American Mi,weralogistVol. 57, pp. 732-750 (1972)

THE CRYSTAL STRUCTURES OF PYROPHYLI,ITE, lTC,

AND OF ITS DEHYDROXYLATE

RoNer,o Wennr,n AND G. W. Bnrxor,ur, Materinls R'esearchLaboratorg, artd, the Department of Geosci'ences,

The Pennsgluania State UniuersitE, [Jni'uersitA Park, 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 - 5.43062 A at 21"C). Some reeordings were made with FeKa radiation. AIsoDebye-Scherrer powder photographs were recorded wiih CuKa and with CoKaradiations. For intensity data, side-packed samples were used (see Brindley and.Wardle,

1970). Overlapping reflections were resolved graphically and intensitiesobtained by measurement of the resolved areas. Basal intensities were recordedfrom 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 differences in samplesfully dehydroxylated over this temperature range.

Srnucrunn ANer,vsrs

Unit CeLIs

The unit cell parameters of pyrophyllite. lTc and its dehydroxylatephase previous reported (Brindley and Wardle, 1970) have beenrefined by the least squares refinement computer program due toEvans, Appleman, and Handwerker (1963), with the following results:

a

bcd.

p

v

Pyrophyllite, 1 Tc

5.1614 + 0.00L A8.9576 + 0.002'9.351r + 0.0018

91.030 + 0.02100.37" + 0.0289.750 + 0.02

425.21 + 0.1, i.'

Pyrophyllite, 1 Tc, dehydroxylate

5.191, + 0.001, ;.9.1221+ 0.00149.4990 + 0.001u

91.170 + 0.02100.21" + 0.0288.620 + 0.02

442.5a + 0.1, ;,'

These data differ only slightly from those previously given but thedeparture of 7 from 90o is an interesting result. In particular it pro-vides indexing for a reflection from the dehydroxylate for which noindices were found previously with y = 90o (see Brindley and Wardle,1970, p. 1266).

Idealiaed, Structu.re of Pgrophgllite, 1Tc

Previous work has shown that pyrophyllite has a 2:1 layer struc-ture, with a dioctahedral Al, O (OH) sheet between two Si, O tetra-hedral sheets. The structural arrangement can be developed with theAl ions at the zero level with respect to c in the unit cell and in con-formity with a C-centered cell. In the ideal model, close pagkedplanes of O, OH anions are placed above and below the level of Alions. The possible arrangements within a one-layer cell (see Fig. 1)are obtained as follows: Three possible octahedral cation sites areoccupied by 2 Al and one vacancy, the latter occupying one of the

7M RONALD WANDLE AND G. W. BRINDLEY

O((

- \

6)'iC,

O(r\r:)

O((ir

\or(

oFrc. 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 compositionO2(OH), the (OH)- ions can occupy one of the positions labelled 1,2,3, and one of the positions l',2',3'. These two planes can be respec-tively above and below the Al plane, as shown by full circles anddashed circles respectively in Figure 1, (arrangement A), or vice versa(arrangement B). The oxygen ions in these planes form the apicesof the SiOr tetrahedra. In the ideal model, the positions of the siliconatoms and of the associated basal oxygen atoms are then fixed. Thetotal number of arrangements within the single layer cell is 3 x 3 x 3x 2 = 5 4 .

The hydroxyl combinations will be labelled as follows:

o

STRUCTURE OF 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 each combination hasthe A or the B anion plane arrangement. Preceding each arrangement1, 2, . . ' 9, a symbol I, II, III, denotes the vacant cation site.

The ideal layer structure conforming most, closely with the observeddiffraction intensities was determined by calculating the intensities forthe first twenty reflections of the 54 possible ideal structures using acomputer program written by Smith (1968, 1967). The atomic scat-tering factors built into the computer program were from self-con-sistent field calculations for Alst and Siai and from a Suzuki curve forO'-; a temperature factor B = 1.0 was used for all atoms. The resultsshowed that only 12 different intensity distributions resulted from the54 arrangements. Table I shows the equivalent Al, OH arrangements,each of which has the A and B arrangement of O, OH anion planes.of these twelve arrangements, one gives an intensity distribution con-siderably nearer than the others to the observed data, namely theequivalent structures IzL, II7A", and 1116A. The detailed intensitydata are given elsewhere (Wardle, 7972). Projections of these equiv-alent structures on (001) show that they differ only in the positionof the origin along the b axis. Arrangement I2A has a center of sym-metry at the origin.

