American Mineralogist, Volume 79, pages80-90, 1994

In-situ study of the thermal behavior of cryptomelaneby high-voltageand analytical electron microscopy Pl,ur,o Mlncos YlscoNcnlos, HaNs-Ruoor,rWnNr Department of Geology and Geophysics,University of California, Berkeley,California 94720, U.S.A. Cnucx Ecrrnn National Center for Electron Microscopy, University of California, LawrenceBerkeley I-aboratory, Berkeley,California 947 20, U.S.A.

AssrRAcr Scanningelectron microscope(SEM) and transmissionelectron microscope(TEM) stud- ies show that cryptomelaneand Cu-rich cryptomelanecrystals from weatheringprofiles in Brazilrangefroml0to200nmindiameterandhaveaspectratiosof l:l0to l:l00between the short and long dimensions. Acicular crystals identified in hand specimen and in the scanningelectron microscope are often composedof bundles of fibers elongatedalong the c axis (tetragonal) or the b axis (monoclinic). In-situ heating with a high-voltage trans- mission electron microscope (HVEM) and an analytical electron microscope (AEM) in- dicatesthat upon heating in vacuum cryptomelanecrystals begin to transform into a mixed and manganositephase at 648 'C. transformations are followed by the loss of K from the cryptomelane tunnel sites. Cu-rich cryptomelane transforms into hausmannitebetween 330 and 515 'C, and manganositebegins to form at 620"C. Cu-rich cryptomelane also loses K and exsolves native copper upon heating, suggestingthat Cu occupies the same site (the A site) as K. Similar mineral transformations are observed when the same samples are heated in air, although the transformations occur at higher temperaturesthan those observedin vacuum. The variation in the thermal stability of hollandite-group suggeststhat the size ofA-site occupantsmay control the mobility of cations along the tunnel direction. Relative grain sizes and the degreeof crystallinity of supergenecryptomelane may also influence the thermal stability of hollandite-group minerals. The applicability of cryptomelane and other hollandite-group minerals to K-Ar and 40Ar/3eArdating is discussedin light of the electronmicroscope investigation results.Ar releasefrom the cryptomelanesamples during lo{r/3eAr vacuum laser heating is also discussedin terms of the phase transformations observedby electronmicroscopy.

INrnonucttoN radiogenic aoAroccupying thesesites are easily exchange- able. Finally, in order to interpret the results of 40Ar/3e Ar Recent 4o{r/3e{r laser-heatingdating has shown that experiments better we must understand the thermal be- K-bearing manganeseoxides (cryptomelane and hollan- havior of hollandite-group minerals. In particular, the dite) are potentially suitable minerals for dating weath- temperature and the mineral transformations associated ering processesby the K-Ar and 4o{r/3eAr methods(Vas- with the Ar releasesteps need to be known to determine concelos et al., 1992). The ubiquitous presenceof these if cryptomelaneretains Ar at normal Earth's surfacetem- datable oxides in weatheringprofiles and soils throughout perature and to determine that thermal degassingof man- the world may provide information on rates of weather- ganeseoxides does not occur during neutron irradiation. ing processes,soil formation, paleoclimates,and land- Cryptomelane and hollandite belong to the hollandite scape evolution. To establish the applicability of cryp- group and have the generalformula (A),-rBrO,..xHrO. tomelane to K-Ar ar.d 40Ar/3eArdating it is necessaryto The A site can be occupied by large cations such as K+, define its composition, structure, and range of stability. Ba2*,Na*, Cu*, Pb2*, Rb2*, Sr2t, Cs2+,etc., whereas It is imperative to determine the size range of crypto- the 16lBsite includesn4ttr*, Mno*, ps:+, dl3+, Sio*, and melanecrystals in order to assessthe possibility of 3eAr Mg'?+(Burns and Burns, 1979).Cryptomelane is the K recoil from the mineral structuresduring the neutron ir- and hollandite is the Ba end-memberof a solid-solution radiation procedure required for 4oArl3eAranalysis. In series(Bystrdm and Bystr6m, 1950).Bystr

elements from the host schists suggestfree mobility of these elements in the weathering solutions. Probable sourcesof theseelements in the weatheringsolutions are metavolcanic horizons interbedded with the metasedi- ments. The difference in compositions between the Cu- rich samples and the K-rich samples and their distinct degassingbehavior during laser-heating+oAr/3eAr analy- sis (Vasconceloset al., 1993)made thesesamples ideally suited for this study.

