Composition Clay Minerals

7/27/2019 Composition Clay Minerals

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doi:10.1144/GSL.ENG.2006.021.01.022006; v. 21; p. 13-27Geological Society, London, Engineering Geology Special Publications

2. The composition of clay materials

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2 . I h e c o m p o s i t i o n o f c l a y m a t e r i a l s

Clay mater ia l s are composed of so l id , l iqu id and vapourphases . The so l id phases are of minera l and organicp h as es t h a t m ak e u p t h e f r am ewo rk o f t h e c l ay m a t e ri a ls .The minera logy can be broadly subdiv ided in to the c layand non-clay minera l s , inc luding poor ly crys ta l l ine ,so-cal l ed 'amorphous ' inorganic phases . By def in i t ion ,minera l s are crys ta l l ine so l ids wi th wel l -ordered crys ta ls t ructures bu t c lay minera l s and o ther inorganic phases inclay mater ia l s are of ten poor ly crys ta l l ine compared tominera l s such as quar tz and fe ldspar .So m e c l ay m a t e r ia l s m a y b e d o m i n a t ed b y o n e m i n e ra lphase , e .g . smect i t e in bentoni tes , opal in d ia tomaceousear ths . However , mos t c lay mater ia l s are composedof heterogeneo us m inera l mix tures . Based on the bu lkm i n e ra l an a l y si s o f o ve r 4 0 0 s am p les , Sh aw & W eav er(1965) repor ted the modal minera logical compos i t ion ofs i li c ic las t i c mud rocks to be:

6 0 % c l ay m i n e ra l s3 0 % q u a r tz an d ch e r t5 % fe l d sp a r4 % ca rb o n a te s1% organic mat ter1% i ron oxidesThere i s a genera l increase in the predominance of c layminera l s in sed imen tary rocks wi th decreas ing gra in s ize

1 0 0 -

8 0 -

~gz.'cot . k l r ~r ~ - - -

(Fig. 2.1) (Blat t e t a l . 1972) . However , i t needs to bes t ressed that , whi l s t c lay m inera l s are usu al ly s ign i f i can t,i f no t p redominant , phases in c lay mater ia l s , o ther minera lphases are usual ly presen t in vary ing amounts and cans igni f i can t ly af fec t the proper t i es and behaviour of themater ia l s .In so i l s , minera l and organic compos i t ional var ia t ionsref lec t the weathered paren t rocks and the phys ica l , chem-ical and b io logical fac tors cont ro l l ing the so i l fo rmingprocesses (see Ch apter 3 ) .The l iqu id and vapour phases , o f which w ater i s usual lythe mos t impor tan t , occur e i ther as 'bound phases ' ,adsorbed onto the surfaces o f the so l id par ti c les , o r as' f ree phases ' wi th in pore spaces .In th is chapter the nature of the minera logical , o rganicand aqueou s phases prese n t in c lay mater ia l s aredescr ibed p lus the nature of swel l ing and ion ic exchangeproper t i es in so l id phases .2 . 1 . C l a y m i n e r a l s

The c la y minera l s are a group of hydrous a lum inos i l ica test h a t a r e ch a rac t e r i s t i ca l l y fo u n d i n t h e c l ay (< 2 g m )fract ions of sed iments and so i l s . The major i ty of c layminera l s have sheet s i l i ca te s t ructures (see Text Box onp. 14).

S a n d I S ilt I C l a yFIG. 2.1. Detrital mineralogy of sedimentary rocks as a function of ~rain size (after Blatt et a l . 1972).


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14 THE COMPOSITION OF CLAY MATERIALS

T h ~ , : s h e ~ i N r i i!i ::if:i.!i !, i :i :i ii ~if: i~i i i::~ii i ii i ii i if i!~ ii ii i il i, ~il i ~iil, !~:: ,:~,::~,::~,:: : iir :;!:i~ili!ii!!i!ii!:?iif: ?i:i :?,:i:;i!:%i!i::~~i! :::liiii::!ii:!!iii: i} i :~i::ii:i::)i:! !i:,:ii!~,:ii : i/:i:i ~:!!i;::i:::i::~::iii ~::i::::i~::!,ii:,!i~i:: ilii ii::ii,,iiii!!::i

FIG. 2.2. Com posite layers in clay mineral s tructures.


