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Clays and Clay M inerals Vol. 29 No. 5 331-340 1981.

K I N E T I C S O F G L S S D I S S O L U T I O N N D Z E O L I T EF O R M T I O N U N D E R H Y D R O T H E R M L C O N D I T I O N S

D A N I E L B . H A W K I N SG e o lo g y /G e o p h y s ic s P ro g ra m , U n iv e r s i ty o f A la s k a , F a i rb a n k s , A la s k a 9 97 0 1

Abstract After a t e m p e ra tu re -d e p e n d e n t p e r io d w h e n l i t t le d i s so lu t io n o c c u r s , t h e d i s s o lu t io n o f rh y o l it i cg lass can be desc r ib ed by dC/d t = k (Cs - C) , where Cs is the concen tra t ion o f d is so lved s i l ica a t sa tu ra t ion ,C is the ins tan tane ous s i l ica conce n tra t ion , and k is a ra te cons tan t eq ua l to 1 .6 10 5 , 3 .0 10 s , and4.5 10 5 se c -1 at 115~ 130~ a n d 1 4 0~ re s p e c t iv e ly , in 2 M N a -K c a rb o n a te s o lu t io n a t 1 k b a r p re s s u re .At 130~ a C~ va lue o f 0.177 M SiO2 is reached in 30 h r , and ph i l l ips i te , c l inop t i lo l i te , and m orden i te beg informing a t 34 , 64 , and 76 h r , re spec t ive ly , in 2 M CO3, 1 :1 NaJK. During g lass d is so lu t ion and zeo l i tefo rm at ion , th e concen tra t ion o f AI as AI(OH)4- is buffe red a t 3 .7 x 10 4 M by an un iden t i f ied phase . T hera t io o f S iO2 to AI(O H)4- a t the on se t o f zeo l i te fo rm at ion is 475 . In 2 M CO3 so lu t ion , ph i l l ips i te c rys ta l -l iza t ion beg ins a t 144 hr a t 115~ a t 34 h r a t 130~ and a t 20 h r a t 140~ Ph i l l ips i te c rys ta l l iza t ion beg insa t 48 h r in 1 .5 M COz, a t 168 hr in 1 .0 M CO3, and in exce ss o f 550 hr in 0 .2 M CO3 a t 140~ In add i t ionto O H - c a t a ly s i s, C O 3 2 - a p p e a r s a l s o to c a t a ly z e th e g l a s s -d i s s o lu tio n a n d z e o l i t e - fo rm a t io n p ro c e s s e s .Thermodynamica l ly , ph i l l ips i te is uns tab le re la t ive to c l inop t i lo l i te and morden i te in s i l ica -r ich a lka l inehydro the rmal so lu t ions . Ph i l l ips i te fo rms f i rs t , fo l lowed by c l inop t i lo l i te , and then morden i te . Ph i l l ips i tefo rm at ion is favo red by runs o f one- wee k dura t ion , tem pera tu res le s s than 150~ and K-r ich f luids. Cl i -n o p t i lo l it e fo rm a t io n i s f a v o re d in ru n s o f m o re th a n o n e w e e k , t e m p e ra tu re s l e s s th a n 1 5 0~ a n d K - r i c hf lu id s . Mo rd e n i t e fo rm a t io n i s f a v o re d b y ru n s o f m o re th a n o n e w e e k , t e m p e ra tu re s g re a t e r t h a n 1 4 0~and N a-r ich f lu ids. In 8 -day runs a t 140~ c l inop t i lo l i te fo rm at ion was favo red by l iqu id : so l id reac tan t(vo lum e : mass ) ra t ios le s s than 1 .0 , mo rden i te by ra t ios f rom 0 .85 to 1 .5 , and ph i l l ips i te by ra t ios g rea te rth a n 1 . 5 . T h e m e c h a n i s m o f fo rm a t io n o f t h e d i f f e re n t z e o l i te s , p a r t i c u la rly p h i l l i p si t e , m a y in v o lv e s i li c a -c y c l i c t e tr a m e rs w h ic h a re a b u n d a n t i n c o n c e n t ra t e d s o lu t io n s u n d e r a lk a l in e h y d ro th e rm a l c o n d i t io n s b u tw h ic h a re a lm o s t a b s e n t i n d i lu te lo w - te m p e ra tu re s o lu t io n s . T h u s , t h e r e s u l t s o fh y d ro th e rm a l e x p e r im e n t sm a y n o t b e d i r e c t ly a p p l i c a b le to z e o l i t e fo rm a t io n a t l o w t e m p e ra tu re s .Key W ords--C l inop t i lo l i te , Disso lu t ion , Kine t ic s , Mo rden i te , Ph i ll ips ite , Syn thes is , V olcan ic g lass , Zeo l i te .

I N T R O D U C T I O NZ e o l i t e s c a n b e r e a d i l y s y n t h e s i z e d i n a f e w h o u r s

u n d e r h y d r o t h e r m a l c o n d i t i o n s , a n d m u c h i n f o r m a t i o ne x i s t s o n s y n t h e t i c z e o l i t e s a n d t h e s o l u t i o n c h e m i s t r yo f z e o l i t e s y n t h e s i s. A m a j o r p r o b l e m , h o w e v e r , a sS a n d ( 1 98 0 ) d i s c u s s e d , i s i n re l a t i n g t h e r e s u l t s o f z e o -l i te s y n t h e s is a t h i g h t e m p e r a t u r e s a n d p r e s s u r e s t o t h ef o r m a t i o n o f z e o l i t e s in n a tu r e . A p r o m i s i n g a v e n u e o fa t t a c k o n t h is p r o b l e m i n v o l v e s k i n e t i c st u d i e s i n w h i c hz e o l i t e s t y p ic a l o f l o w - t e m p e r a t u r e c o n d i t i o n s a r e s y n -t h e s i z e d f r o m n a t u r a l r e a c t a n t s u n d e r c h e m i c a l c o n d i -t i o n s s i m i l a r t o t h o s e i n w h i c h t h e n a t u r a l m i n e r a l s a r et h o u g h t t o f o r m .

