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Pyrophyllite Solid Solutions in the System A|,O,-S|O,-H,O

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Pyrophyllite Solid Solutions in the System A|,O,-S|O,-H,O American Minerulogist, Volume 59, pages 254-260, 1974 PyrophylliteSolid Solutions in the SystemA|,O,-S|O,-H,O Pnrr,rp E. RoseNnenc D ep art ment ol G eolo gy, 14as hing ton Stat e U nio er sity, P ullman, 14as hington 99 I 6 3 Abstract Pyrophyllite solid solutions have been synthesized in the system Al,O'-SiOr-H,O between 400' and 565'C at 2 kbar from a variety of starting materials using sealed gold tubes and conventional hydrothermal techniques. The duration of experiments was 3-9 weeks. Products were characterized by X-ray measurementsand by infrared spectroscopy. Appreciably larger basal spacings were observed for pyrophyllite synthesized from gels than for natural pyrophyllite; small amounts of quartz were present in these samples. Basal spacings of pyrophyllite synthesized from mixtures of kaolinite and quartz approach but do not equal those of natural pyrophyllite. Rehydroxylation of dehydroxylated pyrophyllite under hydrothermal conditions suggests that enlargement of basal spacings cannot be due to dehydroxylation. Substitution of the type Al"* + H* : Sia* is proposed to explain the observed variations. The coupled substitution of Al3* and (OH)- for Sin*and O- results in expanded basal spacingsdue largely to the formation of OH on the basal surface and in increased thermal stability due to the formation of H-bonds between oxygens in adjacent silica sheets.Analyses of natural pyrophyllite also suggest limited substitution of this type. Failure to recognize these substitutions in synthetic samples accounts, in part, for dis- crepancies in the reported thermal stability of pyrophyllite. Phase characterization, often lack- ing, is essentialin experimentalstudies of mineral stabilities. Introduction (Haas and Holdaway, 1973). Pyrophyllite,synthe- sizedin the ternarysystem between 400' and 565'C Since natural pyrophylliteis commonly believed at pressureof 2 kbar, has been characterizedby to deviate only slightly from end-membercomposi- X-ray and infrared techniquesin an effort to resolve tion (Deer, Howie, and Zussman,1962), previous this problem (see Rosenbery,l97l). investigatorshave tacitly assumedthat the theoretical end-memberwas presentin the products of their Experimental Details experiments. Conventionalhydrothermal equipment and tech- During the courseof an experimentalinvestiga- niques were used throughout this study. All experi- tion of compositionalvariations in topaz (Rosen- ments were carried out in sealed, gold tubes and berg, 1972), inconsistencieswere observedin the cold-sealpressure vessels. F/OH ratios of topaz coexistingwith pyrophyllite Starting materials were natural pyrophyllite which were thought to be related to variations in (Glendon, Moore County, North Carolina), pyro- the compositionof pyrophyllite.Simple substitution phyllite gels (Luth and Ingamells,1965) and me- of F for OH could not accountfor the basalspacings chanical mixtures of pyrophyllite composition pre- of thesepyrophyllites, and, therefore,compositional pared usingkaolinite (API No. 9, Mesa Alta, New variationsin the systemwithout fluorine were sus- Mexico) plus quartz (Minas Gerais, Brazil) and pected. recrystallizedA1(OH)3 plus Cab-O-Sil(amorphous An investigationof pyrophyllite compositionsin silica, dried).'Natural pyrophyllite was ground to the systemAl2O:r-SiO2-H2O was clearlyprerequisite lessthan 200 mesh (<74 pm) under alcoholin an to an understandingof fluorinesubstitution. Further- agatemortar; averagegrain size was on the order more, compositionalvariability might account, in of 50 pm with only a small fraction less than 10 part, for the discrepanciesin the upper thermal s,m. Constituentsof the mechanicalmixtures were stability of pyrophyllite reported in the literature ground separatelyto less than 200 mesh under 254 PYROPHYLLITE SOLID SOLUTIONS 255 alcohol in an agatemortar and then mixed together were the same within the limits of error of these in the required proportions by grinding in a vibra- measurements. tory mill for 15 minutes.Grain sizeof the resulting Identification of pyrophyllite polymorphs (Brind- ( mixtures varied from 1 pm to 73 p.m; average ley and Wardle, 1970) was difficult becauseof grain size appearedto be about 10 pm, but was interferencecaused by coexistingphases and be- not uniform for different constituentsof any given cause of the weaknessof diagnostic X-ray reflec- (e.g., mixture quartzcoarser than kaolinite). tions due to orientation effects and poor crystal- The natural pyrophyllite used in this study is linity of many syntheticsamples. However, the poly- probably monoclinic (2M) accordingto the criteria morphic forms of severalsamples synthesized from of Brindley and Wardle (1970); it was found