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THE CO MPOSITIONOF CLAYMATERIALS |

T e t r a h e d r a l l a y e r

O c t r e h e d r a l l a y e r

2 l a y e r ( I : I y p e

KAOLINITE

3 l a y e r ( 2 : 1) y p e

Interlayer sites X X

ILLITESSMECTITESVERMICULITESCHLORITESFIG. 2.3. Gen eral structures of c lay minerals.TABI~E 2.1. Classification o f clay mineralsSheet silicate type Property1:1 ~p___~Kao lin and serpentine Non -swelling2:1 typeIllites Non-swellingChlorites Non-swellingSm ectites (Montm orillonites) SwellingVermiculites SwellingMixed layer clays with smectite/ SwellingvermiculiteMixed layer clays without Non-swellingsmecfite/vermiculitePalygorskite and sepiolite Non -swelling

The r e a r e two type s o f c om pos i t e l a ye r s t r uctu r es in thec l a y m i n e r a ls :9 t he two l a y e r o r 1 : 1 t ype r e p r e se n te d by the ka o l in a ndse r pe n t ine g r oups ;9 t he th r e e l a ye r o r 2 :1 type r e p r e se n te d by the i l l i t e -m ic a , sm e c t i t e , ve r m ic u l i t e a nd c h lo r i t e g r oups

(Fig . 2 .3 and Tab le 2 .1) .2 . 1 .1 . T h e k a o l i n a n d s e r p e n t i n e g r o u p sT h e k a o l i n g r o u p o f m i n e r a l s is t h e m o s t c o m m o n o f th ec l a y m ine r a l s w i th the two l a ye r 1 : 1 type o f s t r uctu r e. Thete r m k a n d i t e ha s a l so be e n use d to de sc r ibe ka o l in g roupm in e r a l s bu t i s no t t he p r e fe r r e d na m e .Ka o l ins ha ve d ioc ta he d r a l s t ruc tu r es w i th , i de a l ly , none t ne ga t ive c ha r ge on the c om pos i t e l a ye r s a nd c onse -que n t ly no c om pe nsa t ing in t e r l a ye r c a t ions ( F ig . 2 . 4 )o r wa te r l a ye r s . The r e a r e th r e e p r inc ipa l po lym o r phs( o r po ly type s ) , ka o l in i t e , d i c k i t e a nd na c r i t e o f ge ne r a lform ula A14Si4Oa0(OH)8. Spec if ic nam es h ave a lso be en

g ive n to ka o l in - r i c h de pos i t s t ha t oc c ur in d i f f e r e n tf o r m a t ion e nv i r on m e n t s ( se e Te x t B ox on p . 16 )Ka o l in i t e c a n r a nge f r om w e l l c r ys t a l l i z e d va r i e t i e sto poor ly c r y s t a l l i ne f o r m s tha t c a n be d i f f e r e n t i a t e dby X - r a y pow de r d i f f r a c t ion a na lys i s ( se e S e c t ion 8 . 2 . 2) .Ha l loy s i t e is a ka o l in g r oup m in e r a l tha t c on ta ins onewa te r l a ye r i n the in t e r l a ye r s i te s , p r odu c ing a 10 ) ~ in t e r -l a ye r spa c ing r a the r t ha n the 7A spa c ing o f t he ka o l in i t estructure.I t s genera l form ula is A14Si4010(OH)8"H20. Re m ov al o fthe wa te r l a ye r c a use s the ha l loy s i t e s t ruc tu re to c o l l a pseto a 7A in t e r l a y e r spa c ing . The c o l l a pse d f o r m i s r e f e r r e dto a s 7A ha l loys i t e ( o r m e ta ha l loys i t e ) a s oppose d tot h e h y d r a t e d 1 0 A h a l l o y s it e . H a l l o y s it e s u s u a l l y h a v e ac ha r a c t e r i s t i c t ubu la r m or pho logy though o the r sphe r o i -da l ha b i t s ha ve be e n r e por t e d ( F ig . 2 . 5 ). T he tub u la rm o r p h o l o g y a r is e s f r o m t h e d i s t o rt i on a n d c u r v i n g o f th e1 : 1 l a ye r s t r uc tu r e f r om w hic h the tubu la r ha b i t de ve lops .The se r pe n t ine g r oup o f m in e r a l s i s t he t r i oc t a he dr a le qu iva le n t o f t he ka o l in g r oup . B e r th i e r ine ( F e 2+, M g)6_x(Fe 3+, A1)xSi4_xAlx)O10(OH)8) s the m os t im po rtan t m em -be r o f t he se r pe n t ine g r oup in s tud ie s o f s e d im e n ta r yr oc ks . I t c om m only oc c ur s in oo l i t i c i r ons tone s a nd inp e d o g e n i c e n v i r o n m e n t s b u t h a s o f t e n b e e n m i s n a m e dc ha m os i t e . C h a m o s i t e i s no t a 7A se r pe n t ine m in e r a l bu t a14A c h lo r i t e m ine r a l ( s e e S e c t ion 2 . 1 .5 ) .2 . 1 .2 . T h e i ll l it e - m i c a g r o u pI l l i t e i s t he t e r m ge ne r a l ly use d f o r a l l c l a y g r a de m ic a sa n d m o s t c o m m o n l y h a s a d i o ct a h e d ra l 2 : 1 o r th r e e - la y e rcomposi te layer s t ruc ture (F ig . 2 .6) .In the i l l i te s t ruc ture sub st i tu t ion of [S i4+ ]iv by [A13 + ] ivand [R3+]w by [ W+ ] v I p r oduc e s a ne t ne ga t ive c ha r ge o fa bou t 0 . 7 - 1 . 0 pe r O10 ( OH)2 f o r m ula un i t . P o ta s s ium i sthe p r inc ipa l i n t e r l a ye r c a t ion a nd l e nse s o f wa te r m a yalso be p resent in the in te r lay er s i tes (FIR. 2 .6) .