T h e p r e s e n t s t u d y i s a n o u t g r o w t h o f c l i n o p ti l o li t e -s y n t h e s i s s t u d ie s r e p o r t e d b y H a w k i n s e t a l (1 9 7 8 ) .S e v e r a l h u n d r e d h y d r o t h e r m a l r u n s w e r e m a d e u n d e rv a r i o u s c o m b i n a t i o n s o f t i m e , t e m p e r a t u r e , a n d c h e m -i c a l c o n d i t i o n s i n w h i c h t h e z e o l i t e s p h i l l i p s i t e , c l i n o p -t i l o li t e , a n d m o r d e n i t e w e r e f o r m e d f r o m v o l c a n i cg l a s s . I n m a n y o f t h e s e r u n s , t h e s i l i c a a n d a l u m i n a c o n -t e n t s o f t h e fl u id p h a s e w e r e d e t e r m i n e d a f t e r v a r i o u st r e a t m e n t t i m e s . F r o m t h e s e d a t a, r a t e c o n s t a n t s f o r t h ed i s s o l u ti o n o f v o l c a n i c g l as s a n d f o r t h e f o r m a t i o n o ft h e d i f f e r e n t z e o l i t e s w e r e o b t a i n e d . A m o d e l s u i t a b l ef o r c o m p u t e r s i m u l a t i o n o f th e p r o c e s s o f g l a ss d i s s o -

l u t io n a n d z e o l i t e f o r m a t i o n w a s d e r i v e d . F i n a l l y , s o m es p e c u l a t i o n s o n t h e m e c h a n i s m o f z e o l i t e f o r m a t i o n a n da p p l i c a ti o n s o f e x p e r i m e n t a l r e s u lt s t o n a t u r al s y s t e m sa r e p r e s e n t e d .

E X P E R I M E N T A LS y n t h e s i s

T h e s y n t h e s i s c o n d i t i o n s o f t hi s s t u d y w e r e s i m i l art o t h o s e r e p o r t e d b y H a w k i n s e t a l ( 1 9 7 8 ) . T h e f o r -m a t i o n o f p h i l l i p s i t e , c l i n o p t i l o l i t e , a n d m o r d e n i t e f r o mv o l c a n i c g l a s s w a s s t u d i e d a s a f u n c t i o n o f t i m e , t e m -p e r a t u r e , m o l a r i t y o f t h e c a r b o n a t e s o l u t i o n , r at i o o f N at o K i n t h e s o l u t i o n , a n d s o l i d : l i q u i d r a t i o o f t h e r e a c -t a n t s. R e c o n n a i s s a n c e s t u d i es s h o w e d t h a t o v e r t h er a n g e o f 0 . 0 2 - 2 k b a r , p r e s s u r e d i d n o t a f fe c t th e c o u r s eo f t h e r e a c t io n s ; t h e r e f o r e , s u b s e q u e n t s t u d ie s w e r ec a r r i e d o u t a t 1 k b a r p r e s s u r e . T h e c h e m i c a l c o m p o s i -t i o n ( w t . ) o f t h e st a r t i n g v o l c a n i c g l a s s i s ( S h e p p a r da n d G u d e , 1 96 8) S i O 2 , 7 2 . 7 8 ; A 1 2 0 3 , 1 1 . 89 ; F e 2 0 3 , 0 . 5 5 ;F e O , 0 . 9 9 ; M g O , 0 . 2 2 ; C a O , 0 . 5 5 ; N a 2 0 , 3 . 0 3 ; K z O ,5 . 3 1 ; H z O + , 3 . 86 ; H 2 0 - , 0 . 2 1 ; T o t a l , 9 9 .3 9 .

T h e r e a c t i o n s w e r e c a r r i e d o u t in w e l d e d , 2 . 5 - r a m i . d .g o l d c a p s u l e s t h a t w e r e 2 c m i n l e n g t h . T w e n t y m i l li -g r a m s o f a s h a n d 2 5 / x l it e r o f t h e c a r b o n a t e s o l u t i o nw e r e u s e d . E a c h e x p e r i m e n t w a s r u n in d u p l i c a te . A s

Copyright O 1981 The Clay Minerals Society 331

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332 Hawkins lays and lay Minerals

, ; i t 1~ . H II Ip c m2i 5 C4 C

3 9~ 2 c

2 : 1 N a : K

=~ sO

4 03 O2 0, S

I Bp ~ m p

i "I l [ l I ~ I . [ I ~ J l ' , ,p r II

1 5 ~1 : 1 N a : K 1 : 2 H a : K

I I

;I ~ i ~ I I iI p~m p m " ~ m

I II 1 4 ~ I

I

. , I l l m l r n l l l ~I ~ l|:iill[ ll - _ I NI ~ l / ~ : I / / I [ m l I4 8 1 6 I 1 61 3 ~

I l i } I I l [ I I ', , . J - h I I ~ I r i l l , n n I m I mP c m p c m p c m p c m I p c m p c m p c m p c m I p c m P c m p c m p c m2 4 8 1 6 2 4 8 1 6 2 4 8 1 6D A Y S

Figure 1. Relative quantities of phillipsite p), clinoptilolite c), and mordenite m) formed in 2 M alkali carbonate solution,2:1, 1:1, and 1:2 in Na:K at 130~ 140~ and 150~ and 1 kbar.

many as four capsules were placed in single cold-sealpressure vessels; the vessels were sealed; and the de-sired pressu re-temperature conditions reached with aTem-Pres HR-4B hydrothermal unit. After an appro-priate time dictated by the experimental design, thepressure vessels were quenched, and the capsules wereweighed to test for leakage during the run. The con tentof each capsule was transferred to a 0.45-/xm Milliporefilter and washed three times with distilled water to re-move soluble carbonates. The washed product wasthen mounted on a glass microscope slide for X-raypowder diffraction XRD) analysis. Relative quantitie sof the various zeolites were estimated from the XRDpatterns.Solution studies

To determine the effect of time, temperature, andcarbonate molarity on the dissolution rate of the glassand on the rate of formation of the various zeolites, ad-ditional hydrothermal runs were made at 115~ 130~ and

140~ in 2.0 M carbo nate solution , 1:1 Na: K, and at140~ in 2.0, 1.5, 1.0, and 0.2 M car bon ate soluti on,also 1:1 in Na and K.

