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Clays and Clay Minerals , Vol. 28, No. 5, 373-380, 1980.

C R Y S T A L C H E M I S T R Y O F B O E H M I T E

R O D N E Y T E T T E N H O R S T A N D D O U G L A S A . H O F M A N N zDepartment of Geology and Mineralogy, The Ohio State UniversityColumbus, Ohio 43210

Abstract --Thirty two boehmites, synthesized at temperatures ranging from room temperature to 300~were examined by scanning electron microscopy, transmission electron microscopy, electron diffraction,X-ray powder diffraction, differential thermal analysis, and infrared spectroscopy. The results show thatboehmite exhibits a continuous gradation in crystallite size ranging from single octahedral layers or a fewunit cells to about 65 unit cells in the y-direction. This conclusion suggests that the term pseudoboehmiteis inappropriate for finely crystalline boehmite. Finely crystalline boehmite contains more sorbed waterthan coarsely crystalline boehmite; this water is commonly intercalated between octahedral layers, usuallyrandomly but sometimes regularly. The regularly interstratified boehmite gives rise to a diffuse " long spac-ing" X-ray diffraction reflection. Calculated 020 X-ray diffraction peaks approximate closely those ob-served experimentally when a range of crystallite sizes is taken into account.Key Words---Boehmite, Crystallite size, Pseudoboehmite, Synthesis, X-ray powder diffraction.

INTRODUCTIONThe primary purpose of this study was to ascertain

the level of agreement that could be attained betweenexperimental and calculated X-ray powder diffractionprofiles for boehmite. Boehmite exhibits an extremevariation in crystallite size (Papre e t a l . , 1958; Hsu,1967). Further, it can be synthesized from its elemen-tary constituents so that its chemistry can be con-trolled. The substances that have been produced andstudied here are probably more chemically pure andexhibit a wider range of crystallite sizes than any pre-viously studied. X-ray powder diffraction data weresupplemented by electron diffraction and microscopy,differential thermal analysis, and infrared spectroscopyin order to characterize fully synthetic boehmites andto discern the property variations among them.

Boehmite synthesized at low temperatures exhibitsbroad diffraction peaks, contains more water, and hasa higher surface area than boehmite synthesized at hightemperatures (Calvet e t a l . , 1953; Pap6e e t a l . , 1958).These differences prompted Calvet e t a l . (1953) to cointhe term p s e u d o b o e h m i t e for this substance. The na-ture of pseudoboehmite, however, is not clear. Variousinterpretations of its structure and chemistry and as-pects of its diffraction patterns noted by previous in-vestigators must be explained before pseudoboehm iteis understood. Pap6e e t a l . ( 1 9 5 8 ) suggested that pseu-doboehmite was not crystalline but had an atomic ar-rangemen t similar to boehmite with a short range orderwhose character is intramolecular or intramicellar.They further suggested that the broad diffraction peaksresulted from a poorly ordered arrang ement of elemen-tary sheet-like units. Hsu (1967) suggested that pseu-

LPresent address: Owens-Coming Fiberglas Corporation,Granville, Ohio 43023.

doboehmite was an in completely dehydrated boehmite.Lahodny-~arc e t a l . (1978) stated that the material theymade at tempe ratur es as high as 80~ was pseudo-amorphous and not crystalline boehmite. Papre e t a l .(1958) noted that the intensities of the diffuse X-raypowder diffraction reflections did not decrease signifi-cantly at high angles, a feature comm on for gels. Theynoted that these high angle reflections were somewhatsharper than the low angle reflections. They also ob-served that the first reflection at the lowest diffractionangle, corres pondi ng to the 020 reflection of boehmite ,was absent from some patterns although the other re-flections remained unchanged. The latter observationwas particularly evident for synthetic samples that hadbeen incompletely washed of salts. The d-spacing of thefirst pseudoboehmite peak is much larger than that ofboehmite and has been attributed to excess water in in-terlayer positions, corresponding to interlayer water inclay minerals.

The present findings explain many of the observa-tions made in previous studies and reconcile some ofthe conflicting interp retatio ns. Also, a new feature ofboehmites with small crystallite size has been discov-ered. The major conclusion is that differences in theX-ray diffraction patterns of boehmites can be ex-plained mainly by variations in crystallite size and,therefore, a ny distinction between boehmite and pseu-doboehmite is arbitrary.

EXPERIMENTALS y n t h e s is p r o c e d u r e

Boehmite was synthesized by the method of Hsu(1967) by adding 100 ml of a 0.6 N NaOH-4 N NaCIsolution dropwise into a 250-ml beaker containing 100ml of 0.2 M AIC13 ove r a period of 30 min. Th e solut ionwas stirred constantly with a magnetic stirrer with the