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FIG. 2.4. C rystal structure of kaolinite (A14Sc40l0 (OH)8).

Illite Ko.s(R 3+,.65R2+0.35) (Si3.55A1045)O,0(OH)2Muscovite KA12(Si3A1)Olo(OH)2Serici te is a lso used to general ly signify fine-grainedmicacaeous mate r ia l of an inde te rmina te na ture thatma y inclu de i l l i te but a lso other c lay and sheet si l icateminera l s .Glauconi te (sensu stricto) is a dioctahedral Fe-richi l l it e of genera l form ula

K (R 2+067R3+,.33) (Si3.67Alo.33)Olo(OH)2 w it hFe 3+ >> A1, M g > Fe 2+ and Fe 3+ > Fe z +However , ' g lauconi te ' i s a l so used to desc r ibe any green(pe l le ta l ) c lay mate r ia l i r re spec t ive of composi t ion(see Text Box on p . 18) . The te rm 'g laucony ' has beenproposed as a genera l t e rm when the spec i f ic minera lcomposi t ion i s not known (Mi l lo t 1970) .

Ce ladoni te i s the more Mg-r ich equiva lent of g lauco-nite o f ideal f orm ula K (Mg , Fe 2+, Fe 3+)Si40~o(OH)2.Unl ike g lauconi tes tha t a re most of ten found in modernand anc ient sediments , ce ladoni tes a re most commonlyform ed in assoc ia tion w i th the a l te ra tion of volcanic ,usually basal t ic , rocks.2.1.3. SmectitesSmecti tes are three-layer or 2:1 c lay minerals (Fig. 2.7)tha t a re most commonly d ioc tahedra l but t r ioc tahedra lva r ie t ie s do exis t . They hav e a laye r charge o f 0 .2-0 .6 pe rO10(OH)2 uni t of s t ruc ture , which i s offse t by hydra tedinte r layer ca tions , pr inc ipa l ly Ca and Na .The hydra t ion of the in te r layer ca tions causesthe interlayer crystal l ine swell ing that characterizesthe smecti tes. Water is sorbed into the interlayer si tes


6/16

THE COMPOSITION OF CLAY MATERIALS 1/composed predominant ly of montmori l loni te and/or toindicate genes is of montmori l loni tes f rom al terat ion ofvolcanic ash. In the UK Fullers Ear th is used in a s imilarway to bentoni te though in Nor th A meric a Ful lers Ear thcan be composed of palygorski te ra ther than smect i te(montmorillonite).Dioctahedral smectites:M ont mo rillo nite s R+y(A12_y R2+y)Si4Olo(OH)z'nH20Beidellite R+yAlz(Si4_yAly)O10(OH)2"nH20Nontronite R+yFe3+2(Si4_yAly)Olo(OH)z'nH20Trioctahedral smectites:Saponite R+x_y(Mg3_y(A1,Fe 3+)y)(Si4_x Alx)Olo(OH)2"nH20Hectorite R+x_y(Mg3_yLiy)Si4Olo'(OH)z'nH202.1.4. Vermiculite