For solution analyses, the hydrothermal proceduredescribed above was followed except that 40.0 mg ofglass and 50.0/zliter of carbonate solution were addedto gold capsules, 4 cm long and 2.5 mm i.d. R uns weremade in duplicate, with two capsules per vessel. Uponcompleti on of a run and after weighing the capsules, thesealed capsule was rinsed with distilled water andplaced in a 25-ml evaporat ing dish to which was added1 ml of 2 M carbona te, 1:1 Na:K , solution and 2 ml ofdistilled water. Each capsule was opened while im-mersed in this solution, and the contents of the evap-orating dish were tra nsferred to a F0-ml plastic syringe.The mixture was filtered through a 0.45-~m filter intoa calibrated 10.0-ml polystyrene tube; the solid waswashed twice with distilled water, and the volume ofthe solution was then brought to 10.0 ml with distilledwater. The tubes were capped and saved for analysis

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Vol. 29, No. 5, 1981 Kinetics of glass dissolut ion and zeoli te formation 3334 0

3 0

i 2 0

1 0

p c m p c m p c m p c m p c m ) r0 . 1 7 0 . 3 3 O . S O 0 ,6 7 0 . 8 3 1 . 2 5 1 ]c m p c m p c1 , 67 2 . 5 0 5 . 0 0L I Q U I D : S O L I D R A T I O ( j u l /m g )2 0 , 8 - -

~ 1 0 1 6 . 6)

I

I I I I I4 3 2 1 0

L o g a H 4 S i O 4 )Figur e 9 . Logar i thmic ac t iv i ty d iagr am depic ting equil ib ri -um- phase r e la t ions among phi l l ips i te , c l inopt i lo l i te , and mor -denite in 2 M alkal i-carb onate solut ion.

r a t i o i s d o m i n a t e d b y t h e i n i t i a l l i q u i d c o m p o s i t i o n .F u r t h e r m o r e , n o z e o l i t e s f o r m e d u n t i l s a t u r a t i o n w a sr e a c h e d , t h u s n o p h a s e s r e m o v e d K o r N a p r i o r to s i li c as a t u r a t i o n . A s a r e s u l t , t h e l i q u i d t r a j e c t o r y s t a r t s a t l o gN a / K = 0 a n d p a r a l l e l s th e a b s c i s s a u n t i l s a t u r a t i o n .U p o n f o r m a t i o n o f p h i l l i p si t e a n d c l i n o p t i lo l i t e ( b o t hK - r i c h z e o l i te s ) , t h e N a / K r a t io i n c r e a s e s , l e a d i n g t ot h e f o r m a t i o n o f m o r d e n i t e . A t t h e e n d o f t h e h y d r o -t h e r m a l r u n , w h e t h e r m o r d e n i t e o r a c o m b i n a t i o n o fm o r d e n i t e a n d c l i n o p t i l o li t e d o m i n a t e d e p e n d s u p o nt h e c o m p o s i t i o n o f th e s e p h a s e s ; t h e d e t e r m i n i n g f a c-t o r s a r e t h e r e l a t i v e si l ic a c o n t e n t a n d t h e N a / K r a t ioo f t h e t w o z e o l i t e s . P h i l l i p s i t e w a s u n s t a b l e a t t h e h i g h -s i l i c a a c t i v i t i e s ; h o w e v e r , o n l y a t 1 50 ~ w a s t h e i n s t a -b i l i t y o f p h i l l i p s i te w i t h r e s p e c t t o m o r d e n i t e a n d c l i-n o p t i l o l i t e ( ? ) a n d c l i n o p t i l o l i t e w i t h r e s p e c t t o m o r d e n -i t e e v i d e n t ( F i g u r e 1 ).Effect of carbonate concentrationon glass-dissolution rate

T h e g l a s s - d i s s o l u t i o n r a t e w a s a f f e c t e d b y t h e c a r -b o n a t e c o n c e n t r a t i o n ( F i g u r e 6 ); h i g h e r c a r b o n a t e c o n -c e n t r a t i o n s c a u s e d f a s t e r d i s s o l u t i o n r a t e s . T h i s e f f e c t

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Vol. 29, No. 5, 1981 Kinetics of glass dissolut ion and zeoli te formation 337Table 2. Observ ed ingest ion t imes for glass dissolut ion andmaximum si l ica concentrat ions for var ious temperatures ,ca r bona te concent r a t ions , and hydr oxide- ion ac t iv i ty se t s.

M a x i m u mT e m pe ra - S i O2t u re O H ) C O 32- ) Inge s t i on c onc e n t r a t i on~ mole/liter) mole/liter) tim e hr) mole/liter)

5 0.15 2.0 48 0.135140 0.17 1.0 72 0.030. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .130 0.20 2.0 10 0.176140 0.21 1.5 30 0.089

E f f e c t o f s o l i d : l i q u id r a t i o o f r e a c t a n t sF i g u r e 2 s h o w s t h a t t h e s o l i d : l iq u i d r a ti o o f t h e r e a c -

t a n t s s t r o n g l y a f f e ct s t h e r e l a t i v e q u a n t i t y o f d i f f e r e n tz e o l i t e s fo r m e d a t a s p e c i fi c t i m e d u r i n g h y d r o t h e r m a lr u n s i n 2 M C O 3 , 1 :1 N a : K , a t 1 40 ~ a n d 1 k b a r , 8 d a y sd u r a t i o n . T h e r e l a t i v e q u a n t i t y o f z e o l i t e s f o r m e d u n d e rh i g h l i q u i d : s o l i d r a t i o s r i g h t s i d e F i g u r e 2 ) c o r r e -s p o n d s t o t h a t f o r m e d d u r i n g e a r l i e r s t a g e s o f z e o l i t eg r o w t h u n d e r l o w e r li q u i d : s o l id r a t io s c f . F i g u r e 1 f o rw h i c h l i q u i d : s o l i d i s 1 . 25 ). T h e z e o l i t e a s s e m b l a g e so b s e r v e d u n d e r l o w l i q u i d : s o l i d r a t i o s m a y r e p r e s e n ta m o r e m a t u r e a s s e m b l a g e . T h e r e a s o n f o r t h i s e f f e c ti s n o t c l e a r .

m a y b e d u e i n p a r t t o t h e c a t a l y t i c e ff e c t o f h y d r o x i d ei o n O H ) o n t h e d i s s o lu t i o n o f s i l i c a F y f e e t a l . , 1978).C a r b o n a t e i t s e l f , h o w e v e r , s e e m s a l s o t o h a v e c a t a -l y z e d t h e r e a c t i o n .