Copyright 9 1980, The Clay Minerals Society 373

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3 7 4 Te t t e n h o r s t a n d Ho f m a n n Clays and Clay Minerals

b u r e t t e t i p p o s i t i o n e d c l o s e t o th e o u t e r e d g e o f t h eb e a k e r t o o b t a in m a x i m u m d i s p e r s i o n. T o r e m o v e r e-s i d u a l N a C I , t h e g e l a t i n o u s p r e c i p i t a t e w a s d i l u t e d i m -m e d i a t e l y w i t h 1 60 m l o f d i s t i l l e d - d e i o n i z e d w a t e r ,s h a k e n v i g o r o u s l y , a n d c e n t r i f u g e d . T h e s u p e r n a t a n tw a s d e c a n t e d , a n d t h e p r o c e s s w a s r e p e a t e d u n t il th ew a s h i n g s w e r e c h l o r i d e f r e e a s d e t e r m i n e d w i t h a 1 0%s i l v e r n i t r a t e s o l u t i o n . I n s o m e o f t h e s a m p l e s t h e f ir std i l u t i o n w a s m a d e w i t h a 1 1 m i x t u r e o f w a t e r a n d m e t h -a n o l . T h e a l c o h o l w a s h c a u s e d n o o b s e r v a b l e d i f f e r -e n c e s i n t h e p r o d u c t s .

C r y s t al g r o w t h w a s a c c o m p l i s h e d b y h e a t i n g t h ew a s h e d , g e l a t i n o u s p r e c i p i t a t e s i n s e a l e d c o n t a i n e r s i no v e n s o r f u r n ac e s p r e h e a t e d t o t h e t e m p e r a t u r e d e -s i r ed . P o l y e t h y l e n e b o t t l e s w e r e u s e d u p t o 8 0 ~ a n dh y d r o t h e r m a l p r e s s u r e v e s s e l s w e r e u s e d a b o v e 8 0 ~T h i r t y - t w o s a m p l e s w e r e s y n t h e s i z e d b e t w e e n r o o mt e m p e r a t u r e a n d 3 0 0 ~ i n - 2 5 ~ i n c r e m e n t s ( T a b l e I ).S i x t e e n s a m p l e s w e r e h e a t e d f o r 1 d a y ; 1 5 s a m p l e s w e r eh e a t e d f o r 2, 4, 6, o r 8 d a y s ; 1 s a m p l e w a s d r i e d i m -m e d i a t e l y a f t e r p r e c i p i t a t i o n . F o l l o w i n g h e a t t re a t -m e n t , t h e r e ac t i o n v e s s e l s w e r e q u e n c h e d i n w a t e r , a n dt h e g el s w e r e r e m o v e d . T h i r t e e n s a m p l e s w e r e f re e z e -d r i e d ; 19 s a m p l e s w e r e o v e n - d r i e d a t 12 5~ A f t e rd r y i n g , a l l s a m p l e s w e r e g r o u n d f o r 1 5 m i n a n d s t o r e di n s t o p p e r e d v i a l s i n a d e s i c c a t o r . T h e f r e e z e - d r i e ds a m p l e s w e r e g r o u n d m e c h a n i c a l l y ; t h e o v e n - d r i eds a m p l e s w e r e g r o u n d b y h a n d .Elec t ron microscopy and e lec t ron d i f f rac t ion

U n d i s t u r b e d s u b s a m p l e s w e r e e x a m i n e d w i t h a C a m -b r i d g e $ 4 -1 0 s c a n n i n g e l e c t r o n m i c r o s c o p e t o a s c e r t a i nt h e i r m o r p h o l o g y . A n u n d i s t u r b e d p o r t i o n o f s a m p l e 3 2w a s e x a m i n e d w i t h a J E M 6 A t r a n s m i s si o n e l e c t r o nm i c r o s c o p e t o r e c o r d a n e l e c t r o n d i f f ra c t i o n p a t t e r n t od e t e r m i n e t h e r e l a t i o n s h i p b e t w e e n c r y s t a l l o g r a p h i ca x e s a n d c r y s t a l h a b i t .X - r a y p o w d e r d i f fr a c ti o n

X - r a y p o w d e r d i f f r a c t i o n ( X R D ) p a t t e r n s w i t h f i l -t e r e d C u r a d i a t i o n w e r e r u n o n a ll s a m p l e s . E x p e r i -m e n t a l c o n d i t i o n s a r e g i v e n b y C h r i s t o p h et al . ( 1 9 7 9 ) .T h e b r e a d t h o f t h e 0 2 0 r e f l e c t io n o f e a c h p a t t e r n w a sm e a s u r e d a t h a l f - m a x i m u m i n t e n s i t y a f t e r s u b t r a c t i n gt h e b a c k g r o u n d ; t h e 2 0 p o s i t i o n o f e a c h 0 2 0 re f l e c t io nw a s m e a s u r e d a t th e m i d p o i n t o f t h e c h o r d a t h a lf - m a x -i -m u m i n t e n s i t y . T h e 1 00 r e f l e c t io n o f p o w d e r e d q u a r t zw a s u s e d a s a s t a n d a r d f o r 20 a nd a s a m e a s u r e o f t h ei n s t r u m e n t a l b r o a d e n i n g . T h e b r e a d t h ( b ) o f t h e 1 00 r e -f l e c t io n o f q u a r t z w a s s u b t r a c t e d f r o m t h e m e a s u r e db r e a d t h o f t h e 0 2 0 r e f le c t i o n ( B ) o f b o e h m i t e t o g i v e th ep u r e d i f f r a c t i o n b r e a d t h ( /3 ), i . e . , / 3 = B - b . T h e c o r -r e c t e d 2 0 a n d / 3 v a l u e s a r e g i v e n in T a b l e 1.Di f f eren t ia l thermal ana lys i s

