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-stretching bands 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/water ratio = 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
under similar conditions with a like enlargement In a study of the effect of iron substitution on of basalspacings (a20 : 0.05.-0.09o) and then the unit-cell dimensionsof talc, Forbes (1969) fully rehydroxylatedhydrothermally (Fig. 1, dashed observedthe enlargementof basal spacingsand arrows). Thus, gel pyrophyllitesare hydroxylated, the presenceof excessquartz. He proposed substi- and dehydroxylatedgel pyrophyllitesare not stable tution of the type FeS** H* : Sia*and accounted under hydrothermalconditions. for the significantly enlargedbasal spacingsof talc Infrared spectroscopywas used to confirm these solid solutionsby suggestingthat protons are added inferences. Dehydroxylated pyrophyllites show no to oxygenatoms bridging Sin. and Fe3*tetrahedra. OH-stretchingband while both natural and gel Overbondingwould then increaseelectrostatic bond pyrophylliteshave strong OH-stretchingbands and lengthsresulting in the elevationof bridging(OH)-. are presumablyfully hydroxylated.OH-stretching Neglectingthe probablelateral displacementof Fes* band intensitiesof natural pyrophyllite are un- and Sin*parallel to the basal sheets,the predicted changedby hydrothermaltreatment. Therefore, de- increasein bondlength (0.12 A) for an overbonding hydroxylationcannot account for observedvaria- of 0.75 would result in a maximum increasein tions in basal spacing. Gel pyrophyllites and their hydroxyl elevationof 0.30 A (Forbes, 1969). rehydroxylatedequivalents consistently show more The observationsand implications in the talc intenseOH-stretching bands than doesnatural pyro- study are clearly qualitatively analogousto those phyllite and thus appear to be OH-rich. In accord for pyrophyllite.Furthennore, the maximum Ad661 with partial rehydroxylation,"rehydroxylated" natu- reported by Forbes (-0.12 A) is only slightly ral pyrophyllites have weaker OH-stretchingbands smallerthan that observedfor pyrophyllite 1-9.14 than natural pyrophyllite and the intensitiesof these A). However, in pyrophyllite,as in other diocta- bandsare proportionalto the measurcdt20 values. hedral sheet structures,apical oxygen atoms ad- No evidencehas been found to suggestthe presence jacent to octahedralvoids are considerablyunder- of hydronium (HrO-) in any of thesesamples. bonded (0.5), thus providing an alternatesite for hydroxyl (Forbes, Interpretation formation 1972). The enlarge- ment of basal spacingsdue to hydroxyl formation Enlarged basal spacings,apparent OH-enrich- at apical sites should be quite small as compared ment, and coexistenceof quartz with pyrophyllite with that produced by hydroxyl formation at basal synthesizedfrom gels suggestthe crystallization of sites and cannot accountfor the observedvariations pyrophyllite solid solutions involving the coupled in basal spacings.Therefore, hydroxyl formation substitutionof Al3. and (OH)- for Sia. and Or- must take place at least partially at basal sites. respectively.Other starting materials apparently re- The thermal stability of pyrophyllite solid solu- sult in less extensivesolid solutionsof the same tions should be higher than that of end-member nature, each having the structural formula pyrophyllite as a consequenceof hydroxyl forma- Al2si4 (oH), (oH) "Al,o1o_" 2. tion on the basal surface.In end-memberpyrophyl- It has been suggestedthat Als. tends to enter lite, bondingbetween adjacent silica sheets is limited tetrahedralsites during the crystallizationof pyro- to very weak van der Waals forces betweenbasal phyllite gels (Roy and Osborn, 1954). Entrapment oxygens.Hydroxyl formation on the basal surface of Al3' in these sites during the crystallizationof should result in the formation of hydrogen bonds fine-grainedor amorphousstarting materials under betweenbasal oxygens, thus strengtheningthe weak- hydrothermal conditions could result in the for- est bonds in the structure which determine its mation of Al-,OH-rich pyrophyllite solid solutions thermal stability. having larger unit-cell dimensions than those of end-memberpyrophyllite. Abrupt expansionof the Natural Pyrophyllite Solid Solutions basal spacingSof gel pyrophyllites at or just above Ten analysesof natural pyrophyllite