Refi,ned Strwchne ol Prophgllite, lTc

The idealized structure has been refined by a trial and error processon the basis of (i) the known crystal-chemical characteristics of.2:ltype layer silicates, and (ii) the fit between observed and calculatedX-ray diffracted intensities, and observed and calculated diffractom-eter patterns.

As shown by Radoslovich and Norrish (1962), a more nearly correct

Tab le 1

Egulvalent alumlnum, hydroxyl arrangements

I r L = l ' 5 = I I | ] - =

I t 3 = ! , 8 = I I , 4 =

T t 6 = ! , 7 = I I , 2 =

f r 2 * I I r 7 = I I I , 6

I r 4 = I f r 3 = f I I , 8

I t 9 = l l l 5 = f I L I

I I , 9 = I I ' I I 5 = f . I I ' 9

I I r S = I f L 3 = I f L 4

T l ' 6 = T I I ; 2 = I I I t ' l

736 RONALD WARDLE AND G, W. BRINDLEY

layer model is obtained by rotating the SiOr groups in each hexagonalring about c'* alternately in opposite directions so that the basalhexagonal ring becomes ditrigonal. The magnitude of this rotationcan be estimated from the observed b parameter' an assumed valuefor the Si-O distance, 1.62;Ft, and a regular tetrahedral geometry' Fora given pair of SiO+ tetrahedra, one in the upper and one in the lowertetrahedral sheet, the rotations may be in the same or in oppositedirect ions, and can be denoted by (*, +), (+, -) , (- , +), (- , -) '

The basal oxygen positions were modified by rotations of i10" for

each of these four schemes and the intensities were calculated for thefirst thirty reflections of the I1A, "' , I9A structures' The resultsshowed that the I2A and equivalent structures gave the best fit withthe observed intensities with rotations (*, -), i.e., in opposite direc-tions for the two tetrahedral sheet's; the sense of the rotations is in-dicated in tr'igure 2.

For dioctahedral layer structures, it is well established (see surveyby Bailey, 1966) that the Al, O(OH) dioctahedral sheets depart fromthe ideal arrangement depicted in Figure I principally in the followingways: the shared octahedral edges are shortened causing a counter-rotation of upper and lower O, OH triads, and the sheet thickness ismodified so that the sheet dimensions parallel io (001) conform withthe o and b parameters. It appears also that the aluminum ions do notdepart from the hexagonal arrangement depicted in Figure 1' If eachtriad OzOH of octahedral anions lies parallel to (001) with equalO-O and O-OH distances, then their coordinates can be related to twoparameters, namely the Al-O, OH bond length and the octahedralsheet thickness.

In the fi.rst stage of refinement, the octahedral sheet thickness washeld constanb at the value 2.14 A given by Rayner and Brown (1966),

and the Al-O, OH bond length was varied to give projections on(001) ranging from 1.55-1.65 A thereby covering the likely value ofthe bond length. Each projected distance gives possible positions forthe O, OH anions, and the ideal model already selected enables theindividual O and (OH) ions to be identified. The oxygen ions are theapical oxygens of the tetrahedral sheets and must be bridged bySi-O-Si bonds. When the tetrahedra are assumed to be regular withSi-O = 1.62 L, this bridging of the apical oxygens imposes twistsand tilts on the tetrahedra and also severe limitations on the projectedAl-O, OH lengths which must lie in the range 1.63-1.64 A. The twistsof the tetrahedra derived from this geometrical approach are of thesame magnitude, :tlQo, and in the same sense as those already ob-tained.

STRUC TA NE OF PYROPHYLLITE

\ - i

- - - \ r i

O 5oro1 +033d0353o . . Jt +0.299

L, (/o,oH +ottg

O A I o o/A

U Q) o,oH -orr3

o Si -0289

Frc. 2. Layer structure of pyrophyllite projected on (001).