A D u p o n t M o d e l 9 0 0 D T A s y s t e m w a s u s e d t o re c o r dt h e d i f fe r e n t ia l t h e r m a l a n a l y s i s ( D T A ) p a t t e r n s o f 7

Ta b le 1 . Ex p e r im e n ta l d a ta f o r s y n th e t i c b o e h m i te s .020 reflectionMethodSample Temp. Duration o f Position Breadthno. (~ (days) drying (~ (~

1 20 0 O 12.90 5.452 20 1 F 12.35 5.663 20 1 O I0.80 5.734 20 2 O 13.86 3.855 20 4 O 13.73 3.816 20 8 O 13.70 3.757 43 1 F 13.30 3.678 60 1 F 12.56 5.669 68 1 O 13.80 3.0510 68 2 O 13.66 3.9311 68 4 O 13.75 3.6512 68 8 O 13.84 3.4113 80 1 F 13.47 3.5414 100 1 F 12.71 5.5515 100 4 F 13.54 4.1816 125 1 F 13.96 2.46

17 125 1 O 14.12 2.4518 125 2 O 14.30 2.0519 125 4 O 14.40 1.3920 125 8 O 14.46 1.3121 150 1 F 14.32 1.2022 175 1 F 14.40 0.7023 200 1 F 14.39 0.5024 200 1 O 14.48 0.7725 200 2 O 14.49 0.8826 200 4 O 14.51 0.4827 200 8 O 14.46 0.6228 250 1 F 14.45 0.2729 300 1 F 14.46 0.0530 300 1 O 14.52 0.1031 300 2 O 14.53 0.103 2 3 0 0 6 F - - - -

F = F r e e z e - d r i e d ; O = O v e n d r i e d a t 1 2 5 ~0 2 0 b r e a d th me a s u r e d a t h a l f - ma x imu m in te n s i ty a n d c o r -r e c te d f o r in s t ru me n ta l b r o a d e n in g ; 0 2 0 p o s i t io n me a s u r e d a tm i d p o i n t o f c h o r d a t h a l f - m a x i m u m i n t e n si t y .

s a m p l e s. A b o u t 5 0 m g o f s a m p l e a n d a n e q u a l a m o u n to f a l p h a a l u m i n a r e f e r e n c e m a t e r i a l w a s p a c k e d i n t og l as s tu b e s . C h r o m e l - a l u m e l t h e r m o c o u p l e s w e r e u s e d ,a n d e a c h s a m p l e w a s h e a t e d i n a n i t r o g e n a t m o s p h e r ew h i c h w a s a d m i t t e d i n t o t h e h e a t i n g c h a m b e r a t a r a t eo f 3 f t3 / hr , T h e h e a t i n g r a t e w a s 1 0 ~ a n d t h e A Ts e n s i t i v i ty w a s c o n s t a n t f o r a ll ru n s .I n f r a r e d s p e c t r o s c o p y

A D i g i la b M o d e l F T S - 1 4 s p e c t r o m e t e r w a s u s e d t or e c o r d t h e i n f r a r e d ( I R ) s p e c t r a o f s a m p l e s 7 , 22 , an d2 9 . S a m p l e 7 w a s d r i e d a t 2 0 0 ~ s a m p l e s 2 2 a n d 2 9 w e r ed r i e d a t 6 5 ~ a f t e r w h i c h e a c h w a s m i x e d w i t h K B r a n dp r e s s e d i n t o a d i s k .

R E S U L T SE l e c t r o n m i c r o s c o p y a n d e l e c t r o ndi f fract ion resul ts

S a m p l e s 7 , 1 4 , 2 1 , a n d 3 2 w e r e e x a m i n e d b y s c a n n i n ge l e c t ro n m i c r o s c o p y . C r y s t a ls w e r e s e e n o n l y in sa m p l e

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Vol. 28, No. 5, 1980 Crysta l chem istry of boehm ite 375

Figure 1. Scanning electron micrograph ofboeh mite sample32.

32. Mo s t o f them showed a b laded hab i t , e longate a longz , and very th in a long y . A separa te por t ion o f sample32 was no t f reeze- dr i ed bu t d i lu ted man yfo ld , a i r -d r i ed ,an d p h o t o g rap h ed . T h e c ry s t a l s i n th i s s amp l e (F i g u re1) showe d a d i f fe ren t hab i t , s imi lar to the rh omb ic , t ab -u l a r h ab i t o f b o eh mi t e d e s c r i b ed b y M ack en z i e et a l .(1971) . The ou t l ine o f mos t c rys ta l s i s dominated by(20 1) , w h e rea s t h o s e s h o w n b y Mac k en z i e et a l . (1971)a r e b o u n d ed b y (1 01 ). T h u s , t h e mo rp h o l o g y o fb o eh m-i t e d ep en d s u p o n i ts me t h o d o f p r ep a ra t i o n , a l t h o ug hal l c rys ta l s a re th in , para l l e l to y .