ExpnnrrvrnxTAl- METHODS The four sampleswere analyzedusing an eight-channel ARL-SEMQ electron microprobe equipped with wave- length-dispersivespectrometers at the Department of Ge- ology and Geophysics at the University of California, Berkeley. The sample current was 30 nA on MgO, the acceleratingvoltage l5 kV, and the beam diameter 3-5 pm. PEG-01 was also arralyzedwith a l5-prmbeam di- ameter, but the results are indistinguishable from those obtainedwith a focusedbeam. Powder X-ray diffraction analyseswere performed with a Rigaku Geigerflex X-ray diffractometer with CuKa ra- diation over the range3 < 20 < 90'with scanspeeds of 1.0.5. and 0.25/rnin. Measurementsof eachsample were repeatedthree to four times, and the averagepeak posi- tions were calibrated againsta Si standardunder the same conditions.Unit-cell parameterswere calculatedusing a modified version of the least-squaresrefinement program LCLS (Burnham, 1962) (Table 2) and confirm all four samples as cryptomelane. Calculations for a body-cen- tered tetragonalunit cell (spacegroup 14/m) and a mono- clinic unit cell (space grotry 12/m) were carried out for comparlson. After petrographic observations and electron micro- probe and scanningelectron microscopeanalysis, suitable thin foils of samplesPEG-01 and F226-l2l were pre- pared by ion-beam thinning for study in the heating stage of the Kratos 1.5-MeVHVEM at the National Centerfor Electron Microscopy (NCEM), Lawrence Berkeley Lab- oratory. The samples were characterizedbefore heating by a seriesof selected-areadiffraction (SAD) patterns and bright-field photomicrographs.The sampleswere heated 'C al a rate of l0 "C/min to a maximum Z of 848 and SAD patterns and bright-field photomicrographswere re- corded throughout this process.The temperature within the heating stage was monitored by a Chromel-Alumel Fig. 1. (a) BackscatteredSEM photomicrographof borryoi- thermocouple, and calibration proceduresindicated that 'C dal cryptomelane(PEG-01); (b) SEMphotomicrograph of acic- the temperature readings were within 5 from actual createdby dis- ular cryptomelanecrystals surrounding cavities sample temperature. solutionof silicategrains; (c) pseudomorphic(p) texturegrading Two foils of sampleF226-l2l and one foil of PEG-01 into massive(m) texture as cryptomelane fills cavities previously 200-kV analyticalelectron occupiedby silicates. were examinedin the JEOL microscopeat the NCEM. The AEM has a Kevex EDX detector and corresponding processingfacilities, which work of crosscutting needles that preserve the original permitted the variations in composition of the samples texture ofthe host rock (Fig. l). Further precipitationof before and after heating to be investigated. A beryllium manganeseoxides in the void spacescreates massive sample holder and grid was employed to obviate spurious manganeseoxide blocks(Fig. 1). K, Ba, and Cu contents X-rays from the holder that might interfere with the sam- in supergenemanganese oxides and the absenceofthese ple analysis.The low energyofthe 200-kV electronsand VASCONCELOS ET AL.: HVEM AND AEM STUDIES OF CRYPTOMELANE 83

Trau2. Unit-cellparameters and structural informationof cryp- PEG-01ELECTRON MICROPROBE ANALYSES tomelane

Spacegroup: l4lm l4lm l4lm Sample: PEG.O1 F40-142 F226-121 a (A) 9.846(16) 9.814(24) 9.834(31) 10 b (A) s.84606) 9.814(24) 9.8i]4(31) 0) l! c(A) 2.860(5) 2.872(9) 2.857(9) + o (") 90 90 90 o"" pf) 90 90 90 + a (') 90 90 90 a .a ? ou aa v(A") 277.24(90) 276.65(136) 276.28(168) s a' aa a o No. of reflections 18 12 10 = aa Specific (calc.) 4.30 4.34 4.38 o4 lft o Aravity aoo /Vote;standard deviationsare in parentheses.