FIG. 2.5. Photomicrograph of tubu lar hab it of halloysite (fromKohyamaet al., 1978, reproduced by permissionof the authors andpublishers).in m onom olecula r sheets (F ig . 2 .8) . N a and Ca-montmori l loni tes can absorb up to three layers a t 90%relat ive humid i ty but in some ins tances a t 100% hum idi ty ,Na-mon tmori l loni tes can absorb more and the 2:1 layersdisperse.Bentoni te is often used syno nymo usly with montmo ri l -loni te but is a rock term rather than a mine ral name. As arock term, i t used to s ignify a comme rcial c lay depos i t

Ver micu lites are similar in structure to the smectites(Fig. 2.9) but have a larger net ne gative charge o n thecomp osite la yer of 0.6-0.8 per O10(OH)2. The p rincip alin ter layer cat ions are hydrated magnes ium. V ermicul i tesexhibi t swel l ing proper ties s imilar to the smect i tesbut to a lesser extent due to the higher layer charge.Vermicu l i tes are t r ioctahedral with the g eneral formula:M g (x-y~/2(Mg, Fe 2+)3_y(A1, Fe 3 +)y

(Si4_x Alx)O10(OH)2"nH202.1.5. Chlorite

The chlorites have a 2:1 type structure with a secondoctahedral layer in the in ter layer s i tes having a net pos i-tive charge to offset the net negative charge on the 2:1layers (Fig. 2.10).

FIG. 2.6. Crys tal structure of muscovite.


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FI6. 2.7. Crystal s tructure o f dioctahedral smecite.

FIG. 2.8. One layer and tw o layer water s tructures in smectite ( from Velde 1992).


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THE COMPOSITION OF CLAY M ATERIALS 19

FI6. 2.9. Crystal structure of vermiculite.

FIG. 2.10. C rystal structu re o f chlorite.

The general form ula of the chlorites is( M g , F e 2+)6_x(A1, Fe 3+)xSi4_xAl~Olo(OH)8

The majori ty of chlori tes are trioctahedral but somedioctahedral and mixed dioctahedral (2:1 layers) -trioctahedral (interlayers) forms are known and can beidentified by X-ray diffraction analysis (see Chapter 8).

2.1.6. Mixed layer clay mineralsMixed layer clay minerals describe those mineral phasesin which differe nt sheet silicate units occu r in the stackin gsequence. G eneral ly the 2:1 clay minerals are most com -monly involved in forming mixed layer clay minerals,though kaolini te-smecti tes and serpentine-chlori tes havebeen reported. The most comm only reported mixed layer


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2 0 THE COM POSITION OF CLAY MA TERIALSI n t e r s t r a t i f i e d

M o d e l

m

S m S m

S m S m

I

l qS m

II

II

E x c h a n g e a t i on s , 2 0 , r g a n i c s) (Exchange ations,H O, organics

K + K+

E x c h a n g e a t i o n s , 2 0 , r g a n i c s

K + K+

K + K+

I n t e r p a r t i c l eM o d e l

1 0 A " S m e c t i te "

1 0 A " S m e c t i te "

2OA " l l l i t e "

3OA " l l l i t e "

/ T e t r a h e d r a l l a y e r sJ L O c t a h e d r a l la y e rs/ "N T e t r a h e d r a l l a y e r s

Fro. 2.11. The nature of i l l ite-smecti te as d efined b y theinterstrat if ied and interpart icle models (af ter Nadeau & Bain 1986).

clays are the i l l i t e-smect i t es and to a much lesser ex ten tthe ch lor i t e-smect i t es . The in ters t ra t i f i ca t ion may berandom, wi th no d i scern ib le pat t ern in the s tack ings eq u en ce , o r o rd e red , e . g . AB AB AB , AAB AAB AA o rAAAB AAB . S p ec i f i c n am es a re a l s o g i v en t o ce r t a i nt y p es o f o rd e red m i x ed l ay e r c l ay m i n e ral s.