have been 550oC, which is accompaniedby a large increase selectedfrom the relatively small number reported in the proportion of qvartz, seemsto be causedby in the literatureon the basisof their apparentre- the exsolution of silica from pyrophyllite solid liability and their close approach to the system solutions leaving small amounts of an unstable AI2OB-SiO2-H2O.Analyses with an alkali-content Al-rich pyrophyllite which decomposesto X-anda- (NazO + KrO) > 0.5 wt percentwere arbitrarily lusite. rejected.An inverse relationshipappears to exist PHILIP E. ROSENBERG between SiOz and both RzOe and HzOt in these studies concerned with the upper thermal stability samples(Fig. 2) suggestingthat the coupled sub- of pyrophyllite-namely, 565oC at 2 kbar (Aramaki stitution proposed for synthetic pyrophyllite orcurs and Roy, 1963),and 378"C at2.4kbar (Haasand in natural materials. Although the extent of this Holdaway, 1973). The previously unsuspected substitutionis small, most natural pyrophyllites may deviationsfrom end-membercomposition described be solid solutions. in this paper are believedto partially explain these Small replacementsof Sin. by Al3- have been discrepancies. recognizedpreviously (e.g.,Deer, Howie, and Zuss- A correlationseems to exist betweenthe nature man, 1962). Moreover, Swindale and Hughes of startingmaterials and the isobaricdecomposition (1968) concludedthat small excessesof alumina temperatureobtained for the reaction and water are present in a pyrophyllite from New pyrophyllite : andalusite 3 quartz water. Zealand (Fig. 2, S) and that Al3* substitutesfor * f Sia* in this material, which is triclinic (Brindley Pyrophyllitessynthesized from fine-grainedor amor- and Wardle, 1970) and has a,L20 of approximately phous starting materials appear stable up to higher 0.805 lcalculated from refined unit-cell dimensions temperaturesthan those synthesizedfrom coarser- (ibid.)1. grained crystalline materials. Pyrophyllite solid solutions may have been produced in these experi- Discussion ments (and in nature) during the hydrothermal Despite differing experimental procedures and crystallization of very fine-grained or amorphous the inadequaciesof individual studies (Haas and starting materials due to the entrapntent of Al3* Holdaway, 1973), it has been difficult to account in tetrahedral sites. The extent of solid solution for the wide range of results obtained in earlier would then be dependenton the proportion of these materialspresent in the startingmixtures' Thermal stability should increase with the extent of solid * solution due to the formation of H-bonds between + '^ oxygensin adjacentsilica sheets. +-b l-A 2 In this light, the decomposition temperature o $ 6 N -c o (565'C at 2 kbar) obtainedby Aramaki and Roy I aO ?; K' (1963) is readily explained.Using gels as starting 4 materials, they unknowingly produced extensive solid solutionssimilar in compositionand in apparent thermal stability to those synthesizedfrom gels in this study. (mixturesof natural minerals) s 32 Startingmaterials j used by Althaus (1969) were subject to extreme t grinding which reduced grain size to the micron rO (1969) o or sub-micronrange. Velde and Kornprobst N o used mixtures of natural and synthetic minerals, l.|. presumablyfinely ground, as well as gels to study 30 rO the decompositionreaction. In the present investi- o of kaolinite * quartz Al ?.3? , gation, finely ground mixtures a r-A a and Cab-O-Sil + gibbsite produced pyrophyllite solid solutions with apparent thermal stabilities comparableto those reported by Althaus (1969) 2A and by Velde and Kornprobst (1969) (-490"C at 6t 62 63 64 65 66 67 by grind- SiO2 (w1."/o) 2 kbar). Degtadationof crystal structures ing, particularly by dry grinding of OH-bearing (Al,O" FIa. 2. Variation of R,O' + Fe"Oa) and H,O* with minerals, is well-known. Disordered or amorphous SiO, in natural pyrophyllite. Circles, analyses from the produced by grinding may contribute an literature: l, 2, 4, 6, Deer, Howie, and Zussman,1962; A-E, material Kodama, 1958; S, Swindale and Hughes 1968; K, King and Al-,OH-rich componentto pyrophyllite crystallized Weller, 1970.Squares, theoretical end-member. under hydrothermalconditions. PYROPHYLLITE SOLID SOLUTIONS