56 57 the highly condensedbeam causedsignificant damageto Wt% Mn sample F226-121, which underwent a phase transfor- mation in the beam. This feature allows mineral trans- formations induced in the sample by electron beam and heating damage to be investigated and compared with 1':t phasetransformations observed during external heating. -aa SamplePEG-01 was much more stable in the electron - aa. beam, and, in spite of extendedinvestigation, no damage 6 ao was observed. ri-.'.'r3: s 06 "'i;: Singlegrains of samplePEG-01 and a Cu-rich crypto- = melanesample (Fl l5- 139.8)from the lgarupeBahia pro- 04 file were further analyzedby aoArl3eAranalysis (Vascon- celos et al., 1993). Sample preparationand irradiation procedures, experimental conditions, and analytical re- sultswere presentedin Vasconceloset al. (1993). Rnsur,rs 44 46 o'*no*uo sz 54 Table I showsmineral compositions and site occupan- microprobeanal- cies calculated from the averageresults of electron mi- Fig.2. Elementalcorrelation from electron ysesof samplePEG-O1. croprobe analysesof each sample (592 points analyzed for samplePEG-0 I , I 98 pointsfor sampleF226-l2l , 100 points analyzedfor F40-142, and 49 points for Fl 15- 139.8).The low totals reportedcannot be accountedfor and 36 cryptomelane samples analyzed by Miura et al. by unanalyzedelements. These sampleswere checkedby (1987)also showed vacancies in the B site,with z ranging EDX and WDS scans,and no other elementswere pres- from 0.300 to 0.153. These vacanciescould allow for ent in significant amounts. Previous analysesof crypto- chargecompensation of the A-site cationsand would pre- melane by other researchershave indicated that signifi- clude the necessityfor the presenceofa lower oxidation cant amountsof HrO (1-3.8 wto/o)can be presentin these state of Mn in the B site. supergeneoxides (Bystrdm and Bystriim, 1950;Mathie- The Mn contents of the samplesanalyzed should also son and Wadsley, 1950; Perseiland Pinet, 1976; Miura decreasewith increasing Si, Al, and Fe competition for et al., 1987; Ostwald, 1988).We concludethat H,O is the octahedralsite. Figures 2-4 illustrate the inverse cor- present in the cryptomelane samples analyzed. relation between Mn and Fe + Si * Al contents in the The cell occupancycalculations shown in Table I sug- cryptomelane samples analyzed, suggestingcompetition gestthat the B site is only partially occupied.In addition, betweenMn and theseelements for the B site. Similarly, charge-balanceconsiderations (based on l6 O atoms) and competition for the tunnel sites may account for the in- the measuredO contents suggestthat all Mn cations pres- verse relationship between K and Cu (Fig. 3) and K + ent in the B site are in the Mna+ oxidation state.Previous Ba * Na * Ca and Cu (Fig. 4). Toledo-Groke et al. authors (Bystriim and Bystrdm, 1950; Burns and Burns, (1990), studyingCu-rich cryptomelanefrom the Salobo 1979;Miura et al., 1987)have suggestedthat the charge profile in Carajis, also noticed evidence for competition of I cations may be compensatedby the presenceof va- between Cu and K for the A sites. Although it is yet not canciesin the B site. In the generalformula proposed by certain whether Cu occupiesthe tunnel sitesor substitutes Bystrom and Bystrdm (1950),A2_"B8-.Xr6, z varies from for Mn in the B sites, the elemental correlations and the 0. I to 0.5 for the minerals analyzed.Twelve hollandite behavior of Cu during heating of the Cu-rich cryptome- 84 VASCONCELOS ET AL.: HVEM AND AEM STUDIES OF CRYPTOMELANE