An e lec t ron d i f f rac t ion (ED) pa t t e rn (Figure 2 , top)was o b ta ined f rom a typ ica l c rys ta l (Figure 2 , inse t ) o fsample 32 . The powder t ings a re due to meta l l i c a lu -mi n u m ev ap o ra t ed o n t o t h e s amp l e fo r r e f e r en ce . T h epat t e rn shows 9 un ique re f l ec t ions which are indexedin the space g roup Amain (Figure 2 , bo t tom) . Cel l d i -men s i o n s d e t e rmi n ed f ro m t h e p a t t e rn ( a = 3 . 6 7 9 A ,c = 2 .799 A) are sma l l e r than those f rom the X-ra y d if -f rac t ion resu l t s o f Chr i s toph et a l . (1979) wh ich are a =3 .6936 ]k, c = 2 .8679 A. The p res enc e o fh k l re f l ec t ionssugges t s d i f fus ion o f in tens i ty para l l e l to b* owing tos ma l l c ry s t a l s i z e an d / o r s t r u c t u r a l d i s o rd e r . Co mp a-r ab l e E D p a t t e rn s o f b o eh mi t e s h o w n i n Mack en z i e e tal. (1971) and Bosmans and Michel (1957) do not showh k l r e f le c t i o ns . A n E D p a t t e rn o f s y n t h e t i c l ep i d o c ro -c i t e , FeO (OH ), which has a s t ruc tu re s imi lar to o r iden-t i c a l w i th b o eh m i t e , g i v en b y M ack en z i e et al . (1971),s h o w s 2 7 u n i q u e r e f l e c t io n s , man y o f w h i ch a r e i n d ex edas hk l .X - r a y p o w d e r d i f f r a c t i o n r e s u l ts

X RD p a t t e rn s s h o w ed n o a l u mi n a h y d ra t e o t h e r t h anboehm i te . A ser i es o f pa t t e rn s i s shown in Figure 3 wi thcrys ta l l i t e s ize increas ing f rom top to bo t tom . An in -d ex ed p a t t e rn r eco rd ed fo r s amp l e 3 2 w as s h o w n i n

Figure 2. Electro n diffraction (ED) pattern (top) obtainedfrom a single crystal (inset) from sample 32. Rings are frommetallic aluminum evapor ated onto the crystal for reference.Schematic ED pattern (bottom) indexed in the space groupAmam.

Ch r i s t o p h et a l . (1979) . The dashed ver t i ca l l ine a t14.5~ repre sen t s the pos i t ion o f the 020 ref l ec t ion fo rb o eh mi t e s s y n t h e s i zed a t 3 00 ~ A n i n c r ea s ed b r ead t ho f th i s r e f l ec t i on w as acco m p an i ed b y a s h i ft o f th e p ea kpos i t ion to smal l 20 va lues , i . e . , as c rys ta l l i t e s i ze de-creases a long y the apparen t d (020) increases . The 020p eak d i s p l acem en t a s a f u n c t i o n o f t h e n u mb e r o f o c -t ahedra l l ayers i s due to the var i a t ion o f F " Lp wi th 20,where F z i s the squ ared modu lus o f the s t ruc tu re fac to ran d L p i s t h e p o w d e r L o ren t z -p o l a r i z a t i o n fac t o r . T h evar ia t ion in pos i t ion and b read th o f the 020 ref l ec t ionw as ex t r em e (T ab l e 1 ), mu ch l a rg e r th an o t h e r l ay e rs t ruc tu res such as c l ay minera l s . A 2~ sh i f t in pos i t ionand bread ths exce ed ing 5~ fo r the 020 ref l ec t ion weremeas u red o n s o me s amp l e s s y n t h e s i zed b e l o w 1 2 5 ~A p lo t o f pos i t ion vs . b read th fo r the 020 ref l ec t ion iss h o w n i n F i g u re 4 . T h e p ro g re s s i v e g r ad a t i o n s h o w n b yt h e s e X RD p a t t e rn s i n d i ca t e s t h a t an y d i s t i n c t i o n b e -t w e e n b o e h m i t e a n d p s e u d o b o e h m i t e i s a r b i t r a r y .Ps eu d o b o eh m i t e i s e s s en t ia l l y b o eh mi t e w i t h s ma l lc rys ta l l i t e s i ze .I n g en e ra l , an i n c r ea s ed h ea t i n g t ime o r t em p e ra t u r e

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376 Tettenhorst and Hofmann C la y s a n d C la y Min e r a l s

* 2 O70 60 5O 40 ~O 20 IOo 2 o

70 60 50 40 30 20 I0' ' ' 4

I

o 2 070 6 0 ,50 4.0 :50 20 tO

3 ~14 ~I

" 2 O70 60 50 4.0 30 20 I0

o20ro ~9 so 4o 3o ~o ~9

~7o so ~0 4q ~ ~9

0 2 070 60 50 49 5 0 ~O I0

. . . . . . 8

o 2070 60 50 qO 5 0 20

2 2

I0

4 ~

020 , 02070 60 ~

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Vol. 28, No. 5, 1980 Crystal chemistry of boehmite 377

14

13

~

12

+

, I J I , I , I , I ,1 2 3 .4 5#(~Figure 4. Position (20) vs. pure diffraction breadth (/3) of the020 XRD peaks for synthetic boehmites for CuK~ radiation.Calculated relationship shown as solid line. Open circles rep-resent experimental data from freeze-dried boehmites; closedcircles represent experimental data from boehmites oven-dried at 125~ closed circles with crosses represent experi-mental data from freeze-dried boehmites subsequently heatedto 200~ The latter heat-treated samples are 7, 8, and 14whose/3 values are 2.89~ 4.80~ and 4.12~ respectively.Experimental positions measured at the midpoint of the chordat half-maximum intensity; experimental breadths measuredat half-maximum intensity and corrected for instrumentalbroadening.

sample 14. Sample 14 was unique in that it showed abroad peak at 4-5~ prior to heating (Figure 5). This"long spacing" peak disappeared on heating. An at-tempt to regenerate the "long spa cing" by rehydrationin a water-filled c~ntainer for one day was unsuccess-ful. It is concl uded that s amples 7, 8, and 14 conta inedwater intercalated between octahedral layers, random-ly along y for 7 and 8, and regularly for 14.