T a b l e 2 s h o w s c a l c u l a t e d h y d r o x i d e - i o n a c t i v i ti e s a sa f u n c t io n o f c a r b o n a t e c o n c e n t r a t i o n a n d t e m p e r a t u r eH e l g e s o n , 1 96 7). A r o l e f o r c a r b o n a t e i s s u g g e s t e d b y

a c o m p a r i s o n o f h y d r o x i d e c o n c e n t r a t i o n i n 2 M C O 3at 130~ w i th tha t in 1 .5 M CO 3 a t 140~ or in 2 M CO 3a t 1 1 5 ~ w i t h t h a t in 1 .0 M C O3 a t 1 40 ~ f o r w h i c h p a i r st h e h y d r o x i d e a c t i v it i e s a r e a b o u t t h e s a m e . B e c a u s et h e r a t e o f s il i c a d i s s o l u t i o n i n c r e a s e s w i t h i n c r e a s i n gt e m p e r a t u r e , i t w a s e x p e c t e d t h a t f o r t h e s a m e O H -va lue , t h e f a s t e r d i s s o l u t i o n r a t e s h o u l d b e a s s o c i a t e dw i t h t h e h i g h e r - t e m p e r a t u r e m e m b e r o f t h e p a i r. T h ed a t a o f T a b l e 2 s h o w t h a t t h e i n g e s t i o n t i m e f o r g l a s sd i s s o lu t i o n w a s s h o r t e r a n d t h a t t h e m a x i m u m s i li c ac o n c e n t r a t i o n w a s l a r g e r fo r t h e h i g h e r - c a r b o n a t em e m b e r o f t h e p a i r . I n m a k i n g t h e s e c a l c u l a t io n s , n oa t t e m p t w a s m a d e t o c o r r e c t th e c a r b o n a t e c o n c e n t r a -t i o n o r h y d r o x i d e - i o n a c t i v i t y f o r t h e v e r y h i g h i o n i cs t r e n g th o f t h e s y s t e m . M o r e r i g o r o u s c a l c u la t i o nw o u l d c h a n g e t h e a b s o l u t e v a l u e s o f th e h y d r o x i d e - i o na c t i v i t i e s , b u t t h e r e l a t i v e v a l u e s s h o u l d b e t h e s a m e ,a n d T a b l e 2 s h o u l d b e v a l i d f o r i l l u s t r a ti v e p u r p o s e s .E f f e c t o f c a r b o n a t e c o n c e n t r a ti o n o nz e o l i t e - f o r m a t i o n r a t e

F i g u r e 6 s h o w s t h a t z e o l i t e g r o w t h i s a fu n c t i o n o f t h ec a r b o n a t e c o n c e n t r a t i o n . K e r r 1 9 66 a) sh o w e d t h a tz e o l i t e - g r o w t h r a t e i s f i r s t o r d e r w i t h r e s p e c t t o t h eq u a n t i t y o f z e o l i te p r o d u c e d a n d t h a t t h e r a t e o f z e o li t ef o r m a t i o n i s d e p e n d e n t o n t h e c o n c e n t r a t io n o f a c ti v es o l u b l e s p e c i e s d i s s o l v e d S i O 2 , A I O H ) 4 - ) . H e a l s os u g g e s t e d t h a t t h e c o n c e n t r a t i o n o f t h e a c t i v e s p e c i e ss h o u l d d e p e n d o n t h e c o n c e n t r a t i o n o f h y d r o x i d e i on .H a y h u r s t a n d S a n d 1 97 7) s h o w e d t h a t p h i l li p s i t e n u -c l e a t i o n a n d g r o w t h a r e s e c o n d - o r d e r r e a c t i o n s w i t hr e s p e c t t o O H - . A l t h o u g h t h e c a r b o n a t e a n d / o r h y -d r o x i d e i o n s c l e a r l y a f f e c t z e o l i t e - g r o w t h r a t e s , t h ep r e s e n t d a t a a r e i n a d e q u a t e t o q u a n t i f y t h e s e e f f e c t s .

T h e a l u m i n u m p r o b l e mA s u r p r i s i n g r e s u l t o f t h e s e s t u d i e s w a s t h e l o w c o n -

c e n t r a t i o n o f A l in s o l u t i o n . T h e A I c o n c e n t r a t i o n w a se x p e c t e d t o i n c r e a s e l i n e a r l y a n d p r o p o r t i o n a l l y to t h el i n e a r i n c r e a s e i n t h e s i l i c a c o n c e n t r a t i o n a s t h e g l a s sd i s s o l v e d , a s f o u n d b y M a r i n e r a n d S u r d a m 1 9 7 0) . T h eA I c o n c e n t r a t i o n a s A I O H )4 - ) o b s e r v e d i n t h e p r e s e n ts t u d y w a s a b o u t 3 . 7 x 1 0 - 4 m o l e / l i t e r. F o r t h e h y d r o x -i d e - i o n a c ti v i t i e s e x p e c t e d i n 2 M c a r b o n a t e a t t e m p e r -a t u r e , t h e c a l c u l a t e d A I O H ) 4 - c o n c e n t r a t i o n i n e q u i -l i b r i u m w i t h g i b b s i t e i s 4 . 2 x 1 0 - 4 m o l e / l i t e r . T h i sp e r h a p s f o r t u i t o u s a g r e e m e n t s u g g e s t s t h a t t h e A I c o n -c e n t r a t i o n i s b u f f e r e d b y g i b b s i t e . A t t e m p t s t o o b s e r v ed i r e c t ly t h is p r e d i c t e d g i b b s i t e p h a s e b y S E M a n dK E V E X a n a l y s i s w e r e u n s u c c es s f u l. M a y et al . 1979)s t u d ie d t h e s o l u b i li t y o f h y d r o x y - a l u m i n u m s o l id s i na l k a l i n e s y s t e m s a n d s u g g e s t e d t h a t a n u n i d e n t i f i e dp h a s e b o e h m i t e ? ) l e s s s o l u b l e t h a n g i b b s i t e c o n t r o l st h e A I c o n c e n t r a t i o n i n a l k a l i n e s o l u ti o n s . H o l d r e n a n dB e r n e r 1 9 79 ) s u g g e s t e d t h a t A l f o r m s a f i n e - g r a i n e dp r e c i p i t a t e t h a t m a i n t a i n s t h e A I c o n c e n t r a t i o n a t v e r yl o w l e v e l s . T h e y w e r e u n a b l e t o l o c a t e o r i d e n t i f y t h i sp h a s e . D e t a i l e d S E M s t u d ie s a n d m i c r o p r o b e a n a l y s e so f th e s o l i d p h a s e s a r e n e e d e d t o r e s o l v e t h i s q u e s t i o n .