F226.121ELECTRON MICROPROBE ANALYSES F4O.142 ELECTRON MICROPROBEANALYSES

12 4A t. l. aa 10 €) o tt l! J 30 + .! .t ah o S 20 , 06 Ott a s = =

t.ta 10 1.tf. aa '.ta. .-.'-. t"l$.r6!r

00 540 545 555 560 570 575 Wt% Mn Wt% Mn

oG i6 ol + 6 Y '.:'* .D r . + -" (E I z + ,.1.1..i.... 5 Y 40 a roaa aa s a =35 a aa aaaa a a a 30 . J'.lr ]" +"3 "" 15 25 t' 34 36 38 to*,*artu 18 20 " *r3ri"u Fig.3. Elementalcorrelation from electronmicroprobe anal- Fig.4. Elementalcorrelation from electronmicroprobe anal- ysesof sampleF226-121. ysesof sampleF40-142. lane samplesdescribed below are suggestiveof Cu occu- Bright-field imagesand SAD patterns for sample PEG- pation of the tunnel sites.Structural considerationsin- 0l are shown in Figure 5. PEG-01 crystalsare prismatic dicatethat Cu, if presentin the A site,must occuras Cu+ and elongatedalong [001] (Fie. 5). Prism diametersvary (r,: 0.96 A1 and not asthe smallerCu2* ion (r, : 0.72 L). from 50 to 1000nm, and their lengthsrange from 500 to The cell-refinementcalculations (Table 2) indicatethat 10000 nm; these prisms display squareand diamond- the cryptomelane samplesanalyzed are either tetragonal shapedcross sections (Fig. 5, arrows).Because ofthe fine- or monoclinic, with B nearly 90'. The powder pattern grained nature of the sample, only ring patterns were resultsrefined assumingtetragonal symmetry Q4/m) (Ta- obtainable by SAD. In order to obtain single-crystalin- ble 2) yield lower residualsthan when fitted assuminga formation, convergent beam electron diffraction (CBED) monoclinic symmetry Q2/m). On the other hand, the patterns were necessary(Fig. 5). The CBED patterns for presenceofdoublets and triplets in the powder pattern of PEG-01 crystalsindicate that the mineral is possiblyte- some samplesis suggestiveof a lower symmetry than 14/ tragonal and elongatedalong the c axis. The CBED pat- m. The resolution of the powder diffractometer is insuf- tern in Figure 5 shows the a*c* plane of tetragonal cryp- ficient to positivelyidentify the spacegroup. Another cri- tomelaneor the a*b* plane of monoclinic cryptomelane terion for distinguishing tetragonal from monoclinic (Fig. 5). It was possibleto obtain lattice fringes(Fig. 5). cryptomelane is the ratio of the averageionic radius of The measuredd value of 7.1 A correspondsto the (l l0) B-site to A-site occupants(Post et al., 1982;Vicat et al., planesof cryptomelane(Fie. 5). In addition to the 7.1-A 1986).When this ratio is >0.48, the structureshould be lattice fringes of cryptomelane, some l0-A httice fringes monoclinic.The ratio of averageionic radiusof B cations were also present.Similar patternswere observedby Tur- and I cations in the samples studied (Table 1) suggests ner and Buseck (1979), who interpreted the l0-A dis- that sample PEG-01 should be tetragonaland the Cu- tances as romanechite intergrown with the 7-A holan- rich cryptomelane should be monoclinic. dite. Chen et al. (1986) also observed similar lattice VASCONCELOS ET AL.: HVEM AND AEM STUDIES OF CRYPTOMELANE 85