Ideally the thickness of the octahedral sheet should be determinablefrom an 001 synthesis but with 007 as the highest observable basalreflection the synthesis did not provide sufficient accuracy. Accordinglydiffracted intensities were calculated for structures with octahedralsheet thicknesses ranging from 2.14 to 2.02 L and a value 2.08'A wasselected which agrees well with the data collected by Bailey (1966).An isotropic temperature factor B was used for all atoms; values ofB ,= 0.5, 1.0, and 1.5 were used and B = 1.0 gave a slightly better fitto the experimental data than the other two. The calculated intensities

737

,\o

RONALD WARDLE AND G. W. BRINDLEY

, + + / + d -

i + o N A i d o N h 3 : : t t :

i € { fi '8 grf i,n E n'H s's 5 R'E p',H'HrErH'E'$'ff S Erf, 5 $

++'+ .---_^-H d d d N 6 m o { N N q o d ) N 6 N n n N N d 6 d o 6 d d + d a N i N

a a

E'N R' i f i f l H $rNE,$ f i H 'drndt 'R,a ' ,d f r N s 'E ' f iTR'$H SH B'S '$

s s g 3 n @ - e s s 5 ^ - t s h s t s @ + N 6

A d N O O N N ; F m 6 O A o 6

$ s3q$ iqBHHpsS qa

s sspsd e$$$ eEi BFf i g$$$9i l$$f , $n Hi{ 11 1 {1 1 r . = , - , ' , = : - t

-1 j j ; ; ; ; . . i ; i ; - . i . . i - i . . i . . i

3

Fl

F

p

A

z

a

A 4z o

z zi Ya^ Ea -

3FP ^Y E< r

z v

a z> o

a 2

. - Hr

- : a

! L

, 2 ,H F

O Z

14

t

F

Fd

Xrid

L.

p N N < ! I N N + N N 6 H H 3 I

x; F 4 e g d 5 S S I g : P 6 F€ 5 € s' .b' $ q { q q I {q a a; ; ; . i - ' . i ' ' i ; j j - : a

: r-;-;-;=;;; o ; a-;-46-A-a * fi 6"'';-r 6? ' - ?;==;-:

It h s s sR€ s R E s R B $ R e € e t s€ $Ea 6 $HH 3 Sn NFgS :3 S € E S s g s s s 5 s 5 g c s E q €. q q q q q q I q 4 4 +, ;. 1 I I nE : : : : ; ; ; ; : - i j j j - i i ; ; . ; . i ; j . ; i -

4 4 A 4 4 d

E €,: fi l$ R 3'H fi rH f, H,S H i'$'ft r$ I H H'S S€ E A * In'f; g rfi '*'H fi

d d a + F .

n S b o o d J + + N N o N

$$s $ $E € s aBHsRg s*E s++ q€ i o ; o i d o ; d o i N o N d d d d

6 o $ @ 3 E 3 F € ! f i H S q q qR . € . q E q q E q q l q q \ \ q q to 5 + $ J J O O O 6 0 A N N O N N

''--i- *S d I e€ g 6 - * p 9 B g - - j R - f t $ * t g - =

5R?FRf i 6pSqSf i EHHsp **R nAf l Re i? gHi { q q"i

+ -' + i i + "i

d -i "i

di ni 6i o; o; .; o; ai ai d d oi d oi o; a; oi oi N N N d

g g : g E E'6 E i ' i H $ E, i i ' i ' i i ' ,3 'H f i E R' f i H E' i H i H f l H

t $ l l i t ir ln:::: i : l i i : i t i : lgl*is99:9:; o o o a j o o o N N N N N N o N i H d i

H

H

H

STRUCTURE OF PYROPHYLLITE ?39

for the optimum arrangement are given in the form of a comparisonof observed and calculated intensities (Table 2) , and, also, followingSmith (1968), as a comparison of the observed and computer-simu-lated diffraction patterns; the latter was drawn using a CalcompPlotter (Figure 3).

The finally selected atomic coordinates which are consistent with acenter of symmetry at the origin, the interatomic distances, and thebond angles are given in Table B. Figure 2 shows a projection of thelayer structure on (001), with the basal oxygen networks indicatedby dashed lines. The rotations of the SiOa groups away from thehexagonal arrangement are readily seen ,and are in opposite senses,as stated earlier. The angle of rotation measured from the. .projectionis ilOo. The tilt of the tetrahedra, indicated by the positions of thesilicon atoms with respect to the apical oxygens, is close to 4o. Figure4 shows a projection of adjacent basal oxygen networks viewed alongc* and the a-parameters of the oxygens are indicated.

