American Mi,weralogist Vol. 57, pp. 732-750(1972) THE CRYSTAL STRUCTURES OF PYROPHYLI,ITE, lTC, AND OF ITS DEHYDROXYLATE RoNer,oWennr,n AND G. W. Bnrxor,ur, Materinls R'esearch Laboratorg, artd,the Department of Geosci'ences, The Pennsgluania StateUniuersitE, [Jni'uersitAPark, Pa' Agsrnect INrnoouctroN material may have some advantages despite the limitations imposed by the powder diffraction technique. ExpnmunNtal crushed powder passing a 200-mesh screen was used. Diffractometer powder pattems were recorded with Ni-filtered cuKa radiation at l/4 (2d),/min and 732 STRUCTURE OF PYROPHYLLITE 733 with 1"(2d)/inch chart paper, and were calibrated rvith respect to silicon powder (o - 5.43062A at 21"C). Some reeordings were made with FeKa radiation. AIso Debye-Scherrer powder photographs were recorded wiih CuKa and with CoKa radiations. For intensity (see Brindley and .Wardle, data, side-packed samples were used 1970). Overlapping reflections were resolved graphically and intensities obtained by measurement of the resolved areas. Basal intensities were recorded from samples oriented from a slurry. Dehydroxylations were carried out to con- stant weight in platinum thimbles in air at temperatures ranging from 550'C (500 hr) to 950"C (5 hr). The diffractometer traces show no differencesin samples fully dehydroxylated over this temperature range. Srnucrunn ANer,vsrs Unit CeLIs The unit cell parametersof pyrophyllite. lTc and its dehydroxylate phase previous reported (Brindley and Wardle, 1970) have been refined by the least squares refinement computer program due to Evans,Appleman, and Handwerker (1963),with the followingresults: Pyrophyllite, 1 Tc Pyrophyllite, 1 Tc, dehydroxylate a 5.1614+ 0.00L A 5.191,+ 0.001,;. b 8.9576+ 0.002' 9.1221+ 0.0014 c 9.351r + 0.0018 9.4990+ 0.001u d. 91.030 + 0.02 91.170+ 0.02 p 100.37" + 0.02 100.21" + 0.02 89.750 + 0.02 88.620 + 0.02 v 425.21 + 0.1, i.' 442.5a + 0.1, ;,' These data differ only slightly from those previously given but the departureof 7 from 90o is an interestingresult. In particular it pro- vides indexing for a reflection from the dehydroxylate for which no indiceswere found previouslywith y = 90o (seeBrindley and Wardle, 1970,p. 1266). Idealiaed,Structu.re of Pgrophgllite,1Tc Previouswork has shownthat pyrophyllite has a 2:1 layer struc- ture, with a dioctahedral Al, O (OH) sheet betweentwo Si, O tetra- hedral sheets.The structural arrangementcan be developedwith the Al ions at the zero level with respectto c in the unit cell and in con- formity with a C-centeredcell. In the ideal model, close pagked planes of O, OH anions are placed above and below the level of Al ions. The possiblearrangements within a one-layer cell (seeFig. 1) are obtained as follows: Three possibleoctahedral cation sites are occupiedby 2 Al and one vacancy,the latter occupyingone of the 7M RONALD WANDLE AND G. W. BRINDLEY O( -\ ( 6)'i \ C,o r( O(r\ r:) o O( (iro Frc. 1. Possible positions in. arrangement A of upper and lower hydroxyls shown by full a.nd dashed Iarge circles respectively, and possible positions of octahedral cation vacancies shown by small circles. three sites labelled I, II, IIL In the anion planes of composition O2(OH),the (OH)- ionscan occupyone of the positionslabelled 1,2, 3, and one of the positionsl',2',3'. Thesetwo planescan be respec- tively above and below the Al plane, as shown by full circles and dashedcircles respectively in Figure 1, (arrangementA), or vice versa (arrangementB). The oxygen ions in these planes form the apices of the SiOr tetrahedra.In the ideal model,the positionsof the silicon atoms and of the associatedbasal oxygen atoms are then fixed. The total number of arrangementswithin the single layer cell is 3 x 3 x 3 x2=54. The hydroxyl combinationswill be labelledas follows: STRUCTUREOF PYROPHYLLITE 735 1 = (1,7'),2: (1,2'),3: (1,3'),4 = (2,I,),8 = (2,2,),G,=(2, 3'),7 = (3, 1'), 8 = (3, 2') and,g: (8, B,),and eachcombination has the A or the B anion plane arrangement.Preceding each arrangement 1, 2, . ' 9, a symbol I, II, III, denotesthe vacant cation site. The ideal layer structureconforming most, closely with the observed diffractionintensities was determinedby calculatingthe intensitiesfor the first twenty reflectionsof the 54 possible ideal structures using a computerprogram written by Smith (1968,1967). The atomic scat- tering factors built into the computer program were from self-con- sistentfield calculationsfor Alst and Siaiand from a Suzuki curve for O'-; a temperaturefactor B = 1.0 was usedfor all atoms.The results showedthat only 12 different intensity distributions resulted from the 54 arrangements.Table I showsthe equivalentAl, OH arrangements, each of which has the A and B arrangementof O, OH anion planes. of these twelve arrangements,one gives an intensity distribution con- siderably nearer than the others to the observeddata, namely the equivalent structures IzL, II7A", and 1116A. The detailed intensity data are given elsewhere(Wardle, 