tq, tYq

ut --roA "" T 45'c

ooooo ooooo Fig.5. (a)BF imageof crosscuttingnetwork of cryptomelane crystalsin PEG-01.Elongated prismatic and rod-shaped crystals are typical habitsfor this cryptomelanesample. Arrows show the crosssectional shape ofa crystaloriented perpendicularly to the planeof the photograph;(b) SAD ring patternof cryptome- lane(see Table 4 for indexedd valuesfor ring pattern);(c) mi- crobeamdiffraction pattem of tetragonalcryptomelane crystals alongthe b axis; (d) indexingof SAD patternshown in c; (e) TEM imageof properlyoriented cryptomelane crystals showing predominantlywell-ordered 7.1-A fringes [(110) planes of cryp- tomelaneland a few l0-A fringes[possibly (100) planes ofpsi- lomelaneor (001)planes of todorokitel. irregularities in synthetic cryptomelane crystals formed epitaxiallyafler birnessite. Bright-field images and SAD ring patterns of sample F226-l2l before and after phase transformationsare shown in Figure 6. Diffuse SAD ring patterns for sample F226-l2l (Fig. 6) indicate that singlecrystals of crypto- melane are much smaller than the diameter of the elec- tron beam (- I pm). These cryptomelane crystals are acicularand have diametersranging from l0 to 120 nm; Fig. 6. (a) BF image of crosscuttingnetwork of cryptomelane their lengthsmay reach 1000nm. needlesin sampleF226-l2l before heat-inducedphase transfor- mation; (b) SAD ring patterns of cryptomelane crystals; (c) BF Mineralogical transformations during heating and image of granoblastic texture formed by intergrown hausman- electron beam damage nite, manganosite,and dispersed native copper in the sample Indexing of SAD patterns taken for the two samples after heat-induced phase transformation; (d) SAD ring pattern phases during the heating experiment (Fig. 7 and Table 3 for of newly formed after heat treatment. sampleF226-l2l and Figure 8 and Table 3 for sample PEG-01) indicated that the two samplesexhibit slightly l2l) (Fig. 7 and Table 3) underwent phase transforma- different behaviors during the HVEM heating experi- tions at lower temperatures, as indicated by the almost ments. The fibrous and prismatic cryptomelane crystals complete disappearance of the cryptomelane lines in the in the K-rich sample(PEG-01) were surprisingly resistant SAD pattern for the sample aI T : 620 "C. to phase transformations, persisting in vacuum up to Z Both samples underwent sudden phase transforma- : 648"C (Fig. 8 and Table 3). The Cu-rich sample(F226- tions during in-situ heating investigations, but not all 86 VASCONCELOS ET AL.: HVEM AND AEM STUDIES OF CRYPTOMELANE

T = 140.C T = 330'C T = 515'C

620'C 715'C

Fig. 7. (a-h) SAD ring patterns depicting mineral transformations in sample F226-l2l during in-situ HVEM heating. Mineral paragenesesare summarized in Table 3. Temperature and some diagnostic reflections are indicated. C : cryptomelane; H : hausmannite;M : manganosite.Several diffraction patterns were taken at the same temperature (d-f and g-h). Note that at 620 "C hausmannite disappearsduring prolonged heating and manganositebecomes the dominating phase. crystalsunderwent transformation at the sametime. The tween cryptomelane crystalspossibly accelerateor retard difference in behavior between the two samplesand the the transition from low- to high-temperaturephases. In different behavior of cryptomelanecrystals in a single addition, other factors, such as grain size, crystal mor- samplesuggest that small differencesin compositionbe- phology and orientation, surface properties, and kinetic

Fig. 8. (a-h) SAD ring patterns illustrating mineral transformations in sample PEG-01 during in-situ HVEM heating. Mineral paragenesesare shown in Table 3. Temperatureand some diagnosticreflections are indicated. C : cryptomelane;H : hausmannite; M : manganosite.At 648 'C (c) hausmannite crystallizesand later transforms to manganosite(e). Notice coarseningat 848 'C as continuous rings (g) changeto a spot pattern (h). VASCONCELOS ET AL.: HVEM AND AEM STUDIES OF CRYPTOMELANE 87