An aly si,s of Pg rophyllite, 1 T c, D ehy dr orgl,ateThe similarity of the powder diffraction patterns of the initial

PYROPHYTLITE ITc

F\c. 3. Experimental (B) and computer-simulated (A) diffractometer patternsof pyrophyllite, lTc.

740 RONALD WARDLE AND G. W. BRINDLEY

fable 3

AtoElc CooralLnates, IDteratoElc dl.stances and augles for Pyrophylllter

Itc.

Atool.c Coorallnates xr Yr zi -x, -Yr -zi ll2 ! x, ll2 ! y, z'

Center of syoretry at (0, 0, 0). Space group Cf'

0b slgniftes o)rygens I'n basal netnorks; 0 are apical o)<ygeus'

al 0.500

s l (1 ) 0 .748

s1(2) 0 .759

0(1) 0 .671

o(2) o.72L

0 .167 0 .000

0 .000 0 .289

0 .331 0 .289

0 .004 0 .113

0.319 0.113

ff, and angles.

0E 0.221 0.186 0.113

ob (1 ) o .os5 0 .387 0 .353

ob (2 ) 0 .724 0 .167 0 .353

ob (3 ) 0 .550 0 .448 0 .335

Iateratonlc distances r

3104 tetrahedla:

A106 octahedra:

0(0n) parallel to laYers'

0(0H) tnclined to layers'

ehbred octahedral edges'

neao Si-0 ' 1.62

Eean O-O' 2.64; O-S1-O' 109.5o

nean Al-0(0H) ' 1.94

nean 0-0, 2.85; O-A1-O' g4o

mean O-0, 2.82; O-A1-0' 93o

nean 0-0, 2.49; 0-A1-0, 8Oo

Interlayer dLstances: (Dlstances > 3.4R are onltte'l)

Ob( l ) . . .0b(6) correspon<l to oxygens label led l , ' ' '5 ln Flgure 4 '

Ob(5), Ob(4) and 0b(6) are re lated by the center of synnetry to 0b(1) '

0b(2) and 0b(3) tespect lvely.

ob(2) - ob(5) , 3 .12

o b ( 3 ) - o b ( 4 ) , 3 . 1 2

o b ( 3 ) - o b ( 6 ) , 3 . 3 2

ob (1)

0b (2)

0b (2)

- 0 b ( 4 ) ,

- 0 b ( 4 ) ,- 0b (5 ) ,

) . J t

3 . 0 7

3 . 3 7

mineral and of its dehydroxylate indicates that dehydroxylation pro-

duces only small structural changes. Two approaches to the structure

analysis have been considered. (i) A direct approach examines the

effect of the dehydroxylation on the 001 Fourier synthesis. (ii) An

intuitive approach considers that adjacent pairs of (oH)- anions (see

Figure 2) react to form O'- and HzO and that the residual O'- ions

remain in the layer structure either distributed over the original (on;-

sites, or in midway positions at the a-level of the Al ions.

STRUCTU RE OF PYROPHYLLITE

t

Frc.4. Projection of adjacent basal oxygen networks viewed along c*. Uppernetwork solid circles, lower network dashed circles. Oxygens labelled l, 2, ... 6refer to Ob(l), ... , Ob(6) given in Table 3; z-coordinates (X1000) are shown.

The 001 syntheses of the initial and dehydroxylate forms of themineral are shown in Figure 5. For the latter synthesis, the signs ofF' (00r) were taken first to be the same as those for the initial mineral.The resulting electron density curve showed a diminution of the elec-trons associated with the O, OH ions and a corresponding increase ofelectrons associated with the Al ions aL z = 0 which points to removalof OH ions and location of the residual oxygen at the a = 0 level.This result is consistent with the intuitive approach mentioned earlier.Accordingly, the signs of F(002) were recalculated with pairs of (OII)-

741

742 RONALD WARDLE AND G. W. BRINDLEY

0 0.1d(oor)

Frc. 5. One-dimensional Fourier syntheses of pyrophyllite, full line, and its

dehydroxylate, dashed line'

ions replaced by a residual O'- ign lqcated in the plane a = 0' Only

the sign of F(004), a weak reflection, required changing. The resulting

Fourier synthesis is shown by the broken line in Figure 5. The resolvedpeak areas have been normalized with respect to 100 electrons in the

combined SiaO6 peak. For the initial pyrophyllite, 55 electrons are

found in the Oe(OH)e Peak and25 electrons in the lz(Ll| peak as

compared. with 60 and 20 respectively from the atoms involved. The

total electrons, howevet, are 180 as expected. For the dehydroxylate,36 electrons are found in the O+ peak and 32 electrons in the % (A1+Oz)

peak, as compared with the expected values 40 and 30 respectively;

the total electrons are 168 as compared with 170 expected.The model which emerges from the replacement of 2(OH)- ions by

