof UO+.4 H,zO,then the with the author. ON STUDTITE AND ITS COMPOSITION t7l tened, the birefringencebeing lower on the plane of tion are markedly decreased.Further investigations flattening and higher if the crystals are turned on are plannedto determinethe natureof this evidently edge.'B doesnot differ markedly from 7 which leads new mineral species. to the conclusionthat the opticalsign is negative.X is References normal to the plane of flattening;Z is panllel to the elongatiron.Anomalous bluish grey interferencecolors Cnmst, C. L., eNo L R. Crenx (1960) Crystal cbemical can be observedunder crossednicols. studies of some uranyl oxide hydrates.Am. Minetal. 45, I 026-l 061. The mineral, in contrastto the associated stud- ConoruNrE, E. H. P. (1961) Its preparation and "-UOs: tite and uranophane,is insolublein cold or hot 1 :1 thermal stability. l. Inorg. Nucl. Chem. 23,285-286. HNOg and I :1 HCl. It is thus easilyseparated from (1966) Thermodynamic properties of hexavalent them. uranium compounds.ln, Thermodynamics,Vol. II. Intern. A samplepurified by acid treatmentwas analyzed Atomic Energy Agency, Vienna, pp. 483-495. AND P. AuNc (1963) Thermal decomposition of by the electronmicroprobe. Only Si and U could be hydrated uranium peroxides. Rec. Trauaux Chim. Pays' detected.Whether U is an essentialconstituent of Bas, E2.257-263. the mineral,however, is questionable.It might well ^LNoA. A. VeN oEn GrEssEN(1963) Pseudo- be that the acid treatmentwas not sufficientto re- morphic decomposition of uranium peroxide into UO.. move all of the associateduraniferous minerals. The l. Inorg. N ucl. Chem. 25, 553-555. AND - (1965) Texture and reactivity of properties of the mineral suggestthat it is not a uranium oxides. in, Reactiuity of Solids, Ed. G.-M. uranyl silicate.Uraniferous silicates are dissolvedor Schwab. 5th International Symposium, Munich, 1964. decomposedby acids, contrary to the behavior of Elsevier Publishing Co., Amsterdam, pp. 456-466. this mitieral. DEBETS,P. C. (1963) X-ray diffraction data on hydrated The powder data are listed on Table 6. The pat- uranium peroxide.l. Inorg. Nucl. Chem.25,727-730. DEsrAs,A., J. F. Vlrs, eNo C. GurnrvIN (1958) Min- tern is not closely related to that of any known 6raux d'uranium du Haut Katanga. Les Amis du Mus6e mineral. A sampleheated for two hours at 300"C royal du Congo belge, Tervuren (Belgique). furnishedthe same'powderpattern as before. The Fnoxorr, C. (1958) Systematicmineralogy of uranium and opticalproperties remained unchanged as well. Heat- thorium. U.S. Geol. Suro. Bull. 1064, Washington. ing to 900'C also doesnot lead to a major change Gurrrrurx, C. (1958) [see Destas, Vaes and Guillemin]. Kxrz, !. J., eNo E. ResINowrrcu (1951) The Chemi.xry in the powderpattern; however, the indicesof refrac- ol Uranium. Dover Publ. Inc., New York. Srro, T. (1961) Uranium peroxide hydrates. Natutwissen- schaften,4E,668. T.ur,n 6. X-ray Powder Data for the Unknown Mineral (1961) Thermal decompositionof uranium peroxide d(A) r atil r d(i) r hydrates.N aturwissenschaften,4E, 693. VrEs, J. F. (1947) Six nouveaux min6raux d'urane prove- 2 11.)d 9 1.984 3 1.285 (Katanga). La billietite (uranate 7.30 LO r.951 I 1,257 1 nant de Shinkolobwe 6.19 9 t.924 4 1.245 I dlffuse hydrat€ de Ba), la vandendriesscheite(uranate hydrat6 5.79 1 1 .895 1 L23O 2 dlffuse 4 t.862 4 r.220 0.5 de Pb), la masuyite(uranate hydrat6 de Pb), la richetite 4 .99 4 1.837 3 I.207 0.5 dlffuse (uranate hydrat6 (?) de Pb), la studtite (carbonate 4,56 3 1 .817 1 1 .194 0.5 hydrat6 d'urane) et la diderichite (carbonate hydrat6 4.26 r.175 0.5 1.r84 0.5 3.88 L,752 0.5 r.r77 1 d'urane). Soc. giol. BelgiqueAnn. 70, 8212-8225. 3.67 | ,72r 3 r.169 2 WlrrNr,t, K. (1960) Ein Vorkommen von Uranocircit im 3.54 r,706 2 1 .161 r 3.50 r.695 2 t.r5l 2 siidlichen Schwarzwald.Neues lahrb. Mineral. Monatsh. 3.42 1.656 3 dif fuse 1 .141 I (1960), 116-119. 3.20 |,629 o-5 1.132 0.5 3.08 1.615 LI22 2 very diffuse (1963) Uber die Barium-Uranylphosphatmineralien 2,99 1 597 1 .109 2 diffuse Uranocircit .I, Uranocircit II, Meta-Uranocircit I und 2.89 t,569 dlffuse 1.093 0.5 dlffuse Meta-Uranocircit II. von Menzenschwandim siidlichen 2.71 1.539 1,083 2 2.67 1.513 1 .078 1 Schwarzwald. Jh. geol. Landesantt Baden-lViirttemberg, 2.55 1.498 0.5 1 .071 r 6, ll3-135. 2.49 r.463 3 diffuse ]. 064 2 very diffuse (1967) Mineralien des Granitsteinbruchsvon 2.44 r,437 0,5 1.050 0.5 very diffuse Die 2,41 2 L.426 2 1.043 0.5 diffuse Menzenschwand im siidlichen Schwarzwald. Der Auf- 2,39 2 1.418 2 1 .039 0.5 2,32 3 L.396 I 1 .031 1 very diffuse scltlup,18, 151-162. 2.29 2 1.386 2 L018 0,5 diffuse ZecnenIrsr.w,W. H. (1948) Crystal chemical studiesof the 2.26 1 1.372 1 r .008 2 5l-seriesof elements.I. New structuretypes. Acta Crystal- 2.2r 3 1.359 1.004 0.5 vety diffuse 2.16 0.5 1.348 I 0 .998 3 Iogr. 1,265-268. 2,L1- 3 1.330 2 diffuse 0.992 0.5 Manuscript receiued, April 2, 1973; accepted 2.O1 3 1.318 2 dlf fuse 0.990 5 diffuse 2.02 5 1.305 I 0.981 1 diffuse lor publication, Iuly 13,1973.