Tlale 3. Mineralparagenesis during heating in HVEM \,

SampleF226-1 21 140.C 330"C s15.C 620.C 620"C 620.C 715.C 715"C .wi c(+) c(+) B(-) H(+) H(+) M(+) M(+) M(+) H(+) M(-) M H(-) H(-) H(-) ^ Sample PEG-01 60.c 490.c 648.C 648.C 648"C 781"C 848.C c(+) c(+) H(+) H(+) H M(+) H CM M H(-) MC Note; C : cryptomelane,B : bixbyite,H : hausmannite,M : man- ganosite;+ : abundant,- : minor. constraints,may also retard the phase transitions ob- servedand must be evaluated. During heatingwith a heatingstage, the Cu-rich cryp- tomelanesample transformed to hausmanniteand native copper. Hausmannite crystalsare coarsergrained than the startingcryptomelane needles (Fig. 6b). As the phase transformationsprogressed, the crosscuttingnetwork of acicularcryptomelane crystals (Fig. 6a)was progressively replacedby a granoblastictexture (Fig. 6b), in which coarsergrained idiomorphic crystalsof hausmanniteand manganositeare joined at l20 anglesand native copper grains,<0.005 pm, occur dispersedthroughout the sam- ple. Texturalevolution of the samplesduring heatingsug- geststhat crystalsoriented perpendicularly to the sample surface recrystallize more readily than the crystals ori- ented parallel to it (Fig. 6, arrow). This observationis consistentwith recrystallizationby breakdownof the tun- nel structure due to migration of I cations out of the tunnel sites. Electron beam damage produced phase transformations similar to external heating (Fig. 9b). Hausmannite and native copper suddenly crystallized during cryptomelanebreakdown (Fig. 9b). It is possible that the temperaturesreached in the electronbeam were high enough to promote the phase transformations observed. EDX semiquantitativeanalysis of sample F226-I2l before and after phase transformation by electron beam damageshowed the lossof K and Cu from the structures as it transformed from cryptomelane to hausmannite.The loss of theseelements was accompa- nied by a relative enrichmentin Al and Co, whereasMn remainedrelatively unaffected. This is interpretedas ev-

---J Fig. 9. (a) BF image of crosscuttingnetwork of cryptomelane needlesin sample F226-l2l before phase transformationin- duced by electron beam; (b) BF image of granoblastic texture formed by intergrown hausmannite,manganosite, and dispersed native copper in the sample (arrow) after phasetransformation induced by electronbeam; (c) BF image of sampleF226-l2l showing granoblastictexture and single-crystalmicrobeam dif- fraction patternsof the newly formed hausmannite(d) and man- ganosite(e) crystals. 88 VASCONCELOS ET AL.: HVEM AND AEM STUDIES OF CRYPTOMELANE