O'- in a midway position involves a five-fold coordination of Al by

oxygen. Supporting evidence for this model has been obtained from

measurements of the AlK, fluorescence wavelength which has beenshown to vary almost linearly with the average Al-O distance in a

number of crystals for which the average Al-O distances are ac-curately known (Wardle and Brindley, 1971). These measurementslead to an average Al-O distance of about 1.80 A in pyrophyllite de-hydroxylate as compared with 1.70-1.74 A- tot AlrY and 1.9G-1.94 for

AIvr. Evidently a mean A1-O distance of 1.80 A could correspond withAlv or with a l:1 mixture of AlrY and Alvr. A mixture of four-fold andsix-fold coordinated Al ions cannot be ruled out )r priori, but since the

co

atCo!o

u

05o2

\ oto\

\\

AI

o,oH

II

LI

II

\\ I

II I

STRUCTURE OF PYROPHYLLITE 745

environments of all OH-OH pairs are identical in the pyrophyllitestructure, it is not unreasonable to suppose that the residual o*ygun,also will have identical environments.

-

A detailed model has been developed with the, residual oxygens inthe Al plane at z = 0, with five Al-O distances of approximaiely t.g0A, and o-o distances in the range 2.85*8.10 A. These conditions in-volved displacements of the Al ions towards the va.cant octahedrarsites and related movements of the o ions; arso adjustments of thesio+ tetrahedra were necessitated by the movement of the apicaroxygens and the assumed regular form and size of the tetrahedra.various atomic coordinates were derived to conform with these re-quirements; those finally chosen as giving the best agreement betweenobserved and calculated intensities (Table 4) and between observedand computer-simulated diffraction patterns (Figure 6) are risted inTable 5. The resulting structure has a center of symmetry and the co-ordinates in Table b are based on axes having th! origin at the centerof symmetry. The labelling of the basal oxygens Ob (l), . . . , Ob (6) inTable 5 is consistent with that used in Tanre B and Figure 4. tr'igure 7shows the Al-o sheet of the dehydroxyrate structure projected on!9otl. The basal oxygen networks are similar to those depicted inFigure 2 and therefore are omitted. from Figure z in order to showmore clearly the structure of the Al-o sheet. The relations betweenadjacent basal oxygen planes also are similar to those shown in Fig-ure 4.

Drscussror

Struchne Reliabititg

The reliabilitv factor.E = r lr'(obs) -F(carc) l/rP(obs) is not

applicable to the present analysis because of the many unresorvedreflections. A sirnilar factor B', defined as folrows, thereiore has beenused:

R, : D l.I(obs)'/, _ r(calc),/,1/Dl(obs),/"For unresolved groups of reflections, .I(obs) and rikewise -I(calc) arethe sums of the intensities of the unresolved reflections.

For the pyrophyllite lTc structure, .8, based on the first thirty_onecalculated reflections for the ideal structure is 24.0 percent. For thesame reflections, with the tetrahedral groups rotated =10 degrees butwith no change in the octahedrar groups, R' = 7z.g percentlFor thefinal model, with modified octahedrar and tetrahedral sheets, g, forall the reflections listed in Tabre 2 becomes 10.6 percent. For the de-

RONALD WARDLE AND G. W. BRINDLEY

6 d d d + N d F 6 N

6 d d t-i-; ^c? ;: t*eet ol]-?;-i ;;-;; 6

E,f i,tsB ts'5 n 5 *,fi ,tfi 'E H'H:fl'ErW',S"Hrfl E'$ $'E

Q d o d d d d + d F

9 S R 3 A

;-;--83;F;-e-46-il;-i N N o a . :?-f;' 6^i :;;-: *

E H $ $f; H'tsrff $'f; 'fl ,3 i'$rH'p i'f; 's'ff HH tsE:HrtsE'fl *'S fi H E'i'H

F O H 6 O O / N N N

s s s 6 e E d g 8 ) q $ € 3 $ 8 f r [ ]dr in i,i E. , i o ; d J d d o ; a ; o i N

d N ; - i T ; - i ; i ^ i - . ? ^ i ' 6 / + d C i * ' - ' ' i - l - '

E f i 'E!*,R 5 fi,f l 'H'N',E fi 'E H!fr d g'f if i ft ' ,$ g'5'?Eri 'f, '$

3 5 F g s d 3 o N o + N S € i . * I I 9 : o d 1

A + . - _ . F o _ .

