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In-Situ Study of the Thermal Behavior of Cryptomelane by High-Voltage And

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In-Situ Study of the Thermal Behavior of Cryptomelane by High-Voltage And 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 hausmannite and manganositephase at 648 'C. Mineral 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 minerals 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<im and Bys- addition, it is necessaryto investigateordering ofthe tun- tritm (1950)showed that ,4 cationsoccupy a tunnel struc- nel site occupantsto determineif cations,HrO, and the ture formed by edge-sharingB-O octahedra.Electrostatic 0003-004x/94l0I 02-0080$02.00 80 VASCONCELOS ET AL.: HVEM AND AEM STUDIES OF CRYPTOMELANE 8l repulsion betweenthe I cations requires that the A sites TABLE1. Microprobe analyses of manganeseoxides be only partially occupied.Bystrtim and Bystrdm (1950) Monoclinic Tetragonal have proposed that the tunnel cations occupy a special pEG-01 position alongthe tunnel, which shouldbe 0, t/2,0in the F226-121 F40-142 F115-139.8 monoclinic hollandite structure and 0, 0, t/zin the tetrag- Si4+ 0.06 0.05 0.03 0.05 onal cryptomelane structure. Charge-balance require- Al3* 0.52 0.18 0.08 0.13 Fe3* 0.02 0.01 o.22 0.00 ments indicate that lower valence cations must occupy Mn4+ 55.05 56.44 56.43 57.57 the B sitesto compensatefor the presenceof the A cations Co,* o.25 0.20 0.14 0.00 (Post 0.00 0.00 0.00 0.05 et al., 1982;Post and Burnham, 1986;Vicat et al., Cu* 2.80 1.51 1.40 0.00 1986).Vacancies in the B siteshave also beenproposed Ca2* 0.11 0.10 0.17 0.12 as a charge-balancemechanism (Bystrdm and Bystr6m, Na+ 0.08 0.13 0.09 0.07 K+ 3.01 3.04 2.98 4.91 1950).Among the common K-bearingmanganese oxides, Ba2* 0.14 0.36 0.01 0.55 hollandite is the most promising datable group because S12+ 0.04 0.14 0.08 0.13 of its large range of stability and its occurring P5+ o.21 0.20 0.24 0.44 as a hy- o*"" 35.29 34.65 34.79 34,75 pogenephase in hydrothermal environments (Hewett and O""n 34 01 34.37 33.62 35.71 Fleischer,1960), as metamorphic minerals in high p-Z Total 97.58 97.01 96.66 98.77 environments(Roy and Purkait, 1965;Miura et a1.,1987), B cations Si4* 0.o2 0.01 0.01 0.01 and as supergenephases in soils and weathering profiles Al3* 0.15 0.05 0.02 o.07 (Hewettand Fleischer,1960). In addition, the 2 x 2 tun- Fe3* 0.00 0.00 0.03 0.00 nel structure of hollandite-group minerals is more likely Mno* 7.56 7.67 7.66 7.54 Co2+ 0.03 0.03 0.02 0.00 to retain K and Ar than the more open structuresof other 0.00 0.00 0.00 0.01 K-bearing manganeseoxides, such as romanechite(2 x 3 Total 7.76 7.76 7.74 7.63 tunnels), todorokite (3 x 3 tunnels), and birnessite (lay- A cations Cu* 0.33 0.18 (Burns 0.08 0.00 eredstructure) and Burns, 1979;Post and Veblen, Ca2* o.02 0.02 0.03 0.02 1990). Na+ 0.03 0.04 0.03 0.02 Thermogravimetric analysis (TGA) and K+ 0.58 0.58 0.57 0.90 differential- Ba,* 0.01 0.02 0.00 0.03 thermal analysis (DTA) of manganeseoxides have indi- Sr'?* 0 00 0.01 0.01 0.01 cated that dehydration reactions occur at 200-400 .C, Total 0.97 0.85 0.72 0.99 and dehydroxylationoccurs at higher temperatures(-650 P5* 0.05 0.05 0.06 0.10 "C) (Kulp and Perfetti, 1950; Ljunggren,1955). Heating Total cations 8.78 8.66 8.60 8.72 MolecularWeight 724.92 720.43 7't7.55 717.56 experimentsin open air have indicated that cryptomelane CalculatedS.G. 4.35 4.32 4.31 transforms into bixbyite at 600 "C, into tetragonal haus- RB 0.60 0.60 0.60 0.60 mannite RA 1.19 1.21 1.26 1.31 at 825 "C, and finally into cubic hausmanniteat RB/RA 0.50 0.50 0.48 0.46 1050(Faulring et al., 1960)or at I160'C (VanHook and Keith, Note.'elemental weight percentagesand formulas basedon 16 O atoms. 1958).DTA analysesby Kudo et al. (1990)showed S.G. : specific gravity in grams per cubic centimeters. that tetragonalcryptomelane transforms into a monoclin- ic structureat 7 l0 'C in air and into hausmannitethrough bixbyite at 850-950 "C. enriched in K+ and Mn2* fill open spacesprovided by To investigate the mineralogical and crystallographic miarolitic cavities in the pegmatites,promoting the pre- relationsof supergenepotassium manganese oxides, we cipitation of high-purity cryptomelane inside these cavi- analyzedcryptomelane samples by powder XRD, elec- ties. The autocatalytic nature of manganeseoxide precip- tron microprobe (EM)" SEM, and TEM. We also moni- itation leadsto the continuousadsorption and subsequent tored phase transformations of two of these samples in precipitationof dissolvedMn2+ onto previouslyprecipi- situ, with a heating stage in the HVEM. The degassing tated Mna+-bearing oxide surfaces,forming concentric history of two sampleswas further studied by laser-heat- growth bands (Fig. 1) (Crerar and Barnes, 1,974 Crerur aoAr/3eAr ing analysis,and the degassingsteps are corre- et al., 1972). BackscatteredSEM investigation and mi- lated with the mineralogical transformations observedin croprobe analysis showed that differencesin the compo- thc TEM. sition of botryoidal growth bands in PEG-01 suggests rhythmic variations in the composition of the weathering Frnr,osrrns solutions(Fig. l). We investigated hollandite-group minerals from two SamplesF226-121, F40-142, and Fl l5-139.8 (Table weatheringprofiles in Brazil. The first sample,pEG-01 l) came from laterites developed over the Igarap6 Bahia (Table 1), came from deep weatheringprofiles in the peg- hydrothermal Cu-Au deposit in the Caraj6sMountains, matite fields in the state of Minas Gerais (Vasconceloset easternAmazon (Vasconceloset al., 1993).The samples al., 1992). This sample occurs as intimately intercon- occur as massive manganeseoxides replacing weathered nected botryoidal potassium manganeseoxide nodules schists.Randomly oriented manganeseoxide crystalshave forming a three-dimensional dendrite-like pattern. Dur- precipitated in the pore spacesaround silicate crystals; ing weathering of primary minerals, supergenesolutions after dissolution of the silicate matrix.
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