XRD patterns of samples synthesi zed at 100~ orbelow and subsequen tly heated at 350~ showed nopeaks. The patterns of these heated boehmites whichhad been rendered "X-ray amorphous" contrastedsharply with the broad but recognizable maxima ob-served in the patterns of the unheated materials.

Weak AgCI (cerargyrite structure) peaks were evi-dent on all XRD patterns of samples synthe sized in thesilver-lined bom bs at 150~ or higher. Evident ly, thesilver liners were not inert at these temperatures andreacted with remnant CI . The presence of CI- suggeststhat the washing procedure was incomplete. However,no NaC1 was detected in the XRD patterns. Chlorideions likely were inc orpor ated in the gel which preve nt-ed their complete removal by the washing procedure.The m axim um amoun t of AgC1 was esti mated to be lessthan 4%.

Calculated one-dimensionaldiffraction profiles alongy were made to compare with the experimental data.

2 2 2 0r- i

o 2 e18 16 14 12 10 8 6 4

i

~L

4I-4xOO

2 " Ov

Figure 5. X-ray powder diffraction pattern with CuKa ra-diation of sample 14 showing a " long spacing" peak at 4~176The dashed vertical line at 14.5~ represents the 020 positionof the samples synthesized at 300~

Relative intensities were co mputed at 20 values from4 ~ o 20~ n 0.1~ incr emen ts by multiplyin g he squaredmodulus of the Fourier transform by the appropriatepowder Lorent z-polarization factor. The b-dimension,y coordinates of the individual atoms, and temperaturefactors were taken from Christoph et al. (1979). Exactagreement between calculated and observed data wasnot expected since these calculations assumed infiniteextension in the xz plane, whereas the XRD data fromsamples grown at low temperature s indicated restrictedgrowth in this plane. Better agreement is expected byprofiling in three dimensions , and that analysis has nowbegun.

Diffraction profiles were calculated for crystalliteswhich had as m any as 60 unit cells along y. Becaus e theboehmite unit cell contains two aluminu m octahedrallayers along y, comput ations were made at "half cell"increments also. The 20 and/3 values for the 020 re-flection were measured from the computed profiles aswas done for the experimental patterns. The computedpoints were connecte d with the smooth line shown inFigure 4.

The theoretical curv e essent ially forms a lowerbounda ry for the experimental points. Agreement be-tween theory and expe riment is good for large crystal-lites, i:e., the data at high 20 and low/3, but the agree-ment is poorer for small crystallites. To bring thetheoretical and experimental data into closer accord,the calcula tions were modified tO allow for (a) substi-tution of C1 for OH as suggested by the pr esen ce of CIin the gels, (b) sorbed water molecules between octa-hedrat layers, and (c) a distribution of crystallite sizes

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378 Tettenhorst and Hofmann Clays and Clay Minerals

O2e1 8 1 6 1 4 1 2 1 0 8

Figure 6. Calculated mean X-ray diffraction profile for CuKaradiation along y for boehmite consisting of equal amounts of1, 2, and 3 octahedral layers.

(thicknesses). Subs titut ion of C1 for OH and interca -lation of water between octahedral layers did not im-prove the agreement between experiment and theory.However, XRD patterns computed for a range of crys-tallite thicknesses gave theoretical points above thetheoretical curve shown in Figure 4 and in the regionof the experimental points. For example, the 020 peak(Figure 6) for equal amounts of 1, 2, and 3 octahedrallayers gave 20 = 12.52 ~ and /3 = 4.69~ Another cal-cul ati on gave 20 = 11.95 ~ an d/ 3 = 5.71 ~ for a mode lcont ainin g 75% of two octa hedral l ayers and 25% ofthree octahedral layers. Results from calculated pro-files indicate that bo ehmit es whose /3(020) = 5-6~consist of a significant numb er of crystallites which arecomposed of a single unit cell or a very few unit cellsat mo st along y. Both single uni t cells and single octa-hedral units apparently contribute to the diffractionpattern even though no 020 peak is presen t for the lat-ter, o nly a smooth rise in inten sity as 20 decreases . T heabsence of the 020 peak obs erved by other investigatorsis likely due to the presence of a significant numbe r ofsingle octahedral units in their samples.Di f f eren t ia l thermal ana lys i s

DTA patterns were made on 7 samples. The patternsare arranged in Figure 7 to depict a progressive gra-dation from one to the next. T he ar rangement from topto bottom is in order of increasing crystallite size asdetermined by XRD (Table 1). The low temperatureendo ther m centere d at about 150~ attribu ted tosorbed water, is an important feature of all patterns ex-cept those of samples 23 and 29. Its inte nsit y decrease s,in general, with an increase in both synthesis temper-ature and crystallite size. The low temperature endo-ther m in the pattern s of samples 8 and 15 is merged with

Ar0

. 3

| i i I ! !