S P E C U L A T I O N S O N T H E M E C H A N I S MO F Z E O L I T E F O R M A T I O N

T h e s t r u c t u r e o f p h i l l i p s i t e i s d o m i n a t e d b y 4 - m e m -b e r e d r i n g s o f S i O 4 t e t r a h e d r a B r e c k , 1 9 7 4 ). T h e s t r u c -t u r e o f c l i n o p t i l o l i t e A l b e r t i , 1 9 75 ) i s s i m i l a r t o t h a t o fh e u l a n d i t e a n d h a s a c h a r a c t e r i s t i c c o n f i g u r a ti o n o f4 - a n d 5 - m e m b e r e d r i n g s o f S iO 4 t e t r a h e d r a . T h e m o r -d e n i t e s t r u c t u r e B r e c k , 1 9 7 4 ) i s c h a r a c t e r i z e d b y 5 -m e m b e r e d r i n g s . B a e s a n d M e s m e r 1 97 6) s h o w e d t h a ta t h ig h p H > 10 ) t h e d o m i n a n t d i s s o l v e d s i l ic a s p e c i e si s th e t e t r a m e r S i 4O s O H )4 4 - w i t h l e s s e r q u a n t i t i e s o fS i O O H ) 3 - a n d S i O 2 O H ) 2 2 -. I n s o l u t i o n s o f l o w i o n i cs t r e n g t h , t h e d o m i n a n t d i s s o l v e d s i l i c a s p e c i e s i sS i O O H ) 3 - . T h e t e t r a m e r S i4 0 8 O H )4 4 i s e i t h e r a b s e n to r i s p r e s e n t i n n e g l i g ib l y l o w c o n c e n t r a t i o n s . T h e d o m -i n a n t A l s p e c i e s i s A I O H ) 4 B a e s a n d M e s m e r , 1 9 7 6) .

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338 Hawkins Clays and Clay MineralsThe dominance of tetramers both in concentrated

solution and in phillipsite is striking. It is suggested herethat phillipsite forms by a condensation reaction in-volving the silica tetramers . Be cause tetra mers are themajor silica species, the condensation reaction domi-nates, and phillipsite is the first zeolite to form. Sucha process involving selective removal of the tetramersmight then lead to an increas e in the ratio of monome ricto tetrameric species and then to the easier formationof zeolites such as clinoptiloli te in which 4- and 5-mem-bered tetramer + a monomer) rings are presen t, andfinally to 5-membered str uctures such as mordenite.The role of silica-cyclic tetramers was emphasiz ed byHayh urst and Sand 1977). The present study drawsattention to the abu ndance of the tetrameric species insolution under conditions suitable for zeolite forma-tion.

SEM observations of the products of this study andof natural zeolite ass emblages show pitting of the glasssurface due to dissolution cf. Mumpto n, 1973) alongwith an intimate association of zeolites and glassshards. The pitting suggests that glass dissolutio n doesnot proceed u nifor mly over the surface but is more rap-id at sites of excess surface energy. In the presentstudy, no secondary protective surface could be seenon the glass. Both of these aspects were discussed byHoldr en and Berne r 1979) in their study of feldspardissolution.

The question also arises, why do zeolites form closeto the glass and not so me place in the solution relativelyremote from the glass surface? Zeolites should nucleateanywhere in solution, but growth depends upon thesupply of nutrient s. It is suggested that the zeolites formon or very close to the dissolving phases glass, gibb-site?, boehmite?) in response to the high flux of nu-trients from these phases and not because of somestructural similarity of parent and daughter phases.

APPLICATION TO NATURAL SYSTEMSThe results of this study are most applicable to geo-

thermal systems, especially those such as the Hot DryRock Project Cremer et al., 1980) in which ho t, fresh,comm only glassy rock is fractured and w ater is pumpedthrough the fractures. The rate cons tants from the pres-ent study may be applicable to the rate of dissolutionof such rock and to the kind and quantity of secondaryminerals such as zeolites that form during the devel-opment of the geothermal system.

At present, the results of this study cannot be ade-quately extrapolated to low-temperature conditions,inasmuch as both the glass-dissolution behavior and thedissolved silica species seem different in the hydro-thermal system from those at low temperatures. In thehydrothermal system, large silica monomers such asthe cyclic tetramers are abun dant and probably play animportant role in zeolite formation under these condi-

tions. In the more dilute, low-temperature systems,large monomers are much less abundant. Rate equa-tions derived for hydrothermal conditions involvinglarge monomers will be inapplicable at low tempera-tures where these monomers are essentially absent.

Critical information needed to understand zeoliteformation under all conditions is the a mount and kindof silica monomers and polymers present in solutionand their behavior during zeolite formation. These datacoupled with reliable analys es of the aqueo us fluid andcoexisting zeolites and with more t hermodynamic datafor zeolites will ultimately permit more complete un-derstanding of zeolite formation under natural condi-tions.

ACKNOWLEDGMENTSI thank my colleagues W. D. Harrison, R. A. John-

son, P. A. Andre sen, G. D. Gislason, and C. La ndo fortheir helpful discussions of various aspects of thiswork. I am indebted to S. Swanson, L. C. Hoskins, J.R. Boles, and R. A. Sheppard for their critical reviewsof this paper and to D. R. Kosiur for his comme nts andfor making preprints of his papers available to me.Thanks are extended to D. Glover for his assistancewith the chemical analysis and to M. A. Borchert forher help with the scan ning electron micro scopy. Thisstudy was partly funded by an NS F Institutional Grantto the University of Alaska.

REFERENCESAlberti, A. 1975) The crystal structure of two clinoptilolites:Tschermaks Min. Petr. Mitt. 22, 25-37.Baes, C. F., Jr. and Mesmer, R. E. 1976) The Hydrolysis ofCations: Wiley-Interscience, New York, 337-342.Breck, D. W. 1974) Zeolite Molecular Sieves: Wiley, NewYork, 45-132.Cremer, G. M., Duffield, R. B., Smith, M. C., and Wilson, M.G. 1980) Hot Dry Rock geothermal energy program: An-nual Report; Fiscal Year 1979: Los Alamos Sci. Lab. Rept.LA-8280-HDR, UC-66A~ 239 pp.Fyfe, W. S., Price, H. J., and Thompson, A. B. 1978) Fluidsin the Earth s Crust: Elsevier, New York, 95-110.Hawkins, D. B., Sheppard, R. A., and Gude, A. J., 3rd 1978)Hydrothermal synthesis of clinoptilolite and comments onthe assemblage phillipsite-clinoptilolite-mordenite: in Nat-

ural Zeolites: Occurrence, Properties, Use, L. B. Sand andF. A. Mumpton, eds., Pergamon Press, Elmsford, NewYork, 337-343.Hayhurst, D, T. and Sand, L. B. 1977) Crystallization ki-netics and properties of Na, K phillipsites: in MolecularSieves II, ACS Symposium Series 40, Amer. Chem. Soc.,Washington, D.C., 219-232.Helgeson, H. C. 1967) Thermodynamics of hydrothermalsystems at elevated temperatures and pressures: Amer. J.Sci. 267, 729-804.Helgeson, H. C. 1971) Kinetics of mass transfer among sil-icates and aqueous solutions: Geochim, Cosmochim. Acta35, 421--469.Holdren, G. R., Jr. and Berner, R. A. 1979) Mechanism offeldspar weathering--I. Experimental studies: Geochim.Cosmochim. Acta 43, 1161-1171.