group 14,) (Frenzel,1980). Although under normal pres- tl a) o , sure conditions the tetragonal-cubic phase transforma- o o oqn tion in hausmanniteoccurs between 1050 and I160'C, rc0) it is possible that in vacuum this transformalion may o o occur below 848 "C. Another explanation for the modu- (,60 lated structure is an incipient chemical fluctuation. SAD K-Cryptomelane Sample PEG-01 ring patterns (Figs. 6, 8, 9) and microbeam diffraction o Lab # 3363-06 patterns (Fig. 9e) also show the presenceof manganosite s40 crystals after the heating of cryptomelane crystals in o a "/o4oAr*Released E vacuum. €20 % t'Ar Released tr I Laser-heatingq Ar /3eAr results o 0 The Ar releasehistory of the K-rich and the Cu-rich 100 cryptomelane samples are shown in Figure 10. The de- E b) o gassinghistory of the two samplesis remarkably similar; o o 40Art< 3eAr Oo^ most of the radiogenic and nucleogenic frac- o G tions are releasedat a narrow range of laser power. De- 6 spite similarities in Ar patterns for 6 the the degassing the ooo Cu-Cryptomelane two samples, the K-rich sample requires a higher laser Sample F115-139.8 energy output to releaseits Ar gas.This in Lab # 4315-01 difference de- o gassinghistory may be accounted for by different cou- s40 o/onoAr' O Released pling between each sample and the laser beam. Another .zo t'Ar possible explanation is that the K-rich sample undergoes 6 I o/o Released azv phase E transformation at a higher temperature than the I ooAr/3eAr C) Cu-rich sample. The laser-heating experiment also showsa closesimilarity betweenthe o0Ar*and 3eAr releasecurves (Fig. I 0), which suggeststhat theseisotopes 1.0 20 3.0 Laser Power(Watts) have the same activation energy, i.e., they occupy the samesite in the .This is indirect yet strong Fig. 10. (a) Releaseof Ar isotopesduring laser-heating o0Ar/ evidence that significant amounts of 3eAr are not dis- 3eAranalysis of samplePEG-01 ; (b) Releaseof Ar isotopesdur- placedby recoil out ofthe tunnel sites. ing laser-heating40Ar/]eAranalysis of sampleFl l5-139.8.The curvein a indicatesthat the K-rich cryptomelanesample releases DrscussroN its radiogenicaoAr* nucleogenicseAr and at a narrowrange of The investigation ofsupergenemanganese oxides at the laserpower. The Cu-richcryptomelane sample (b) alsoreleases appropriate scaleand with the proper methodology sug- its ooAr*and seArisotopes at a narrowrange of laserpower, but gests lesslaser energy is requiredto degasthe Cu-richsample. that the oxides we studied are, on the whole, well- crystallized single minerals with only minor occurrences of other intergrown phases.The high degreeof crystallin- idence for the presenceof Al and Co in the octahedral ity and the purity of these minerals, in spite of their ex- sites; the octahedral sites undergo structural rearrange- tremely fine-grained nature, suggestthat they should be ment but remain otherwise undisturbed during the phase suitable for radiometric dating, provided that aoArand K transformation. The loss of K and Cu is attributed to the do not easily diffuse into and out of the mineral structures breakdown ofthe tunnel structure.K and Cu are exsolved and that 3eArrecoil lossdoes not occur. from the tunnel site; Cu forms a distinct phase (native The diffusion of K from cryptomelane structures was copper) (Fig. 9b), and K is lost by volatilization. Native studied by electrodialysis(Sreenivas and Roy, 196l). copper has also been found as a residue together with These authors proposedthat the large tunnel structure,I manganosite in samples fused for K-Ar analysis and in cation disordering, and partial occupancyofthe channel 'C, samplesheated in a furnace to 900 further strength- sites led to low activation energiesfor the I cations, re- ening this hypothesis. sulting in a high cation-exchangecapacily for these min- Hausmannite forms euhedralcube-shaped crystals (Fig. erals. High cation exchangewould also imply loss of K 9b, 9c). Indexed SAD ring patterns (Figs. 7, 8, and 9) and and radiogenic aoAr*, leading to inconclusive dating re- microbeam single-crystal diffraction pattems (Fig. 9d) sults. On the other hand, Vicat et al. (1986) interpreted suggesta tetragonal crystal lattice for hausmannite. The the presenceof a superstructurealong the c axis in syn- noticeablepresence ofa tweed structure (Fig. 9c) in haus- thetic barium titanium hollandite as evidenceofordering mannite crystals may be attributed to the distortion of ofthe tunnel A cations. This superstructureinvolves the the crystal structureduring the phasetransformation from tripling of the cell along the c axis, which Vicat et al. a high-temperature cubic spinel structure into the lower (1986)interpreted as one-dimensionalordering ofK cat- temperature tetragonal hausmannite structure (space ions in the partially filled A sites, following the sequence VASCONCELOS ET AL.: HVEM AND AEM STUDIES OF CRYPTOMELANE 89