15

21

2 3

2 9f

,I I I l,o o 2 b o 3 0 0 4 0 0 5 0 0 6 6 0Temperature (~

Figure 7. Differential thermal analysis patterns of syntheticboehmites. Increment on 2xT scale is 2~

the high temperature end otherm suggesting a continu-ous gradation of bonding energy for the water. The verybroad endotherm centered near 275~ in the pattern ofsample 23 indicates the presence of strongly boundwater molecules. This endothe rm is absent in the pat-tern of sample 29.

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Vol. 28, No. 5, 1980 Cry stal chem istry of boeh mite 379

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Figm-e 8.d i sks . A bso r bance sca le i s l inea r.

T h e h i g h t e m p e r a t u r e e n d o t h e r m , r e p r e s e n t i n g t h el o s s o f h y d r o x y l g r o u p s , i n c r e a s e s i n i n t e n s i t y a n ds h i ft s to h i g h e r t e m p e r a t u r e s f r o m t o p t o b o t t o m . T h i sv a r i a t i o n i s , i n i t s e l f, a n i n d i c a t i o n o f a n i n c r e a s e i nc r y s t a l l i t e s i z e ( S m o t h e r s a n d C h i a n g , 1 95 8, p . 5 2 ). T h ep r o g r e s s i v e g r a d a t i o n o f t h e p a t t e r n s a l s o s u g g e s t s th a ta n y d i s t in c t io n b e t w e e n b o e h m i t e a n d p s e u d o b o e h m i t ei s a r b i t r a r y .I n f r a re d s p e c t r o s c o p y

I R s p e c t r a w e r e r e c o r d e d o n t h r e e s a m p l e s ( F i g u r e8 ). B a n d a s s i g n m e n t s f o r b o e h m i t e w e r e m a d e b y F r i -p i a t e t a l . ( 1 9 6 7 ) a n d R u s s e l l e t a l . ( 1 9 7 8 ) . T h e I R p a t -t e r n s a r e s i m i l a r w i t h r e g a r d t o t h e n u m b e r o f b a n d s a n dt h e i r p o s i t io n s , b u t t h e i r in t e n s i t y a n d b r e a d t h v a r i e s .T h e d i f f e r e n c e s b e t w e e n t h e p a t t e r n s i s p r o b a b l y d u et o v a r i a t i o n s i n p a r t i c l e s i z e a n d t o t h e p r e s e n c e o fs t r o n g l y b o u n d w a t e r m o l e c u l e s in s a m p l e 7 . T h e e n -h a n c e d i n t e n s i t y o f t h e b a n d a t 4 85 a c m c o m p a r e d w i t ht h a t i n t h e p a t t e r n g i v e n b y R u s s e l l e t a l . ( 1978) i s duet o t h e a b s o r b a n c e s c a l e b e in g l i n e a r a n d t h e e l o n g a t i o np a r a l l e l t o z o f t h e f r e e z e - d r i e d c r y s t a l s (V . C . F a r m e r ,M a c a u l a y I n s t i t u t e fo r S o il R e s e a r c h , A b e r d e e n , S c o t -l a n d , p e r s o n a l c o m m u n i c a t i o n ) .

C O N C L U S I O N ST h e p r e s e n t d a t a s h o w t h a t b o e h m i t e a n d p s e u d o -b o e h m i t e a r e c o n t i n u o u s i n t h e i r st r u c t u r e a n d p h y s i c a l

' 485 p r o p e r t i e s a n d t h a t a n y d i s t i n c t i o n b e t w e e n t h e m i s a r -b i t r a ry . P s e u d o b o e h m i t e i s e s s e n t i a l l y f i n el y c r y s t a l l in eb o e h m i t e w h i c h c o n s i s t s o f th e s a m e o r s i m i l a r o c t a -h e d r a l l a y e r s i n t h e x z p l a n e b u t l a c k s t h r e e - d i m e n -s i o n a l o r d e r b e c a u s e o f a r e s t ri c t e d n u m b e r o f u n it c e l l s

ro65

[ ,65-~,~ a l o n g y . I t c o n s i s t s o f a s i g n i f i c a n t n u m b e r o f c r y s t a l -~ ~ , ~~ ~5 ] l i te s w h i c h c o n t a i n a s i n gl e u n i t c e l l a l o n g y o r s i n g lez 9 ~ o c t a h e d r a l l a y e r s . I t c o n t a i n s m o r e w a t e r w h i c h i s c o m -

.... m o n l y i n t e r c a l a t e d b e t w e e n o c t a h e d r a l l a y e r s , n o r m a l -. . . . . l y r a n d o m l y a r r a n g e d , b u t s o m e t i m e s r e g u l a r l y . T h e

l a r g e b r e a d t h a n d a n d h i g h d (0 2 0 ) a r e a r e s u l t p r i m a r i l yo f a s m a l l n u m b e r o f o c t a h e d r a l l a y e r s a l t h o u g h b o t ht h e b r e a d t h a n d d - v a l u e o f 0 2 0 a re i n c r e a s e d b y w a t e rm o l e c u l e s b e t w e e n o c t a h e d r a l l a y e r s i n s o m e s a m p l e s .