E * . g s g n d i i : - . i 6 9 o @ 6 F e i F d F

a s

3::::iii i:ii:li l i i lBlf ili

qq qq A

i tqii iqgg::::sl::g 35I55B}ii3iiS:i

****1*33::::33:::i$lig:9Elll l

s,*s$snr5 € f ls*a lE{€ . q a q3 H. f il i * Y i 1 ; ; ; ; . ; ; a ; " i d d i d o i o i o i " i

* N N N

*:51lIl5ss5:$s 3:t55$n$5:*:::tE g g iEEri 'E E i, i ' f i H'E B E Hri 'f l ' f l 'H R E!f, f; ' f l 'H'f i A!fl

scs s $ q $ E 4q q; ) ; j j . i i ; H -

H

o

o

@ @

F F 9

744

A O E

+ _ u 9 V l

o r E

STRUCTURE OF PYNOPHYLLITE 746

PYROPHYLLITE ITC DEHYDROXYLATE

Frc' 6. Experimental (B) and computer-simulated (A) diffractometer paf,rernsof pyrophyllite, lTc, dehydroxylate.

hydroxylate structure and using all the reflections listed in Table 4,R' :71.2 percent.

On the basis of these results, and the very close fit between theobserved and the calculated diffractometer patterns, Figures B and 6,it is concluded that the derived structures approach the true structures.Further refinement on the basis of X-ray powder data is not war-ranted.

The Structures of PgrophEllite lTc and its Dehydroryl,ate

rn the case of pyrophyllite, the octahedral and tetrahedrar sheetsare similar to those of other dioctahedral layer silicates. of particularinterest are the interlayer relations illustrated in Figure 4. Because theSiOa groups are tilted with respect to (001), one oxygen of each basal

746 RONALD WARDLE AND G. W. BRINDLEY

table 5

AtoEtc Coordlnate3, IsteratoElc dtstabces aail angles for Pyrophylllte'

llc, dehydroxylate.

marked tendency for oxygens of one layer to fit between- two or three

oxygens of the adjacent layer along lines parallel to [110]. This re-

latiJnship may well be a consequence of attractions between Si** ions

Atoulc cooEdinatea. x' lt 2i -xt 'y, -zi !12 t x. U2 ! y' z'

Certer of sy@try at (0' 0' 0). Space grouP cl.

A1 0.552

s1(1) 0.225

sl(2) O.749

Or 0.250

0(1) 0.123

5104 tetrahedra3

Al05 trLgonal bipyraEid:

0r- (1)

0r- (2)

0(1)-0 (2)

0(1)-0(3)

o(2) 0.728 0.292 0.115

ob(1) 0.037 0.378 0.355

ob(2) o.7r7 O.L52 0.355

ob(3) 0.522 0.425 0.320

0.149 0.000

0.486 0.286

0 .312 0 .286

0.250 0.000

0.492 0.115

Interatoolc distances' 8, and agles.

Ee s1-0 , 1 .62

reaa 0-0, 2.65

A1-0r ' 1 .80

@& A1-0 , 1 .82

o r - 0 ( 4 ) , 3 . 1 r

0 r - 0 ( 3 ) , 2 . s 7

o ( 3 ) - o ( 4 ) , 2 . 7 9*o (2 ) -o (4 ) , 2 . 34

o (1 ) -o (4 ) , 3 . 18

o (2 ) -o (3 ) , 3 , 65

o-s1-0, 109.50

0r-A1-o(1) (4)r 1190 .

or-A1-o (2) (3) r 9oo

o(1), (3)-A1-o(2), (4) , looo

o(1), (2)-A1-o(3), (4) , 8oo

o ( 1 ) - A 1 - O ( 4 ) , 1 2 3 0

o ( 2 ) - A r - o ( 3 ) , 1 8 0 0

*shared eclge

Inter layer dlstances: (dtstuces > 3.4R are ontt te ' t )

ob( l ) -ob(4), 3.28

0b (2 ) -0b (4 ) , 3 . 22

ob (3 ) -ob (4 ) , 3 . 23

0(3) and O(4) are related resPectively to O(I) and 0(2) by a ceuter of sye€tr)r '

