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Synthesis of Pyrophyllite Polytypes and Mixed Layers

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Synthesis of Pyrophyllite Polytypes and Mixed Layers American Mineralogist, Volume 64, pages 109I-1096, 1979 Synthesis of pyrophyllite polytypes and mixed layers Dr,NNrsEsEnr GeologyDepartment, University of lllinois at Urbana-Champaign Urbana.Illinois 61801 Abstract Monoclinic, triclinic, and disorderedpyrophyllites have been synthesizedin hydrothermal experimentsconducted at2kbar between355o and 450oC.The monoclinic variety formed at the lowest temperature,and the disorderedvariety formed after a short run time at 360oC. The triclinic polytype formed at higher temperatures.Varying the Allsi ratio of the system did not greatly affect this pattern of reaction. Mixtures of mixed-layer pyrophyllite/smectite and margarite,/smectitewere synthesizedfrom a gel which had a compositionhalfway be- tweenCa-beidellite and pyrophyllite at 375" and 400"C, therebysuggesting that thesephases mav be found in nature. Introduction with increasingtemperature and pressure(Yoder and Pyrophyllite is a geologically important but rela- Eugster,1955; Velde, 1965),and that the formation tively little-studied mineral that has recently been of pseudomonoclinic(D-axis disordered) and triclinic shown to exist in severalpolytypic and mixed-lay- (D-axisordered) kaolinite is a function of both tem- ered forms. Natural 2-layer monoclinic and l-layer perature and the AllSi ratio of the system,with syn- form triclinic pyrophyllites have been analyzed by thesisof the triclinic favored by higher temper- greater (Eberl Brindley and Wardle (1970).They also describeda ature and a AllSi ratio and Hower, with disorderedpyrophyllite which was not amenableto 1975).Given this experience illite and kaolinite polytypes, AllSi ratio detailed analysis. Randomly interstratifed and both temperatureand the were partly-ordered mixed-layer pyrophyllite,/smectites varied in an attempt to synthesizethe polytypes of pyrophyllite. with a range of expandabilitieshave been synthe- sized by Eberl (1979) from natural montmorillonite in hydrothermal solutions containing AlClr. These Experimentaltechniques phaseshave not yet beenfound in nature. Starting compositionsincluded gels with AllSi ra- The present hydrothermal experimentswere un- tios correspondingto pyrophyllite (Al:Si' : l:2), dertaken to discover genetic relationships between pyrophyllite * 2 qtrartz(Al:Si : l:3) and pyrophyl- the polytypes, and to synthesizemixed-layer pyro- lite + Ca-beidellite(Al:Si:Ca : 2.33:3.67:0.165). phyllite/smectite starting from a gel rather than from They were preparedby the method of Hamilton and montmorillonite. It was hoped that the former exper- Henderson(1968), with the exceptionof the gel used iments would prove to be petrologically useful, al- in run 8 (Table l) which was prepared accordingto though there would be the usual problem of applying Luth and Ingamells (1965).Details of hydrothermal relationships found in the chemically simple, short- techniqueshave beendescribed elsewhere (Eberl and term, experimentalsystem to nature. A synthesisof Hower, 1976).Briefly, startingcompositions were run pyrophyllite,/smectite from completely disordered in welded gold tubeswith an equal weight of distilled starting material would indicate (although far from water at 2 kbar, water pressure.Run products were prove) that the phaseis not sinply a metastablereac- mounted on glass slides and X-rayed at l" 20 per tion product of montmorillonite, and would increase minute with Ni-filtered CuKa radiation with a No- the likelihood that it will orte day be found in nature. relco diffractometer. Pyrophyllite polytypes were Synthesisexperiments have revealed that illite identified from peak positions and patterns given in polytypesare related in the serieslMd + lM --+ 2M Brindley and Wardle (1970)and by comparisonwith o*34s4X1't 9/09 I 0- l 09l $02.00 l09l lw2 EBERL: SYNTIIES/S OF PYROPHYLLITE Table L Hvdrothermal runs made at 2 kbar Run Gel Temp Time Run composi t ion ('c ) (days) products pyrophyl I i te 355 38 monoclini c pyrophyl I i te pyrophyl I i te 360 ll disordered pyrophyl I i te pyrophyllite 37s 38 tricl inic pyrophylI i te pyrophyllite 380 t2 tricl inic pyrophylI ite pyrophyllite 4oo 38 tricl inic pyrophylI i te 6 pyrophyllite 430 l2 tricl inic pyrophylI ite 7 pyrophyllite 450 38 tricl inic pyrophyl1 i te, quartz 8 pyrophyllite'\ 380 t69 pyrophyllite, quartz 9 pyrophyllite + quartz 355 38 monoclinic pyrophyI I ite l0 pyrophyllite + quartz 375 38 tricl inic pyrophylI i te, cristobal ite ll py.ophyllite + quartz 380 t2 tricl inic pyrophylI ite (poor) l2 pyrophyllite + q uartz 400 38 tricl inic pyrophylI ite r3 pyrophyllite + quartz cJo t2 tr;cl inic pyrophyllite (poor) r4 pyrophyllite + quartz 45U 38 triclinic kaolinite r5 pyrophylI ite + Ca-beidelI ire 375 38 