z 2 / / ~ / I G o o d a g r e e m e n t b e t w e e n c a l c u l at e d a n d e x p e r i m e n t a l0 2 0 X R D p r o f i l e s is o b t a i n e d w h e n a r a n g e o f c r y s t a l l i t et h i c k n e s s e s i s t a k e n i n t o a c c o u n t . T h e t e r m p s e u d o -b o e h m i t e i s i n a p p r o p r i a t e f o r f in e l y c r y s t a ll i n e b o e h m -i t e a n d s h o u l d b e d r o p p e d f r o m t h e l i t e r a t u r e .

7\ . ' A C K N O W L E D G M E N T S

T h i s s t u d y w a s p e r f o r m e d w i t h th e s u p p o r t o f N a -I I J ~ I. . . . . . . . . . . . . t i o n a l S c i e n c e F o u n d a t i o n G r a n t E A R 7 5 -2 0 5 1 1 . W eWAVENUMBER, crn I x I 0 0 t h a n k t h e O h i o S t a t e U n i v e r s i t y I n s tr u c t i o n a n d R e -

I n fr a re d s p e c tr a o f s y n th e ti c b o e h m it es i n K B r s e a r c h C o m p u t e r C e n t e r f o r g r a n t i n g u s t i m e a n d f a c i l -i t i e s . C a r l M e l t o n t o o k t h e t r a n s m i s s i o n e l e c t r o n m i -c r o g r a p h s a n d t h e e l e c t r o n d i f f r a c ti o n p a t t e rn . D e n n i sF o r e m a n d e t e r m i n e d t h e c e ll d im e n s i o n s f r o m t h e e l e c-t r o n d i f fr a c t io n p a t t e rn . T h e I R s p e c t r a w e r e r e c o r d e du n d e r t h e d i r e c t i o n o f B o b J a c o b s o n .

R E F E R E N C E SBosmans , H . and Miche l , P . ( 1959) E tude de c ri s taux deboeh mite par m icrosco pie et dif f ract ion 61ectroniques: C.R . A c a d . S c i . ( P a r i s ) 249 , 1532-1533.Ca lve t , I S . , Boiv ine t , P . , N o~ l , M . , Thibon , H . , Mai l la r d , A . ,and Ter t ian , R . ( 1953) Cont r ibu t ion ~ l ' ~ tude des ge lsd ' a lumine : B u l l . S o c . C h i m . F r . 20, 99-108.Chr i s toph , G . G . , Cor ba t6 , C . E . , H of mann, D . A . , and Te t -tenhor s t , R . ( 1979) The c r ys ta l s t r uc tur e of boehmi te :C l a y s & C l a y M i n e r a l s , 27, 81-86.F r ip ia t , J . J . , Bosmans , H . , and Rouxh e t , P . G . ( 1967) P r o ton

mobi l i ty in so l ids I : H yd r ogenic v ibr a t ion modes an d pr o tonde loca l iza t ion in boehmi te : J . P h y s . C h e m . 71, 1097-1111.H su , P . H . ( 1967) Ef f ec t o f s a l ts on the f or mat ion of bayer i tever sus pseudoboehmi te : S o i l S c i . 103, 101-110.L a h o d n y - ~ a r c , O . , D r a g r e v i ~ , Z . , a n d D o ~ e n - ~ v e r , D .(1978) Th e inf luence o f the act iv i ty of wate r on the pha secompos i t ion of a luminum hydr oxides f or med by r eac t ionof amalgamated a luminum w i th w a te r : C l a y s & C l a y M i n -e r a l s 26, 153-159.Mack enz ie , R . C . , Fo l le t t , E . A . C . , and M eldau , R . ( 1971)The oxides of i r on , a luminum, and m anganese , Chap te r 11 :in T h e E l e c t r o n - O p t i c a l I n v e s t i g a t i o n o f C l a y s, J . A . G ar d ,ed . , Miner a log ica l Soc ie ty , London, Monogr aph 3 , 315-344.Papre, D. , Ter t ian, R. , and Biais , R. (1958) Recherches surl a c o n s t i t u t i o n d e s g e l s e t d e s h y d r a t e s c r i s t a l l i s r sd ' a lumine : B u l l . S o c . C h i m . F r . , M e m . S e r . 5, 1301-1310.Russe l l , J . D. , Farm er , V. C. , and Lewis , D. G. (1978) Latt ice

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380 Tetten hors t and Hofmann Clays and Clay Minerals

vibrations of boehmite (T-AIOOH): eviden ce for a C~ ratherthan a D~ sp ace group: Spectrochim. Acta 34A, 1151-1153.Smother s, W. J. and Chiang, Y. (1958) Differential ThermalAnalysis: Theory and Practice: Chemical Publishing Com-pany, New York, 633 pp.(Received 6 March 1980; accepted 19 June 1980)