Ob(5) , Ob(4) an<f 0b(6) a re re le ted s in l la r l v to 0b(1) ,0u(2) aud 0b(3) '

o b ( 2 ) - o b ( 5 ) , 3 . 2 8

0b (2 ) -0b (6 ) , 3 . 23

STRUC TU RE OF PYNOPHYLLI?E 741

O olo ooo

the

zO +olrs

Frc. 7. The Al-O sheet of

O -o:ts

dehydroxylate structure projected on (001).

increase indicated an untwisting of the tetrahedral sheets following thereorganization of the octahedtal sheets. The present analysis indicatesthat the expansions arise largely from the rearrangement of the Al ionsand the insertion of oxygen ions in the Al planes. Seen in projectionon (001), the tetrahedral twists are not much changed.

The five-fold coordination of Al is an interesting result. Previousdiscussions involving pyrophyllite dehydroxylate (Heller et aI., lg62;Brett et al., 1970) have mentioned the possibility of a five-fold co-ordination but definite evidence has been lacking. Heller et at. (1962)stated that infra-red evidence precludes four-fold coordination but"does not exclude five-fold eoordination". The form of the five-foldgroup now found is approximately a trigonal bipyramid and is similar

748 BONALD WARDLE AND G. W. BRINDLEY

to that found in andalusite (Taylor, 1929; Burnham and, Buerger,

1961) for which the latter authors found an average Alv-o distance

of 1.836 A and a'bent'pyramidal axis with O-AI-O = 162"' In the

and Buerger.

T he D ehy drorylation R eaction

There has been considerable argument (see review by BteLt, et aI.,

1970) wheiher dehydroxylation of hydrous layer silicates proceeds

by homogeneous or inhomogeneous processes' The reaction in pyro-

phyllite appears to be homogeneous with pairs of (oH)- ions reacting

Iocally to form 02- and. Hzo. An inhomogeneous reaction is visualized

as a migration of protons to favorable reaction sites with a counter-

migration of cations, Al3' ions, to maintain electrical neutrality. on

this hypothesis, oxygen; ions are not lost from that part of the struc-

ture which forms the dehydroxylate, but only from reaction zones

where they combine with protons to form water and liberate Al3* ions

for counter-migration.These mechanisms have been discussed particularly by Nicol (1964)

in relation to a comparable problem involving the dehydfoxylation of

muscovite, which had been studied earlier by Eberhart (1963)"The

latter also obtained 001 syntheses of muscovite and its dehydroxylate'and concluded that a homogeneous reaction replaced 2(OH)- by O'z- in

the plane of the Al ions. Nicol (1964) criticized the conclusion of Eber-

hart and concluded that the i,nhomogeneous mpchanism "explains the

observed data at least as well as the homogeneous one". It is difficult

to reconcile the inhomogeneous mechanism with a coordination of Al

other than six-fold. The AlK" wavelength measurements clearly in-

dicate a ilecrease in coordination; four-fold coordination is precluded

by the infrared data. The available data, including the present struc-

ture analysis, all indicate a five-fold coordination and on this basis it

is considered that the pyrophyllite dehydroxylation reaction (and

probably also that of muscovite) is homogeneous.The increase in c and d(001) on dehydroxylation is related partly

to the increased tilt of the sio+ tetrahedra, which increases the effec-tive thickness of each tetrahedral sheet by 0.037 A, and partly to the

increased thickness of the Al, o sheet by 0.071 A. The total increase,

0.037 + 0.071 + 0.037 : 0.145 A, in the layer thickness is about

identical with the observed increase of 0.150 A in d(001). In other

STRAC TU RE OF PY ROPHY LLITE 749

words, the separation of adjacent basal oxygens shows little change;the average separations of near basal oxygens in Table B, (8.28 A) ,and in Table b, (9.2b A) are nearly identical. The net result of theparameter changes is a 3.g percent increase in unit cell volume withdehydroxylation.