The mixed layer phases may a l so show segregat ionsin to AB sequences w i th separate dom ains o f A or B ra therthan cont inuous ly a l t ernat ing sequences .Gen era l l y m i x ed l ay e r i l l i t e - s m ec t i t e s w i t h m o rethan 45% smect i t e are rando mly in ters t ra ti f ied , those wi th3 0 -4 5 % s m ec t i t e can b e r an d o m o r o rd e red an d w i t hless than 30% smect i t e are ordered (Bethke & Al taner1986).2 . 1 . 7 . S e p i o l i t e a n d p a l y g o r s k i t eAlthough sep io l i t e and palygorsk i te are of ten descr ibedas 2:1 sheet s i l icates , the sheets are not cont inuous butform al t ernate r ibbons of 2 :1 sheet s il i ca te s t ructures(F ig . 2 .12) . Consequent ly they have f ib rous ra ther thanthe typ ica l ly p la ty habi ts o f the t rue sheet s i li ca tes . Co m-pos i t ional ly they are Mg-r ich , Mg being the pr incipalcat ion in the octahedral s i t es bu t in some palygorsk i tesa luminium can be the more common octahedral ca t ion ,i . e . [ A P + ] v , > [ M g 2 + ] v i.

The name at t apulg i t e i s o f ten used for commercia ldepos i t s o f palygorsk i te , bu t pa lygork i te i s the prefer redm i n e ra l n am e .2 . 1 . 8 . S w e l l i n g p r o p e r t i e s o f c l a y m i n e r a l sThe c lay mineral s can be c lass i f ied in to swel l ing and non-swel l ing var ie t i es (Table 2 .1).Water and organic molecules may be sorbed onto thesurface of and in to the in ter layer s i t es of c lay mineral s .The sorp t ion of water and organic molecules in to theinterlayer s i tes is responsible for the characteris t icin t rapar t i c le swel l ing proper t i es of smect i t es and vermi-cul i tes (Table 2.1). A ' dr y ' smec t i te absorbs w ater into thein ter layer s i t es in d i scre te l ayers wi th the w ater forminghydrat ion sheaths around the in ter layer ca t ions . At veryhigh humidi t i es Na-smect i t es when immersed in waterexhib i t osmot ic sw el l ing and m ay com plete ly d i ssocia te .Com plete d i ssocia t ion of the c lay l ayers occurs whe n therepuls ive forces o f the negat ively charge d c lay surfacesexceed the forces of a t t rac t ion between the hydratedin ter layer ca t ions and the c lay par t ic les .Vermicul i t es , hav ing a h igher in ter layer charge thansmect i t es , show less swel l ing and wi l l no t complete lydissociate in water.


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P A L Y G O R S K I T E

L j

T H E C O M P O S I T I O N O F C L A Y M A T E R I A L S 21

S E P I O L I T E

2 7 ~ ,I I

\ ( \ /\ \ f

I o nc o n c n~ T e t ra h e d r a l a y e rO c t a h e d r a l a y e rT e t r a h e d r a l la y e rFro . 2 .12 . Cry s t a l s t ruc tures of pa lygorsk i t e and sep io l i t e .

I l l it es ma y have l enses of water in the in ter layer s i tes ,ou t no t complete l ayers and do not show in t rapar t i c leswell ing.Chlor i t es are non-swel l ing c lay mineral s bu t there haveb een r ep o r ted i n s tan ces o f ' s w e l l i n g ch l o r i t e s '. M o o re &Reynolds (1997) commented that swel l ing ch lor i t es aresuch a poor ly descr ibed phase they were unab le to ca lcu-late their diffract ion characteris t ics . I t seems probab le that' swel l ing ch lor i t es ' a re mos t l ikely to be mixed layerch lor i t e / smect i t es or ch lor i t e /vermicul i t es ra ther than aswel l ing ch lor i te sensu s t r ic to .Hal loys i t es in the hydrated s ta te have a water l ayerbetw een the 7N ka ol in l ayers produc ing thei r character is -t i c 10 /~ in ter layer spacing . K aol in i te in cont ras t tohal loys i t e does no t swel l in the presence o f water thoughincreas ing ly d i sordered kaol in i tes conta in l enses o f water .Sepio l i t e and palygorsk i te have open channel s i t ess imi lar to zeo l i t es (see Sect ion 2 .2 .7) in to which waterand organic compounds can be absorbed or desorbedwi thout s ign i f i can t ly af fect ing the un i t ce l l d imens ionsand thus are no t regarded as swel l ing c lays .In addi t ion to in t rapar t i c le sw el l ing , a l l c lay m ineral scan show in terpar t i c le swel l ing which i s governed bysimilar factors that control intrapart icle swell ing, thati s the nature of the c lay mineral s a nd the nature o fand concent ra t ion o f ca t ions adsorbed onto the c laysurface in the d i f fuse double l ayer and hydrat ion of