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V o l . 2 9 , N o . 5 , 1 98 1 K i n e t i c s o f g l a s s d i s s o l u t i o n a n d z e o l i t e f o r m a t i o n 3 3 9K e r r , G . T . ( 1 9 6 6 a) C h e m i s t r y o f c r y s t a l l i n e a lu m i n o - s i l i -

c a t e s : I . F a c t o r s a f f e c t i n g t h e f o r m a t i o n o f z e o l it e A : J .P h y s . C h e m . 7 0 , 1 0 4 7 - 1 0 5 0 .K e r r , G . T . ( 1 9 6 6 b) C h e m i s t r y o f c r y s t a l l i n e a lu m i n o - s i l i -c a t e s : I V . F a c t o r s a f f e c ti n g t h e f o r m a t i o n o f z e o l it e s X a n dL : J . P h y s . C h e m . 7 2 , 1 8 : 1 3 8 5 - 1 3 8 6 .

K o s i u r , D . R . ( 1 98 1 ) T h e d i a g e n e s i s o f p e l a g i c z e o l i t e s : G e o -c h i m . C o s m o c h i m . A c t a ( i n p r e s s ) .K r a u s k o p f , K . B . (1 9 6 7 ) I n t r o d u c t i o n t o G e o c h e m i s t r y :M c G r a w - H i l l , N e w Y o r k , 6 5 7 - 6 6 3 .M a r i n e r , R . H . a n d S u r d a m , R . C . ( 1 97 0 ) A l k a l i n i t y a n d f o r -m a t i o n o f z e o l i t e s i n s a l i n e a l k a l in e l a k e s : S c i e n c e 1 7 0 , 9 7 7 -

9 8 0 .M a y , H . M . , H e l m k e , P . A . , a n d Ja c k s o n , M . L . ( 19 7 9) G i b b -s i te s o l u b il i ty a n d t h e r m o d y n a m i c p r o p e r t i e s o f h y d r o x y -a l u m i n u m i o n s i n a q u e o u s s o l u t i o n s a t 2 5~ G e o c h i m . C o s -m o c h i m . A c t a 4 3 , 8 6 1 - 8 6 8 .M u m p t o n , F . A . ( 19 7 3) S c a n n i n g e le c t r o n m i c r o s c o p y a n dt h e o r i g in o f s e d i m e n t a r y z e o l i t e s : i n M o l e c u l a r S i e v e s - -P r o c . 3 r d I n t . C o n f . M o l e c u l a r S i e v e s , Z i i r i c h , 1 9 7 3 , J. B .U y t t e r h o e v e n , e d . , L e u v e n U n i v . P r e s s, L e u v e n , B e l g i u m ,1 5 9 - 1 6 1 .

S a n d , L . B . ( 1 98 0 ) Z e o l i t e s y n t h e s i s a n d c r y s t a l l i z a t i o n : i nP r o c . 5 t h I n t . C o n f . Z e o l i t e s , N a p l e s , 1 9 80 , L . V . C . R e e s ,e d . , H e y d e n , L o n d o n , I - 9 .

S h e p p a r d , R . A . a n d G u d e , A . J ., 3 rd ( 1 96 8 ) D i s t r i b u t i o n a n dg e n e s i s o f a u t h i g e n i c s i li c a te m i n e r a l s i n t u f t s o f P l e i s t o c e n eL a k e T e c o p a , I n y o C o u n t y , C a l i f o r n i a : U .S . G e o l . S u r v .P r o f . P a p . 5 9 7 , 3 8 p p .

S i c k s , G . C . ( 1 97 5 ) T h e k i n e t i c s o f s i l ic a d i s s o l u t i o n f r o m v o l -c a n i c g l a s s i n t h e m a r i n e e n v i r o n m e n t : H a w a i i I n s t. G e o -p h y s . R e p t . H I G - 7 5 - 2 3 , 8 2 p p .T a r d y , Y . a n d G a r r e l s , R . M . ( 19 7 4) A m e t h o d o f e s t i m a t i n g

t h e G i b b s e n e r g i e s o f f o r m a t i o n o f l a y e r s i li c a te s : G e o c h i m .C o s m o c h i m . A c t a 3 8 , 1 1 0 4 - 1 1 1 6 .T a r d y , Y . a n d G a r r e l s , R . M . ( 1 97 6 ) P r e d i c t i o n o f G i b b s

e n e r g i e s o f f o r m a t i o n , p a r t I: R e l a t i o n s h i p s a m o n g G i b b se n e r g i e s o f f o r m a t io n o f h y d r o x i d e s , o x i d e s a n d a q u e o u si o n s . G e o c h i m . C o s m o c h i m . A c t a 4 0 , 1 0 5 1 - 1 0 5 6 .

T a r d y , Y . a n d G a r r e l s , R . M . ( 1 97 7 ) P r e d i c t i o n o f G i b b se n e r g i e s o f f o r m a t i o n , p a r t II : R e l a t i o n s h i p s a m o n g G i b b se n e r g i e s o f f o rm a t i o n o f s i li c a t e s, o x i d e s a n d a q u e o u s i o n s .G e o c h i m . C o s m o c h i m . A c t a 4 1 , 8 7 - 9 2 .