Pe3roMe----Tprl,~UaTb~8a 6eMHTbl, CHnTe3rlpoaannble npa TeMnepaTypax, H3MeH:aIOH~HXC~[ OT KOMHaTHOfl~o 300~ 6blUrt nCCJle~oaaHhl C HOMOlUbIO 3JIeKTpOHHOFO c~aunpymmero MHKpOCKOna, 3~eKTpOnuoroTpaHCMHCCHOHHOFO MHKpOCKOIIa,~HqbpaKILH~ ~JIeKTpOHOB, peHTreHOBCKOrO HOpOLUKOBOFOana~I3a, ~b -qbepeHl~Ha~bnoro TepMHqeCKOFO aHa.qn3a, H ~nqbqbaKpacnofi cneKTpOcKonHn. Pe3ynbTaTh~ noKaablBalOT,qTO 6eMI4T npo~IBJI~IeT CIUIOIIIHylO rpa~aIBilo pa3MepoB KpHCTa3UIHTOB, H3MeH~IIOIL~HXC~I OT IIpOCTblXOKTaeJ1pHqecKHX CJIOeB IAJII4 HeMHOFHX eJleMettTapltblX ~qeeK BHJIOTb ~O upnMepno 65 eJleMenTapHblX~lqeeK no y-nanpaBJleHH10. ~TOT BblBOJ~nOJlCKa3blBaeT, qTO TepMHH HceBJ~ofieMHT~IBJlfleTCfl HeCOOTBeTCT-BylOmHM BjI~ TOHKO-KpHCTa.rIJII4qeCKOFO 6eMHTa. TOHKO-KpHCTa.rl.rIHyecKHI4 6eMHT CoJIepxHT 60abmeeKoaHqeCTBO copfiHpOBaHHOH BOJ~H, qeM Fpy60-KpHCTaJIBHqeeKH~ ~eMHT, 3Ta BO~a O6bIKHOBeHHOHaXO~HTC~I Me~KJ1y OKTae~pHqeCKHMH CJIORMH, O0hItIHO fiecnop~i~oqno, HO HHOFJ~a peryagpHo.Pery~mpno BHyTpHHaHJIaCTOBaHHbI~ fieMHT HpHBOJ~HT K peHTFeHOBCKOMy oTpaxentllo c ~Hqbqby3HOHHblM"J]r!IHHHbIM paCCTO~IHHeM." KorJ1a 6epeTc~ BO BHHMaHHe J~nana30H pa3MepoB KpHCTaJIJIHTOB, pacqeTHble020 peHTFeHOBCKHe HHKH HaxoJI~qTC~I B XOpOLUeMCOOTBeTGTBHH C 3KcnepHMeHTaYlbHblMI4 Be.rIHqHHaMH.[E.C.]Resiimee--ZweiunddreiBig, m Temperatu rbereic h von Zimmertemper atur bis 300~ synthetisi erte Boehm-ite wurden mittels Rasterelektronenmikroskopie, Transmissionselektronenmikroskopie, Elektronenbeu-gung, RiSntgenpulverdiffraktometrie, Differentialthermoanalyse, und Infrarotspektroskopie untersucht.Die Ergebnisse zeigen, dab sich die Kristallitengr6Be des Boehmits allm~thlich ~indert und von einzelnenoktaedr ischen Lagen oder einigen Elementarzellen bis zu - 65 Elementarzell en in der y-Richtung reicht.Diese SchluBfolgerung deutet darauf hin, dab der Au sdruck Pseudo boehmi t fiir feinkristallinen Boehmitunangebracht ist. Feinkristallisierter Boehmit enth~ilt mehr adsorbiertes Wasser als grobkristallisierter;dieses Wasser ist gew6hnlich zwische n oktaedris che Lagen eingeschaltet und zwar meist statistisch, edoc hmanchmal geord net. Der regelm~iBig wechselgelagerte Boehmit gibt eine diffuse "lo ng spacing " R6ntgen-reflexion. Berechnete 020 Peaks liegen nahe bei den experimentell beoba chtet en, wenn man eine Variationder Kristallitengr6Be beriicksichtigt. [U.W.]R6sum6---Trente deux boehmit es, synth6tis6es/~ des temp 6ratures s' 6tageant de temp6r ature ambiante300~ ont 6t6 examin6es par mic roscopie 61ectronique balayante, microscop ie 61ectronique par transmis-sion, diffraction 61ectronique, diffraction poudr6e aux rayons-X, analyse thermale diff6rentielle, et sp ec-troscop ie infrarouge. Les r6sultats m ontren t que la bo6hmite exhibe une gradation continuelle de la taillecristalline, s'6tag eant de co uches octa~dres simples ou de q uelques mailles/~ h peu prbs 65 mailles dans ladirection-y. Cette concl usion sugg~re que le terme pseu dobo6h mite est inappropri6 pour la bo6hmite fine-ment cristalline. La bo6hmite ~ fins cristaux cont ient plus d'eau sorb6e que celle a gros cristaux; cette eauest souv ent intercal6e entre des couche s octabdres, habituellement au hasard, mais parfois r6guli~rement.La bo6hmite r6guli~rement interstratifi6e produit une reflection de rayons-X diffuse ~ "longs espace-merits." Des sommets 020 de rayons-X calcul6s sont tr~s proches de ceux observ6s exp6rimentalementlorsqu'un 6tagement de tallies cristallines est consider6. [D.J.]