In trioctahedral layer silicates, dehydroxylation generally occursat higher temperatures than in dioctahedral silicates, and recrystalliza-tion generally occurs at about the same temperature. The presentresults for pyrophyllite, and the probability that a similar processoccurs in muscovite, suggest a possible structural reason for the dif-ferent thermal behaviors, namely that in the trioctahedral mineralsthere is less freedom of movement for cation and oxygen reorganiza-tion when reaction occurs. Consequently higher temperatures andmajor structural changes are involved with the trioctahedral minerals.

Nomenclature

Previously (Brindley and Wardle, 1gZ0) the dehydroxylated phasewas deseribed as an anhydride, but following the recommendation ofBretL et al. Q97$ the term "dehydroxylate" is now preferred as beinga more accurate term. Anhydride is a term more aptly used whenstructural water is removed.

The one-layer triclinic structure of pyrophyllite is now labelled"pyrophyllite, lTc". Similarly the two-layer monoclinic structure canbe labelled "pyrophyllite,2M". By placing lTc and 2M alter ratherthan before the name of the mineral, the indexing of the mineral underttPl' rather than "T" or .(Mt, is assured.

Acrwowlnncunnrs

of computer facilities was made possible by a grant from The pennsylvaniaState University.

R^urnnpNcns

Berrnv, s- w. (1966) The status of clay mineral structures. clavs claa Mi,neral.14. t-23.

Bnnrr, N. H., K. J. D. MecKrr.rzrn, exn J. H. Snenp (f920) Thermal decomposi-tion of hydrous layer silicates and their related hydroxides. euart. Reu.24,185-207.

BnrNnr,nv, G. W. (1971) Reorganization of dibctahedral hydrous layer silicatesby dehydroxylalion. M,i,neral. Soc. Amer. Spec. Pap. t, ZO-ZB.

750 RONALD WARDLE AND G. W. BRINDLEY

AND RoNALD Wenor,p (1970) Monoclinic and triclinic forms of pyrophyllite

and pyrophyllite anhydride. Amer. Mirteral. 55, 1259-7n2'

Bunxrrervr, C. W., aNo M. J. Buincnn (1961) Refinement of the crystal stnrcture

March. 19,63, p.4243.Hnr,r,nn, L., V. C. FAnunn, R. C. MacKnNzru, B. D. Mrrcunr,r,, ern I[' F' W'

Teyr,on (1962) The dehydroxylation and rehydroxylation of trimorpltic

dioctahedral clay minerals. CIag Mineral. BuII. 5, W72,Nrcor,, A. W. (1964) Topotactic transformation of muscovite under mild hydro-

thermal conditions. Clags Clng Mi,neral. 12, 1l-L9.Rerosr,ovtcn, E. W., eNo K. Nonnrss (1962) Cell dimensions and symmetry of

layer lattice silicates. I. Some structural considerations. Amer. Mi,neral. 47,

599-616.Ravxon, J. I{., eNr G. Bnowx (1966) Stmcture of pyrophyllite. Clags Clag

Mi.neral. 13, 73-84.surrrr, D. K. (1963) A Fortran program for calculating X-ray powder diffrac-

tion patterns. Lawrence Radiation Laboratory, Livermore, california.

UCRL-71e6 (Apri l f9$).- (1967) A revised program for calculating X-ray powder difrraction pat-

terns. Lawrence Radiation Laboratory, Livermore, california. ucRL-50264(June 1967).

- (1963) Computer simulation of X-ray difiractometer traces. Norelco Rep.

Apr-June 1968. (Reprinted by the Lawrence Radiation Laboratory. Liver-

more, California, as UCRL-70674).Swrr.roer,n, L. D., axo I. R. Eucsps (1968) Hydrothermal assosiation of pyrophyl-

lite, kaolinite, diaspore, dickite and quartz in the Coromandel Area, New

Zealard. NZJ. Geol. Geopltys. 11' 1163-1185.Tarr,on, W. H. (1929) The stnrcture of andalusite, ALSiO". Z. Krtstallngr' 77,

205-218.Wennr,n, Rower,o (1572) A Struntural Study of Pyrophyllite ITc aln'd i'ts Dehy'

dronglate. Ph-D. Thesis, The Pennsylvania State University.eNn G. W. Bnrrvnr,ov (1971) Dependence of the wavelength of AlKc

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2123-2728.

Manuscnpt receiued, Nouember 29, 1971; accepted Jor ptblicotinm, January 31,

Im2.