pyrophyltite/smectite (30),1,?,mrgarite/smecti te (70) t5 pyrophyllite + Ca-beidellite 400 38 pyrophyl I i telsmect i te, margar i telsmecti te 17 pyrophylI ite + Ca-beidelI i te 450 38 beidellite, kaolinite 'tGel prepared by the methodof Luth and Ingamells (1955). All other gels prepared by the methodof Hamilton and Henderson(l!68) r,*Number in parenthesis refers to percent smectite in the mixed-laver ohase. samplesgiven to our laboratory by Dr. Brindley (Fig. The pyrophyllite/beidellite gelsproduced mixtures l). Mixed-layer pyrophyllite/smectite was identified of mixed-layer pyrophyllite/smectiteand margarite/ from calculatedX-ray patternscomputed with a pro- smectiteat 375" and 400o(runs 15-16),and beidel- gram modified from Reynolds and Hower (1970), lite plus kaolinite at 450oC (run l7). The X-ray pat- and from patternspresented in Eberl (1979). tern for the 375oC run is given in Figure 4 with a computer-simulatedpattern. The simulated pattern, Experimentalresults which was constructedby adding togethercomputed Both monoclinc and triclinic pyrophyllite were profiles fot 70Voexpandable randomly interstratified synthesizedfrom the pyrophyllite gel (runs I and 3-6 margarite/smectite and 30Vo expandable randomly in Table l; Fig. 2). Monoclinic pyrophyllite formed interstratified pyrophyllite/smectite, is a satisfying at the lowesttemperature (355oC, run l), whereasthe match: peak positions correspond exactly with the triclinic variety formed at 3J5"C and above.A short real pattern, although intensities of the low angle run at 360"C yielded disorderedpyrophyllite (run 2), scatteringand the peak at 8.904 are slightly differ- for which no X-ray reflectionsappear on the band- ent. In addition, there is a very weak peak at 2.53A in head beginnng at 4.42A. The long run at 380oC(run the real pattern which is not accountedfor in the cal- 8, Fig. 3) and the highest-temperaturerun (run 7) culated pattern. Basal spacingsused in the calcu- produced pyrophyllite plus quartz, suggestingthat lation were: margarite : 9.6A, pyrophyllite : 9.2A, thesepolytypes do not have the ideal composition,as and smectite: 16.9A,with an equally weightedcrys- will be discussed.The polytype for run 8 (Fig. 3) tallite sizeof l0 to 15 layers.Expandabilities for run could not be determined.If the 4.26A peak for this 16 could not be determined due to poor X-ray in- sample is entirely due to quartz, then the polytype is tensities. the disordered form. A comparison between this peak and the other qtJartzreflection at 3.344, how- Conclusions ever, suggeststhat the 4.26A peak is too intenseto be Monoclinic and triclinic pyrophyllite are related entirely quartz. Thus the peak could also be a tri- through changing temperaturein the hydrothermal clinic pyrophyllite reflection, except that the other systemsstudied here. Monoclinic pyiophyllite is the important reflectionat 4.06A is missing. low-temperature form. Mixtures of monoclinic and Decreasingthe AllSi ratio of the systemdid not triclinic pyrophyllite did not appear in theseexperi- changesignificantly the pattern of reaction (runs 9- ments, although they have been found in nature l3), except for the 450oC run which, surprisingly, (Brindley and Wardle, 1970).Disordered pyrophyl- formed triclinic kaolinite rather than pyrophyllite. lite is related to the monoclinic form by reaction The pyrophyllites formed in runs 9-13 were not as time: it formed after a short run tine at 360"C. well-crystallineas their lower-silicacounterparts. Changing the AllSi ratio of the system did not EBERL: SylrrfiES.IS OF PYROPHYLLITE 1093 "ze Fig. l. X-ray difraction patternsof triclinic pyrophyllite from New Zealand(reference #l of Brindlcy and Wardle, 1970)at the top, and monoclinic pyrophyllite (reference#3) from Honami at the bottom. An: anatase. changethe general pattern of reaction, although the oxygen plane. Evidencefor the Al3* + (OH)- : Si4+ pyrophyllite * 2 qrtartz composition did form kaolin- + O'?-coupled substitution is given by hydroxyl en- ite at 450"C. This product was surprising,since ka- richment for syntheticpyrophyllites as shown by in- olinite was thought to disappearin such a systemat frared analysis,and by chemical analysesof natural 350"C (Eberland Hower, 1975). pyrophyllites which show an inverse relationship be- The formation of pyrophyllite plus quartz from tween Sia* and both RrO, and structural water. He pyrophyllite gel in runs 7 and 8 indicatesthat pyro- found no evidence for HrO* in the structure. Evi- phyllite in these runs may be richer in aluminum dence for the position of the extra H* on the basal than ideal pyrophyllite. Rosenberg(1974) also sug- oxygen plane is founded on enlargedbasal spacings geststhat some pyrophyllites have more aluminum for synthetic pyrophyllites; Rosenbergfeels that H* than the ideal, and also baseshis argument on the associatedwith apical oxygenscould
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