m0m

m0m

s o l u t i o n-I-- - I-+ + - + _++ - + + +

-

+ + + +++ - + +

+ + + ++ +

-I--- t-

- I-

n+

d i s t a n c e f r o m c l a y s u r f a c eF I a . 2 . 1 3 . D i s t r i b u ti o n o f i o n s i n th e c l a y - w a t e r l a y e r o f t h ic k n e s s da n d f o r m a t i o n w a t e r .

surface cat ions (Guven 1993; Guven & Pal las t ro 1993;Low 1992) .In terpar t ic le associa t ions of c lays cont ro l thei r f loccu-la t ion and d i spers ion in natura l waters .2 . 1 . 9 . I o n e x c h a n g e p r o p e r t i e s o f c l a y m i n e r a l sThe 2 : 1 c lay mineral s h ave a net negat ive charge on thei rcom posi te layer s due to cat ion subst i tut ions (e.g. A13+ forSi4+; FEZ+; M g 2+ fo r A P +, Fe 3+). In m inerals such asi l l i t es and smect i t es the net negat ive charge i s o f fse t byin ter layer ca t ions and in the ch lor i t es by the in ter layeroctahedral sheet . Kaol ins ideal ly have neut ra l com pos i telayer s t ructures , bu t in rea l i ty l imi ted cat ion subs t i tu t ionm a y o ccu r p ro d u c i n g a v e ry s m a l l n e t n eg a t i v e ch a rg e o nt h e co m p o s i t e l ay e r o f f s e t b y a s m a l l n u m b er o f i n te r l ay e rcat ions . W hen suspended in water som e of the in ter layercat ions on the surface of the c lay par t i c les go in to so lu t ionp ro d u c i n g a n eg a t i v e l y ch a rg ed c l ay s u r f ace s u r ro u n d edb y a d i f fu s e ' d o u b l e l ay e r ' o f h y d ra t ed ca t io n s (F i g . 2 . 1 3 )(van Olphen 1977) .


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2 2 T H E C O M P O S I T I O N O F C L A Y M A ' I E R IA L SO H

S iO. . . HA ( - O H +/ \O . . .H

S i\ o .

/ O - . . . H O HS i

\ o/A I - O - . .. N O N

\o/S i

O - . . . H O HIn ac id so lu t ionp o s i t i v e l y c h a r g e dAnion exchanger

O _ A I ' I - 2/S i \?A I - O - A I + 2\/ oS i \

O - A I + 2

O HS i

) oS i \O H/ /

In a lka l ine so lu t ionn e g a t i v e l y c h a r g e dCa tion exchanger

/ O A I 0 3 4S i

\oA [ - O A I O ~ 4

/S i

\ O A I O ~ 4In ac id so lu t ion in thep r e s e n c e o f A I 3 + i o n sp o s i t i v e l y c h a r g e dAnion exchanger

In a lka l ine so lu t ion in thep re s e n c e o f A I O 4 " io n sn e g a t i v e l y c h a r g e dCation exchanger

Flc . 2 .14 . Edg e s i t e an ion exch ange as a func t ion of pH ( f r om Yar iv and Cros s , 1979) .