Investigators using the more sensitive single solid solutions, if actually formed in this study, crystal (crystal-growth) method have observed could be stable or metastablewith respect to the much lower decomposition temperatures-namely, end-member;sufficient amounts of disordered ma- 410'C :t L5" at 1.8 kbar (Kerrick, 196g) and terial may have been present in starting mixtures, 370"-393"C at 2.4 kbar (Haas and Holdaway, despite careful treatment, to produce limited solid 1973)-than those obtained by the synthesis solution. method. A direct conparison of the resultsobtained The effect of compositional variations on the using these two methods is not possible because thermal stability of pyrophyllite is probably not factors other than the nature of starting materials large within the range of natural com,positions.Haas may be involved. However, the fact that the lowest and Holdaway (1973) have shownthat the natural decompositiontemperatures reported to date (Haas triclinic pyrophyllite solid solution from New Zea- and Holdaway,1973) wereobtained using materials land, analyzed by Swindale and Hughes ( 1968, which were subjected to moderate grinding and Fig. 2, S) and studied by Brindley and Wardle then elutriated to remove the finest particles is con- (197A), undergoesapparent decomposition at about sideredto be significant.Since lower decomposition 410'C and 4.8 kbar. Andalusiteweight gains are temperatures imply compositions closer to that approximatelythe same as for the monoclinic pyro- of the end-member,it seemslikely that end-member phyllite from Indian Gulch, California, under sim- pyrophyllite is not a stable phase under the condi- ilar conditions. Unfortunately, this pyrophyllite, tions of the present investigation.Reversals of the which was used as a starting material by Haas and decomposition reaction at,higher temperatures Holdaway (1973) and by Kerrick (1968), has (e.g., Kerrick, 1968) probably involve pyrophyllite not been adequately characterized. A chemical solid solutions. analysis(Haas and Holdaway, 1973) suggeststhat Haas and Holdaway (1973) report spurious it may be a solid soltuion, but the analysisappears weight gains of andalusitecrystals within the sta- to be in error. If the Indian Gulch pyrophyllite is bility field of pyrophyllite which they attribute to a solid solution, then the stability limit of the end- "a reaction of unknown nature." The unknown re- member should lie at even lower temperaturesthan action could be the formation of pyrophyllite solid those reported by Haas and Holdaway (1973). solutionsaccording to the equation, Pyrophyllite solid solutions have been shown to exist in this study, but their formation mechanism : Al, Si4O10(OI{), Al," Sia_3"Al"Oro_6"(0H)"(OH)r_" and stability with respect to the end-memberhave not yet been firmly established.While a P-7 sta- + 3XSiO,. bility field for compositionsclose to that of the end- Observedweight gains of andalusitemay have been member is possiblein the light of the presentstudy, due to the growth of quartz on thcse crystals. the apparent dependenceof the extent of solid Kerrick (1968) has carefully documentedan ex- solution on the nature of starting materials implies ample of quartz growth on andalusite.The fact that metastabilityand it seemsmore likely that all solid the spurious weight gains are observed only at solutions are metastablewith respectto the end- lower pressures(e.g., 2.4 kbar, Haas and Hold- member. Experiments are now underway to test away, 1973) is in accord with the formation of these inferences. solid solutions having larger molar volumes than the end-member. Furthermore, the frequency and Conclusions magnitudeof theseweight gains seemsto be slightly In experimentalmineralogy it is usually assumed greater in the presenceof excessAl3. (rmw re- that, if an end-member mineral composition is action, Haas and Holdaway, 1973) than in the used as'a startingmaterial and productsappear to absenceof excessAl3. (rnqw reaction,ibid.). This contain only that mineral, then the composition is also consistentwith the explanationoffered. Small of the syntheticmineral is that of the end-member. weight gains due to the formation of pyrophyllite The results of this study should serve to caution solid solutions would be observableonly within the against such assumptions. stability field of pyrophyllite close to its decomposi- Starting with end-member composition, it has tion temperaturewhere andalusitewould otherwise proved to be difficult or impossible to synthesize be expectedto lose weight very slowly. pyrophyllite theoreticalpyrophyllite even in experimentsof rela- 260 PHILIP E. ROSENBERG tively long duration, due to the intervention of sub- some aluminum silicates' Geochim. Cosmochim- Acta, stitutionsof the type A13.