W o l l a s t , R , ( 1 97 4 ) T h e s i l ic a p r o b l e m : i n T h e S e a 5 , E . D .G o l d b e r g , e d . , W i l e y , N e w Y o r k , 3 5 9 - 3 9 4 .R e c e i v e d 1 0 N o v e m b e r 1 9 80 ; a c c e p t e d 2 0 A p r i l 1 9 8 1)

P e a m M e - - F I o c ~ e n e p H o ~ a , 3 a BH C ~ ll Re ro O T T e M n e p a T y p h l , B O a p e M ~ K O T Op OF OBblCTyHalOT MaJlble paCTBO-peHH n, pac Tao peH He p1402inToaoro C TeK. rl a MO;~KeT 6b lTb onH cal{o KaK dC /d t = K(C s - C ) , r~ le C s =KOHIJ[eHTpaUH~I paCT OpeHHOrOKpeM ne3eM a npH aacb~ UleHHn, C = MFHOBeHHaH KoHR eHTpaUH~I Kpe Mne -3eMa, 14 g = nOC TOgHHaH cgo poc TH , paaHa~l 1 , 6 10 -5 , 3 , 0 10 -5 , 4 , 5 10 -5 ceK -1 npH 115 ~ 130~a 1 4 0~ C O OT ae TC Ta eH H o a 2 M N a - K g a p 6 o u a T n h l x p a c T n o p a x n p H ~ Ia aa eH H u 1 g f a p . H p n 1 3 0 ~a e 2 i n q u u a C s 0 , 1 7 7 M S i O 2 ~ 1 o c T n r a e T c a n T e q e n u e 3 0 q a c o n n n a q n H a e T c ~ l o 6 p a a o n a n n e ~ H Y l2 IH n CH T a,K2IHHOI1TH~O2IHTa [4 Mo p~e Hn Ta na 34 , 6 4, 14 76 qa cy COOTBeTCTBeHHO B 2 M CO 3 1:1 N a/ K . Bo np eM apacT[4OpCH14a cTe g2ia n 06 pa ao [4 an aa I Ieo.r114Ta KO141~eHTpaRHH AI g ag A I(O H )4 - aMopT14314poaaHa up143, 7 10 -4 M He[4~e14T[4dp14IIUpOBaHHO fiq b a a o f i . O T u o m e n 1 4 e S i O z 14 A I ( O H ) 4 - u p 1 4 n o aa 2 ie a1 4 14 o t p a -3oaa14na aeo 2 iaT a pa[414o 475 . B 2 M C O 3 pac Ta op e gp ncT a . ~ 14a a t ln a qb14~]2 innc14Tanaq14naeTc~l na 144 qacyup14 115 ~ Ha 34 qa cy np14 130 ~ n Ha 20 qa cy np n 140 ~ KpHcTa2ia143au.aa qbH2i~14nC14Tan a q a n a e r c a H a4 8 q a c y a 1 , 5 M C O n , H a 1 68 q a c y a 1 , 0 M C O 3 , 14 n o c a e 6 o n c e 5 5 0 q a c o[ 4 [4 0 , 2 M C O 3 n p H 1 4 0 ~~O140~H14Te~b140 r KaTa .q143y O H - HO,qB JIHeTC H TaK)Ke C O 32- , qTO6b l KaTa2in314ponaTb npo l I eC C blpaCTBOp e1414a cT eg 2i a n otpa3O BaH 14H IIeo2114Ta. @HJI2IH14C14TRBYLqeTcgTepMo~HHRM14qeCKHHecTafILrlbHbIMn o OTHO IIIe14HIO K KJ114HOnT142IOJIHTy14 Mop~eHHTy B rpeM ne ae M oto raT b~ X IReYlOqHblX r14RpOTepMa21hHblXpa cT ao pa x . C na qa aa 06 pa ay eT cg (])H212IHIIC HT, aa HaM c ae ay eT K2IHHOHTH2IO2IHT H HOTOM MopR eHHT.O 6p aa oa an nm qb142i~14ncnTa cn oc o6 cT ao B a ~a yc2 ioam a OnblTOB IIpO~102I )KHTe2IbHOC TblO O) lHOf i ue~ le2 innpH TeM nep aTy pe MeHee 150~ n ~ a K-6 oraT hlX X~14~lKOCTefi. O 6p aa oa an m o KaHHOnTn2IO2IHTa6 a a r o -np14HTCTBOaa.qH yC2IOa14H OHhlTOB HpO~IOJIX ~14TeYlhHOCThlO o 2ie e o~ m o~ Helical4 14p14 T e u n e p aT y p e Me He e150~ n / I~H K-6ora ThlX ~14~KOC Tef i. Otpaao aaH1 4Fo Mop~IeH14Ta cn oc o6 cT ao na an yC2IO[4H~I onbITO a14poRo2I)K14Te2IbHOCTh~O 6o ae e O~HOfi ne ~ ea u up14 Te M ne pa Ty pe 6 o2iee 140~ 14 ~2ia Na -6o raT blX X~14~ZO-CTefi . Bo ap eM g 8-~14ea14oro n ep1 4o~ a 14p14 140 ~ o 6 p a a o s a u n ~ K2114HOrlT1422IO2114TaCHOCO6CTBO14a210COOTnome1414e )KH~KOCTh TBep~lbII~ pea reH T ( ot ~e M :M acc a) M ene e, qeM 1,0, Mop~lenaTa---COOTnomeHHe OT0,8 5 ~Io 1,5 14 qbn~2114nc14Ta---COOTnomeHHe o nc e 1,5. MeXaHHaM ot pa ao sa ~m H paa2114qnb~x I I[eO2IHTOB, Boco6e14nocT14 qba~2innCHTa, MoX~eTBK~1~OqaTbgpeMH eaeMH hle u~agaHqecgH e TeTpaMepb~, ~OTOpb[e M14oro-qHC ~eHnh~ a gOalI enTp14posanHb~X pa cT ao pa x up14 n le2 ioqub~x rn~ lpoTepMa~lbHb~X ycao[4m ax , HO KOTOpbIenoqT140TC yTCT[4y~OT B paa taa2 ieHHb lX HnagOTeMl lepaTypHblX pa cT ao pa x . TaK14M o6 paa oM , peay2 ibTaTb~rn~poTepMa2ibHblX 3KC HepHMeHTOB He MOFyT 6b lTb Henop e~lcTaen Ho HC HO2Ib3OB aHbl npH 06paa oaanH 14a e o 2 i H T a n p [ 4 n H 3 r H X T e M n e p a T y p a x . [ E . C . ]