7972).Projections of theseequiv- alent structures on (001) show that they differ only in the position of the origin along the b axis. ArrangementI2A has a centerof sym- metry at the origin. Refi,nedStrwchne ol Prophgllite,lTc The idealizedstructure has beenrefined by a trial and error process on the basisof (i) the known crystal-chemicalcharacteristics of.2:l type layer silicates,and (ii) the fit betweenobserved and calculated X-ray diffracted intensities, and observedand calculated diffractom- eter patterns. As shownby Radoslovichand Norrish (1962),a morenearly correct Table 1 Egulvalent alumlnum, hydroxyl arrangements IrL = l'5 = II|]- = II,9=II'II5=f.II'9 It3 = !,8 = II,4 = IIrS= IfL3= IfL4 Tt6 = !,7 = II,2 = Tl'6= TII;2= IIIt'l fr2*IIr7=III,6 Ir4 = Ifr3 = fII,8 It9 = lll5 = fILI 736 RONALDWARDLE AND G, W. BRINDLEY layer model is obtainedby rotating the SiOr groupsin eachhexagonal ring about c'* alternately in opposite directions so that the basal hexagonalring becomesditrigonal. The magnitude of this rotation can be estimatedfrom the observedb parameter'an assumedvalue for the Si-O distance,1.62;Ft, and a regular tetrahedralgeometry' For a given pair of SiO+tetrahedra, one in the upper and one in the lower tetrahedral sheet,the rotations may be in the same or in opposite directions,and canbe denotedby (*, +), (+, -), (-, +), (-, -)' The basal oxygen positionswere modified by rotations of i10" for eachof thesefour schemesand the intensitieswere calculatedfor the first thirty reflectionsof the I1A, "' , I9A structures'The results showedthat the I2A and equivalent structures gave the best fit with the observedintensities with rotations (*, -), i.e.,in oppositedirec- tions for the two tetrahedral sheet's;the senseof the rotations is in- dicatedin tr'igure2. For dioctahedrallayer structures,it is well established(see survey by Bailey, 1966)that the Al, O(OH) dioctahedralsheets depart from the ideal arrangementdepicted in Figure I principally in the following ways: the sharedoctahedral edges are shortenedcausing a counter- rotation of upper and lower O, OH triads, and the sheetthickness is modifiedso that the sheetdimensions parallel io (001) conformwith the o and b parameters.It appearsalso that the aluminumions do not depart from the hexagonalarrangement depicted in Figure 1' If each triad OzOH of octahedral anions lies parallel to (001) with equal O-O and O-OH distances,then their coordinatescan be related to two parameters,namely the Al-O, OH bond length and the octahedral sheetthickness. In the fi.rst stage of refinement,the octahedral sheet thickness was held constanbat the value2.14 A givenby Raynerand Brown (1966), and the Al-O, OH bond length was varied to give projectionson (001) ranging from 1.55-1.65A thereby coveringthe likely value of the bond length. Each projecteddistance gives possiblepositions for the O, OH anions,and the ideal model already selectedenables the individual O and (OH) ions to be identified.The oxygenions are the apical oxygensof the tetrahedral sheets and must be bridged by Si-O-Si bonds. When the tetrahedra are assumedto be regular with Si-O = 1.62 L, this bridging of the apical oxygensimposes twists and tilts on the tetrahedra and also severelimitations on the projected Al-O, OH lengthswhich must lie in the range 1.63-1.64A. The twists of the tetrahedra derived from this geometricalapproach are of the same magnitude,:tlQo, and in the same senseas those already ob- tained. STRUCTA NE OF PYROPHYLLITE 737 \-i ---\ri o ,\ O 5oro1+033d0353 o.. Jt +0.299 +ottg L, (/o,oH OAI oo /A -orr3 U Q) o,oH o Si -0289 Frc. 2. Layer structure of pyrophyllite projected on (001). Ideally the thickness of the octahedral sheet should be determinable from an 001synthesis but with 007 as the highest observablebasal reflectionthe synthesisdid not providesufficient accuracy. Accordingly diffracted intensities were calculated for structures with octahedral sheetthicknesses ranging from 2.14to 2.02L and a value 2.08'A was selectedwhich agreeswell with the data collectedby Bailey (1966). An isotropictemperature factor B was used for all atoms; values of B ,= 0.5, 1.0,and 1.5 were usedand B = 1.0 gavea slightly better fit to the experimentaldata than the other two. The calculatedintensities RONALD WARDLE AND G. W. BRINDLEY 3 AdNOONN;Fm6OAo6 Fl $ s3q$iqBHHpsS qa F ,++/+d- p i+oNAido Nh3::tt: A g$$$9il$$f, ssspsd e$$$r. =, -,',eEi =: -t BFfi $nHi z { 11 1 {1 1 -1 j j ; ; ; ; ..i ; i ; -.i..i -i ..i..i i €{ fi'8grf i,n E n'H s's 5 R'E p',H'HrErH'E'$'ff SErf, 5 $ a + N N 6 HH A4 p N N <! I N N 3 I zo x ; F 4 g I g:P e .b' $d q5 S Sq q a6 aF € 5 € s' { j j I {q .i -' .i ''i ; -:a zz ; ; ; : r-;-;-;=;;; o ; a-;-46-A-a * fi 6"'';-r 6? ' - ?;==;-: iY a I ^E a- Re € e Sn t h s s sR€g s RE s RB g$ q €.ts€q q$Ea q q q6 q$HH q 3 4 +,;.NFgS: 3 S € E S s s s s 5 s 5 c s E I 4 44A44d1 I I n 3F E ::::;;;;:-ijjj-ii;;.;.i;j.;i- P^ YE In'f;g rfi '*'H <r E€,: fi l$ R3'H fi rHf, H,SH i'$'ft r$I HH'S S€ EA * fi N zv dda + .