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Kaolinitization of Biotite: Rem Data and Implications for an Alteration Mechanismt

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Kaolinitization of Biotite: Rem Data and Implications for an Alteration Mechanismt American Mineralogist, Volume 72, pages 353-356' 1987 Kaolinitization of biotite: rEM data and implications for an alteration mechanismt JuNc Ho AHNr2 DoN.ql-o R. PBlcon Department of Geological Sciences,The University of Michigan, 1006 C. C. Little Building, Ann Arbor, Michigan 48109,U.S.A. Ansrucr Partially kaolinitized biotite from Brighton, New Znaland, was studied by using trans- mission and analytical electron microscopy (rErraand aer',r).Kaolinite occursin two distinct modes: (l) as 50-300-A-thick packetsof layersinterstratified within biotite and (2) as two- layer units (14 A) irregularly interlayered within biotite. Some two-layer kaolinite units terminate at single biotite layers (10 A), implying a reaction of one biotite layer to two kaolinite layers. Structural and chemical considerationssuggest an alteration mechanism consisting of complete dissolution of biotite and crystallization of kaolinite at linear boundaries. Tunnel-like structures inferred at reaction boundaries may serve as conduits for diffusion of reactant and product ions. Such linear imperfections may be significant in mass transport during the alteration of large rock volumes. yield images of the interfaces where biottte alters INrnooucrroN direct to kaolinite. craw et al. (1982) indicated that kaoliniza- phases, Biotite can react to form a variety ofdifferent tion in thesesamples appears to have been accompanied depending on alteration and weathering environments, by hrgh activity of COr, resulting in partial loss of epidote (Walker, including vermiculite 1949), montmorillonite and producing rutile and/or anatase at the expense of (MacEwan, (Veblen Ferry, 1983;Yau 1954),chlorite and sphene, without oxidation of siderite. Alteration was gibbsite(Wil- et al., 1984;Eggleton and Banfield, 1985), therefore not a simple weathering process. son, 1966),or kaolinite (Wilson, 1966;Stoch and Sikora, 1976;Muenier and Velde, 1979; Craw et d., 1982).Al- ExpnnrunNTAL METHoDS though kaolinitization is a common biotite alteration Sampleswere first preparedas thin sectionsoriented approx- phenomenon that has been investigated by optical mi- imatelyperpendicular to (001)of the partiallyaltered biotites. croscopy and powder X-ray diffraction (xno) (e.g., Wil- Followingoptical examination, selected areas were ion-milled son, 1966; Stoch and Sikora, 1976:'Craw et al., 1982), and thencarbon-coated. Samples were observed at 100kV in a little is known about the textures,chemistries, and struc- JEoLJEM-loocx scanning-transmission electron microscope (sreIr'r) tures involved in the alteration process.We therefore have fitted with a solid-statedetector for X-ray energy-dispersive carried out a rEM and aev study at high resolution in analysis.One-dimensional lattice fringe images were obtained in Iijima order to directly characteize the mechanism of alteration rErr,robservations by using only 00/ reflectionsfollowing (1978). of biotite to kaolinite. andBuseck partially The samples used in this study are kaolini- TEM OBSERYATIONS tized biotites from the lower limit of the biotite zone of Kaolinite occurs in two modes. The first consists of the Otago schist at Brighton, approximately 16 km south packetsofkaolinite layersinterstratified with biotite (Fig. of Dunedin, New Zealand. Craw et al. (1982) described ofthe packetsvaries from 50 to 300 A similar samplesfrom the samelocality as containing bio- i). fn. thickness less than the spatial resolution of nEru tite having a curious greenishcolor and low KrO contents and is therefore (=300 aeu analysesofinterstratified packets (4.4-8.5 wto/o).Using xno and electron-microprobedata, analysis A). yield high Al and Si contents characteristic Craw et al. (1982) showed that the "green biotite" ac- of 7-A hyers low K, Fe, and Mg contents due to con- tually consistsofan intimate intergrowth of metamorphic of kaolinite, and tamination by in the analyzedarea (Fig. 2a). This biotite and secondarykaolinite that is not fully resolvable "biotite with the electron-microprobe data of Craw optically or with the electron microprobe. Their data im- is consistent (1982) the possibility that the specific plied that rEM-AEMstudies of such intergrowths might et al. and excludes 7-A hyers imaged by reu are trioctahedral minerals such as serpentine or berthierine. Interfaces between biotite 'Contribution no. 428, Mineralogical Laboratory, Depart- and kaolinite along (001) are parallel, and they display ment of Geological Sciences,University of Michigan, Ann Ar- no strain contrast. Parallel intergrowth featuresofkaolin- bor, Michigan 48109,U.S.A. '?Present address: Department of Geology, Arizona StateUni- ite and wonesite were previously reported in a reu study versity, Tempe, Arizona 85287, U.S.A. of alteredNa-rich biotite by Veblen (1983). 