to each starting mixture are known with reasonable contain small amounts of quartz and traces of certainty. kaolinite by X-ray diffractometryand optical micro- Infrared spectrophotometrywas used to study the scopy. Portions of this specimenhaving X-ray dif- OH-stretchingbands of natural and synthetic pyro- fraction maxima correspondingonly to pyrophyllite phyllite. Sampleswere prepared using conventional were selectedfor chemical analysis.Although HzO. KBr pellet techniques(Lyon, 1967) and,run on a wasnot determined,major-element chemistry (SiO2, Beckman IR-8 infrared spectrophotometerwith a Al2Os, FezOa) was found to be virtually identical blank KBr pellet in the referencebeam. Since in- to that of a specimenfrom the same locality re- dividual crystals of synthetic pyrophyllite could not ported by Hendricks (1938; see also Deer, Howie, be resolved under the microscope,measurements and Zussman,1962, p. 118), and shownin Figure of optical properties were not possible. 2 (No. 4). Other constituents,determined by semi- quantitative spectro-chemicalanalysis, were present Experimental Results in trace amountsonly. \ Samplesof thesemixtures weie sealedalong with Synthesis excesswater (solid/waterratio = 2/l by weight) The basal spacings and polymorphic form of into gold capsulesand subjectedto temperatures natural pyrophyllite are essentiallyunchanged after between400' and 575'C at 2 kbar for periodsof hydrothermaltreatment for periods of 34 weeks. 3 to 9 weeks.After quenching,products were identi- (Fig. 1, open squares).All A2d valuesare slightly fied by X-ray powder diffractometry and by optical less than that of the starting material but the ap- microscopy. parent changeis very small and may not be real. Basal spacingsof pyrophyllite were determined However, a somewhatlarger changein basal spac- by measurementof the X-ray reflection at approxi- ings was observed in an experiment of 60 days mately 29.1" 20 CuK(I l2M(006) or IZc(003); duration at 475"C (Fig. 1, filled square). t2A for Brindley and Wardle, l97O; Wardle and Brindley, this sample (0.831'-+ 0.005") is significantly 19721 againstannealed, internal standardsCaFz or lower than that of the startingmaterial (0.860' -+ CdF2 which have conveniently located reflections 0.005'), suggestingthat natural pyrophyllite does at 28.3O'(20) and 28.7O"(2d) respectively.a.20 undergo an expansionof basal spacingsunder the values,which are all reported as measurementswith conditionsof these experiments. respectto CaF2,were obtainedby averagingat least Basal spacingsof pyrophyllitessynthesized from two measurements(scan rate, l/4" min) per re- kaolinite, quafiz mixtures (Fig. 1, triangles)at and flection and are known to within 0.01"(24). A above 50OoC very closely approximatethose of precisionof I 0.005'(2d) was attainedfor well- the natural material. Below this temperatureL20 crystallizedpyrophyllites by averagingfour or more values obtained for pyrophyllite synthesizedin ex- measurements. perimentsof 3-4 weeks duration (Fig. 1, open Natural pyrophyllite from Glendon, Moore triangles) approach but do not equal the value for County,North Carolina,used as a startingmaterial natural pyrophyllite; basal spacingsundergo rela- -+ in this study,was found to have a a;20of 0.860 tively little changein experimentsof longer duration 0.005'. Samplesfrom two other localitiesin North (Fig. 1, filled triangles)and remain larger than for - -F Carolinawere also measured (L20 0.853 0.005. natural pyrophyllite. Severalof thesesynthetic pyro- -+ and 0.856 0.005') to providesome assurance that phyllites were found to be monoclinic or a mixture this starting material was not unusual;A2d values of monoclinic and triclinic forms. Decomposition 256 PHILIP E. ROSENBERG oo productsof an experimentat 560'C as the starting o material for an experimentat 425"C. After 65 days o the products were indistinguishablefrom those of d experimentswith gels at 425"C. Pyrophyllite has also been synthesizedfrom a GN mixture of gibbsite and Cab-O-Sil (Fig. 1, hexa- o o l----------------F,--)ra----) O AE gons). Pyrophylliteand quartz were the only prod- U E ucts of experimentsabove 450oC, but at lower F 450 D------- ----- - --) o AAD temperaturesboehmite and tracesof diasporewere E. r also present. A20 values at higher temperatures o- l-----------------r) o AA tr u closely approachthose of natural pyrophyllites.Con- D siderably larger basal spacings were recorded at NATURAL YLATEO OEHYOROXYLATED NATURAL lower temperatures,but their rapid decreasewith increased duration of experiments (Fig. 1, filled o .o8 .t6 ?4 32 40 .4A 56 .64 .72 .80 88 .95 hexagons) suggeststhat given sufficient time L20 a2e[2e2y16651 o.11" 19s31 -2Ogqp. I values would approach those obtained for mixtures (Fig. tri- FIc. 1. Variation in A20 (202a<w>or Lrc(@sr'20canz)of syn- of kaolinite and quartz 1, triangles).The thetic pyrophyllite
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