K-K-n-K-K-tr. Orderingof thel cationsimplies restrict- CoNcr-usroNs ed mobility of the cationsalong the tunnel direction,an The range of crystal sizesobserved by electron mlcros- important feature for the K and Ar retentivity hollan- of copy indicatesthat the supergenemanganese oxides stud- dite-groupminerals. Restricted mobility of I cationshas ied, even the well-crystallized ones, are very fine grained proposed (1980), also been by Sinclairet al. who noticed (crystaldiameters are generallysmaller than I pm). In- presence the ofsquare-shapedbottlenecks along the chan- situ observation of mineral transformations during heat- nels. These bottlenecksrestrict cation motion and explain ing of supergenemanganese oxides suggeststhat these presented the high resistanceto leaching by hollandite- phaseshave a broad range of thermal stability, even in (Sinclair type structures et al., 1980).Cation mobility along vacuum. The large range of thermal stability of crypto- the tunnel structure must be a function of the cation ra- melane in vacuum suggeststhat the mobility of the I greatermobility lower dius, and one should expect and cation is fairly restricted and requires a large activation activation energyfor smaller cationsoccupying the A sites. energy.The refractory nature of the cryptomelanecrystals (Sinclair Someauthors et al., 1980;Post et al., 1982;Vi- during in-situ heating-stageexperiments also suggeststhat cat et al., 1986)have that A shown some of the cations cryptomelane should not be affectedby the temperatures position are displaced from the special in cryptomelane. reachedinside the nuclear reactor during neutron irradi- Vicat (1986)proposed x et al. that K is shifted0.125 c ation(Z - 180'C). away from the 0072position. Postet al. (1982)have shown We postulate that the temperaturerequired for the that the smaller cations (Na and Sr) form shorter A-O mineral transformations observed in the TEM, crypto- contactsand will be displacedfrom the (000) position melane + hausmannite-' manganosite,correspond to occupied by K. Structural energy calculations also show the temperaturesin which radiogenic and nucleogenicAr that smaller tunnel cations are displacedfarther from the isotopes are releasedfrom the mineral structures. This special positions than larger cations (Post and Burnham, indicates that Ar releaseby hollandite-group minerals is 1986).The model calculationsalso indicatethat the en- not a volume diffusion phenomena.The identicaldegas- ergy minimum calculated for smaller cations is broader sing patterns for radiogenic o0Ar* and nucleogenicseAr and shallower than for larger cations, suggestingthat the observedin the laser-heating4oAr/3eAr experiments sug- smaller cations have larger temperature factors (Post and gestthat theseisotopes occupy the samesite in the crystal Burnham, 1986).The in-situ mineraltransformations ob- latticeand that 3eArdoes not recoil out ofthe tunnel sites served above suggestthat K-rich cryptomelane are more during neutron irradiation in any significant quantities. thermally stable than Cu-rich cryptomelane, consistent The very fine-grained nature of the samplesstudied, on with the prediction ofa larger activation energyfor larger the other hand, suggeststhat 3eAr recoil is theoretically calionsoccupying the A site. possible and should be monitored in future experiments. Ar diffusion from mineral structures is controlled by the energy required to distort neighboring atoms and to AcxNowr-rncMENTS passage (McDougall allow of Ar and Harrison, 1988).The ResearchCouncil (CNPq) : We acknowledgethe support ofthe Brazilian large atomic radius of Ar (R 1.96 A; imposesstructural to P.V. (grant 2O0392/88-3/GL)and ofthe National ScienceFoundation constraintsin the tunnel site that impede the migration to R.w. (gant EAR-91-04605).We also are thankful for the accessto the of Ar out of these sites unlessthe other A-site occupants facilities of the NCEM and the help with the HVEM provided by Doug (K, Ba, Na, Cu, etc.)are alsodislocated. This processwill Owen. We gratefully acknowledgeJohn Donovan for helping with the Tim Teagefor samplepreparation, and Ron promote electronmicroprobe analysis, the collapse of the hollandite tunnel structure Wilson for helping with the SEM studies.This project has also benefited and will favor the formation of bixbyite, hausmannite, from discussionswith Tim Becker, Garniss Curtis, and Paul Renne. We and manganosite.The heating-stageEM experiments also thank the garimpeiro Orilio Vaz de Aguilar from Divino das lar- suggestthat K loss and possibly Ar degassingfrom the anjeiras for help with the collection of manganeseoxide samples from in particular Augusto fine-grained network of cryptomelane crystals occur by pegmatites,and the staffof CVRD mining company, Kishida, JoseLuzimar do Rego,and Paulo Seryio,for their help and field the breakdownofindividual needlesor fibers.This pos- support. The final manuscript also benefited from insightful comments tulated Ar degassingmechanism differs from volume dif- and suggestionsby J Post and D. Veblen. fusion models postulated for Ar loss from other mineral structures(McDougall and Harrison, 1988)and is similar RnrpnnNcns cITED to the mechanismsproposed for Ar releasefrom biotite Burnham, C.W. (1962) httice constant refinement.Carnegie Institute of and amphiboleby Gaber et al. (1988). WashingtonYear Book, 61,132-135. The laser-heatingexperiments have shown that, in spite Burns, R.G, and Burns, v M. (1979) Manganeseoxides. Mineralogical of a similar behavior betweenthe K-rich and the Cu-rich Society of America Reviews in Mineralogy, 6, l-46. Bystr6m, A., and Bystrdm, A.M. (1950) The crystal structure of hollan- cryptomelanesamples, these samples release their tunnel- dite, the related manganeseoxide minerals, and a-MnOr. 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