0003404x/87/03044353$02.00 353 354 AHN AND PEACOR: KAOLINITIZATION OF BIOTITE 0.00 1.00 2.00 3.00 4.00 5.00 5.00 7.00 ENEBGY(KEV} Fig. l. Latticefringe image of biotite and interstratifiedpack- etsofkaolinite layers.(001) interfaces between biotite and ka- olinite areparallel. A second mode of kaolinite occunence that provides direct insight into the alteration processeswas also ob- served (Figs. 3, 4). This mode invariably occurs as pairs of kaolinite layers that are randomly interlayered within biotite. Electron-difraction pattems from such areasex- hibit intense streaking along c* due to random mixed layering (e.g.,Fig. 3). However, severalrepeats of ordered sequencesconsisting oftwo-layer kaolinite units and sin- gle biotite layers occur locally (Fig. 3). AEManalyses of biotite containing such pairs oflayers show a significant 0.00 t.00 2.00 3.00 4.00 5.00 6.00 7.00 decreasein K, Fe, and Mg, and increase in Al and Si ENEBGYKEV) content compared to biotite that is free of interlayering, indicating that the interlayered 7-A hyers are kaolinite (Figs.2a, 2b). In addition, twoJayer kaolinite units frequently are ob- servedto terminate at a singlebiotite layer (Fig. 4), where the termination is inferred to extend as a linear feature at some angle to the lattice fringe images. As shown in Figure 4, there is some image contrast associatedwith these terminations, apparently reflecting strain in re- sponseto the replacementof a single biotite layer (10 A) by two kaolinite layers (14 A). DrscussroN The layer transition boundariesbetween a singlebiotite Iayer and two kaolinite layers representa reaction front 0.00 2.00 lnn 4.00 Enn 6.00 7.00 and imply that one T-O-T unit of biotite must changeto ENEBGYKEV) two T-O units of kaolinite, with the addition of a gibbs- Fig. 2. Energy-dispersiveX-ray spectraof (a) interstratified iteJike octahedral sheet and the reversal in orientation packet ofkaolinite layers, (b) biotite free ofinterlayering ofka- of one tetrahedral sheet of a T-O-T unit. Concomitant olinite layers, and (c) biotite containing interlayered kaolinite chemical changesinclude replacement of the single Fe- layers. Small Mg, K, and Fe peaks in spectrum (a) are due to and Mg-rich trioctahedral sheet by two aluminous di- contamination by biotite in the analyzed area. Low intensities octahedral sheets.Furthermore, all Al in the tetrahedral ofpeaks ofI! Mg, and Fe in spectrum (c) relative to spectrum sheetsmust be replacedby Si, assumingthat the kaolinite (b) indicate that the interlayered 7-A hyers are kaolinite. layers are compositionally ideal. If the biotite has ap- proximately equal fractions of Fe and Mg, as shown by the electron-microprobe data of Craw et al. (1982), the l.5Fe+2 * l.5Mg+'z + 6H+, implying a net gain of HrO, reaction of one biotite layer to two kaolinite layers can Al, and Si and loss of K, Mg, and Fe (if other biotite/ be approximatedby K(Fe, rMg,,)SirAlOr0(OH), + kaolinite ratios are involved in the reaction, specific ele- 3Al+3 + Si*o + 6HrO : 2AlrSirO5(OH)4+ K+ + ments can be eliminated from the reaction). AHN AND PEACOR: KAOLINITIZATION OF BIOTITE 355 tures at the reaction interfaces.The necessaryaddition of H2O, Al, and Si and loss of H, K, Fe, and Mg require a pathway for transport. The linear boundary is inferred to serve as such a conduit especiallyinsofar as there is an apparent absenceofany other structural defects.Similar pathwaysfor transport ofions and fluids were postulated at chain-silicate reaction interfaces (Veblen and Buseck, 1980) and at chlorite-phlogopite interfaces (Yau et al., 1984).Veblen (1985) has discussedthe importance of such extended defects in mineralogical processes.Once initiated at a crystal surface,such a linear reaction bound- ary would be self-perpetuatingand would causealteration of a single layer of biotite within an otherwiseunaffected sequenceofbiotite layers. Becausebiotite is altered per- vasively over several tens of meters at Brighton, such defect-controlledtransport is inferred to be an important mechanism of ion and fluid transport within individual crystalsand to pervasively involve at leasttens of meters Fig. 3. Latticefringe image of biotite and interlayeredka- of rock. olinitelayers (inset diftaction pattern).Kaolinite invariably oc- cursas pairs oflayers. If the reaction mechanism for pairs of kaolinite layers also occurs for interstratified packets ofkaolinite layers, the number of kaolinite layers in the packets should be Such drastic structural and chemical changessuggest multiples of two. However, the number of layers in ka- that the biotite structure may be completely disarticulat- olinite packets cannot be accurately counted becauselarge ed; i.e., kaolinitization of biotite in these samples may portions of lattice fringe imagesof kaolinite
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