8/13/2019 Kinetics of Glass Dissolution

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340 Hawkins lays and lay MineralsResiimee- -Nach einem temperaturunabh~ingigen Zeitabschnitt, in dem die Aufl6sung gering ist, kann dieAufl6sung yon Rhyolithglas durch dC/dt = k(Cs - C) heschrieben werden, wobei Cs die Konzentratio n angel/~stem SiO2 bei S~ittigung und C die augenblickliche SiO~-Konzent ration ist; k, e ine Geschwindigkeit s-kons tante betr~igt 1,6 10 5, 3,0 10 -5, und 4,5 10 5 sec ~ bei 115 ~ 130~ und 140~ in 2 M Na-K-Karbonat l6sungen und bei 1 kbar Druck. Bei 130~ wird ein Cs-Wert yon 0,177 M SiO2 in 30 Std erreicht;Phillipsit, Klinoptilolith und Mordenit beginnen nach 34, 64 bzw. 76 Stunden, sich in einer 2 M CO3, 1:1Na/K zu bilden. WS.hrend der Aufl6sung des Glases und der Zeolithbildung wird die Al-Konzentration alsAI(OH)4- durch eine nicht ident ifizie rte Phase bei 3,7 10 -4 gepuffert. Das SiO2 zu AI(OH)4- Verh~iltnisbetr~igt zu Beginn der Zeolithbildung 475. Die Kri stallisation von Phillipsit beginnt in 2 M CO3-L6sung nach144 Stunden bei 115~ nach 34 Stunden bei 130~ und nach 20 Stunden bei 140~ In 1,5 M CO3 beginntdie Phillipsitkristallisation nach 48 Stunden, in 1,0 M CO3 nach 168 Stunden , und n ach fiber 550 Stundenin 0,2 M COa bei 140~ Zus~itzlich zur Kat alysieru ng durch OH - schein t auch CO32 die Aufl6sung desGlases und die Zeolithbildung zu katalysieren.Thermo dynamis ch ist Phillipsit im Vergleich zu Klinoptilolith und Mordenit in SiO2-reichen alkalischenhydrot hermalen L6sungen instahil. Phillipsit bildet sich zuerst, d anach ent steht Klinoptilolith und d anachMordenit. Die Phillipsitbildung wird dutch eine Reaktionszeit yon 1 Woche, Temper aturen unter 150~und K-reichen L6sung en beg/instigt. Klinoptilolith bildet sich bevorzugt bei Reaktionszeiten fiber einerWoche, Temperaturen un ter 150~ und K-r eichen L6sungen. Die Mordenitbildung wird durch Reaktions-zeiten fiber einer Woche, Temperaturen fiber 140~ und Na-r eichen L6sungen begfinstigt. Bei einer Reak-tionszeit yon 8 Tagen und bei 140~ wird die Bildung yon Klinoptilolith durch ein L6sung /Festsubs tanz-Verh~iltnis un ter 1,0 begfinstigt, die yon Mordenit durch Verh~iltnisse yon 0,85 his 1,5 und die yon Phillipsitdurch Verh~iltnisse fiber 1,5. Der Bildungsmechanismus der ve rschied enen Zeolithe, vor allem der yo nPhillipsit, kann mit zyklischen SiO2-Tetrameren zusammenh~ingen, die in konzentriert en L~sungen unteralkalischen hydrot hermalen Bedingungen hS.ufig sind, aber in verdfinnten niedrig temperi erten L6 sungennahezu fehlen. Aus diesem Grund k6nnen die Ergebnisse der hydrothermalen Experimente nicht direktauf die Zeolithbildung bei niedrigen Temperaturen angewende t werden. [U.W.]

R6sum6--Apr~s une p6riode d6pendan te de la temp6rature pendant laquelle se passe peu de dissolution,la dissolution de verre rhyolitique peut ~tre d6crite comme dC/dt = k(C s - C), off Cs est la concent ratio nde la silice dissoute au point de saturation, C est la concentration instantann6e de silice, et k est uu tauxconst ant 6gal 5. 1,6 l0 ~, 3 l0 -5, et 4,5 10 5 sec-1 5. ll5OC, 130oc, et 140~ res pecti vem ent, dansune solution carbonate 2 M Na-K 5. l kbar de pres sion. A 130~ une valeur pour Cs de 0,177 M SiO2 estatteinte en 30 heures, et de la phillipsite, de la clinoptilolite, et de la mordenite commencent 5. se formeren 34, 64, et 76 heure s resp ectiv ement, dans 2 M COa, l:l Na/K. Pendant la dissolu tion du verre et laforma tion de z6olite, la concentra tion d Al en tant qu Al(OH)4 est amoindr ie 5. 3,7 10 4 par une phasenon-identifi6e. La proporti on de SiO2 5. AI(OH)4- au d6but de la formation de la z6olite est 475. Dans unesolution 2 M CO3, la cristallisation de phillipsite commence apr~s 144 heures 5. 115~ apr~s 34 heur es 5.130~ et apr~s 20 heures 5. 140~ La cristallisation de phillipsite com mence apr~s 48 heure s dans 1,5 MCO3, apr~s 168 heu res dans 1,0 M CO3, et apr~s plus de 550 heures dans 0,2 M CO3 5. 140~ En plus dela catalyse d OH , COa 2 semble aussi catalyser les proc6d6s de dissolution de verre et de formation dez6olite.Thermod ynamiqu ement parlant, la phillipsite est instable en comparaison avec la clinoptilolite et la mor-denite dans des solutions hydrothermales alkalines riches en silice. La phillipsite est form6e en pr emierlieu, suivie de la clinoptilolite et puis de la mordenite. La formation de phillipsite est favoris6e par desexp6riences d une s emaine, des temperatures sous 150~ et des fluides riches en K. La formation declinoptilolite est favoris6e par des exp6riences de plus d une semaine, des temp6ratures sous 150~ et desfluides riches en K. La for mation de mordenite est favoris6e par des exp6riences de plus d une s emaine,des temp6ratures plus 61ev6es que 140~ et des fluides riches en Na. Dans des exp6 riences de 8 jou rs 5.140~ la forma tion de clinoptilolite 6tait favoris6 e par des taux de r6action liquide : solide (volume : masse)plus ba sq ue 1,0, celle de la mordenite par de s taux de 0,85 5. 1,5, et celle de la phillipsite par des taux plus61ev6s que 1,5. Le m6canisme de formation des diff6ren tes z6olites, particuli~rement de la phillipsite, peutimpliquer de t6tram~res silice-cycliques qui abonden t dans des solutions concentr6es sous des cond itionshydrot hermales alkalines, mais qui sont quasi absentes darts des solutions dilu6es 5. basses temp6ratures.Ainsi, les r6sultats d exp6r iences hydr othermales ne peuvent pas ~tre directement appliquables 5. la for-mation de z6olite 5. de basses temp6ratures. [D.J.]