+ Ht : Sia*.The enlarged 21,99-109. Dnnn, W. A., R. A. Howrc, lNp J' Zussum (1962) Rocft- observedfor the resultingpyrophyllite basalspacings forming Minerals, Vol. Iil. Wiley, New York, pp. 115- solid solutions are believed to be due mainly to r20. the formation of (OH)- on the basal surface;in- Fonsps, W. C. (1969) Unit-cell parameters and optical prop- creasedthermal stability probably results from the erties of talc on the join Mg,SLO.(OH).Fe.SinOro(OH). 54, 1399-1408. formation of H-bonds betweenoxygens in adjacent Am. Mineral. (1972\ An interpretation of the hydroxyl contents to form silica sheets.Solid solutions are believed of biotites and muscovites.Mineral' Mag. 3Er 712-720' metastablyfrom fine-grainedor amorphousstarting Hem, H., lNo M. J. Horolwev (1973) Equilibria in the materials due to the entrapment of Al3- in tetra- system Al,O-SiOr-H"O involving the stability limits of hedral sites.Many natural pyrophyllites also deviate pyrophyllite and thermodynamic data of pyrophyllite' significantly from end-member composition. Am. l. S,ci.273,449464. S. B. (1938) On the crystal structure of talc previously unsuspectedpyro- HBxpntcrs, The existence of and pyrophyllite. Z. Kristallogr. 99,264. phyllite solid solutionscan accountfor discrepancies Krnnrcr, D. M. (1968) Experiments on the upper stability in the reported thermal stability of pyrophyllite in a limit of pyrophyllite at 1.8 kilobars and 3.9 kilobars general manner. Unfortunately, it is not possible water pressure. Am. I. Sci. 2661204-214- (1970) I-ow-temperature to reevaluate previous experimental investigations KrNc, E. C., eNo W. W. Wpnnn entropiesat 298.15" K of goethiteand and/or products heat capacitiesand at presentbecause starting materials pyrophyllite. lJ.S. Bur. Mines, Rep. Inrsest.7369' have not, as a rule, been adequatelycharacterized. Kooerurl, H. (1958) Mineralogical study of some pyrophyl- Reexamination of the natural and synthetic pyro' lites in Japan.Mineral' I' lapan,21236-244' phyllites of previous investigations,in the light of Lurrr. W. C., eNo C. O. INclurns (1965) Gel preparation experimentation' the presentstudy, might prove to be very instructive. of starting materials for hydrothermal A m. M ineral. 50, 255 -260' However, a definitive study will probably be neces- LvoN, R. I. P. (1967) Infra-red absorption spectroscopy'J' sary to resolve this long-standingproblem. Zussman, F;d,, PhysiCal Methods in Determinatioe Min- eralogy, Academic Press,New York, pp. 371403, Acknowledgments RosBNssnc. P. E. (1971) Compositional variations in synthetic pyrophyllite. Geol. Soc. Am. Annu. Meet', Financial support was provided by National Science Progr. Abstr., pp. 688-689. Foundation Grant GA-4483. The author is indebted to Mrs. (1972) Compositional variations in synthetic topaz' Albina Mellot for valuable technical assistance. Am. Mineral. 57, 169-187. Roy, RusruM, eNo E. F' OssonN (1954) The system References AI,O.SiO.H,O. Am. Mineral. 39' E53-885' SwrNplre, L. D., lNo I. R. Huorps (1968) Hydrothermal Ex- Arrneus, EcoN (1969) Das system Al,O.SiO.HrO. association of pyrophyllite, kaolinite, diaspore, dickite' FiiLr die perimentelle Untersuchungen und Folgerunge and quartz in the Coromandel area, New Zealand' J' N' lahrb. Petrogeneseder metamorphenGesteine, Teill. Geol. Geophys. New Zealand, l|, 1163-1183. Mineral. Abh. lll, 7 4-ll0. Vnrop, B., rNp J. KonNptossr (1969) Stabilit€ des silicates (1963) polymorph ARAMAKT,S., lNo Rusrurvr Rov A new d'alumine hydrat6s. Contrib. Mineral. Petrol.2l' 63-74' of Al,SiO" and further studies in the system ALO'-SiO- Wenorr,, Roxerp, eNo G. W. BnrNorBv (1972) The crystal H,O. Am. Mineral. 48, 1322-1347. structure of pyrophyllite, l?c and of its dehydroxylate' (1970) BnrNornv, G. W., eNo RoNero Wenors Monoclinic Am. Mineral. 57, 7 32J 50. and triclinic forms of pyrophyllite and pyrophyllite anhydride. Am. Mineral. 55, 1259-1272' Manuscript receiued,Ianuary 3a, 1973; accepted Cenn, R. M., eNo W. S. Ftrp (1950) Synthesisfields of for publication,October 25 ' 1973.