O n t h e e d g e s o f th e c l a y p a r t i c l e s t h e d i s r u p t i o n o f t h ec l a y s t r u c t u r e p r o d u c e s b r o k e n b o n d e d g e s i t e s w h i c hm a y b e n e g a t i v e l y o r p o s i t i v e l y c h a r g e d . T h e n a t u r e o ft h e c h a r g e i s d e t e r m i n e d b y t h e p r e s e n c e o f c e r t a i n i o n s ,n o t a b l y H + , O H - , A 1 3+ a n d A 1 0 4 5 - , w h o s e p r e s e n c e i sp H d e p e n d e n t , b e i n g n e g a t i v e l y c h a r g e d i n a l k a l i n es o l u t i o n s a n d p o s i t i v e l y c h a r g e d i n a c i d i c s o l u t i o n s( F i g . 2 . 1 4 ) . T h i s p H d e p e n d e n t c h a r g e a c c o u n t s f o r o n l ya s m a l l p e r c e n t a g e o f t h e t o t a l c h a r g e i n i l l i te s a n ds m e c t i t e s , b u t i s m o r e s i g n i f i c a n t f o r k a o l i n s a n d c h l o r it e s .I n a d d i t i o n t o c a t i o n e x c h a n g e i t i s a ls o p o s s i b l e t o h a v e

a n i o n e x c h a n g e b u t u s u a l l y t o a l e s s e r e x t e n t t h a n c a t i o ne x c h a n g e .T h e i o n s p r i n c i p a l l y a d s o r b e d o n t o t h e c l a y s u r f a c e sa n d , t o a l e s s e r e x t e n t , o n t h e e d g e s o f th e c l a y p a r t i c l e s,c a n b e e x c h a n g e d w i t h o t h e r a v a i l a b l e i o n s . T h i s g i v e sr i se t o th e p h e n o m e n o n o f c a t io n / a n i o n e x c h a n g e . T h ed e g r e e t o w h i c h e x c h a n g e r e a c t i o n s t a k e p l a c e d e p e n d s o nt h e n a t u r e o f t h e c l a y m i n e r a l s a n d t h e c o n c e n t r a ti o n s o ft h e c a t i o n s / a n i o n s i n v o l v e d a n d o f o t h e r i o n i c s p e c ie s .O x i d e s a n d o r g a n i c m a t t e r c o m m o n l y c o a t c l a y p a r t ic l e sa n d a l s o h a v e i o n e x c h a n g e p r o p e r ti e s , w h i c h a r e s t r o n g l y


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THE COMPOSITIONOF CLAY MATERIALS 23TABLE 2.2. Cation exchange capacities of some clay m inerals(meq/lOOg) at p H = 7 (af ter Dre ver 1982)Smectite 80-150Illite 10-40Kaolin 1-10Chlorite < 10dependen t on pH, and can a l t er the ca t ion /an ion ex changecapaci t i es of associa ted c lays in natura l sys tems .Ion exchange i s a dynam ic process govern ed by the lawof mass ac t ion . Ge nera l ly , the greater the concent ra t ionof a par t i cu lar ca t ion in so lu t ion , the more read i ly i t wi l lrep lace equivalen t ca t ions adsorbed onto the c lay surfacesand in ter layers . Cat ions of h igher valence w i l l usual lyrep lace those o f lower valenc e and , i f o f the same valence ,smal ler s i zed ca t ions wi l l rep lace l arger s i zed ca t ions .How ever , i t i s impor tan t to no te that in the d i f fuse l ayer ,the ca t ions wi l l be hy drated and i t i s the s ize o f thehydra ted ca t ion that needs to be cons idered .In swel l ing c lay minera l s , the in ter layer ca t ion s i t eswi l l a l so provide ex change s i tes , whe reas for o ther c laysonly the ou ter surface s i tes wi l l be involved . T his expla insthe h igher ca t ion exchang e cap aci t ies o f the smect i t escom pared to the i l l it es (Table 2 .2) .Because the edge s i t es are a l so involved in ion exch-ange a nd as the nature of the charge on the edg e s i t es is pHdependent the ca t ion exchange capaci ty of ind iv idual c layminera l s are a lso pH dependen t .

2 . 2 . N o n - c l a y m in e ra lo g y2.2.1. Qua rtz and chertQuartz i s commonly presen t in c lay mater ia l s , p redomi-nant ly in the s i l t g rade mater ia l (Fig . 2 .1) . I t i s genera l lydet r it a l , bu t some m ay be b iogenic or au th igenic in or ig in .Si l i c i f i ca t ion may occur dur ing ear ly d iagenes i s wi ththe d i sso lu t ion o f b iogenic s i li ca , a poss ib le source forthe l a ter inorganic precip i t a t ion of quar tz (M i l lo t 1970).Diagenef ic modi f ica t ion of c lay minera l s may a l so causethe mobi l i za t ion o f SiO2 wh ich ma y reprecip i t a te as aquar tz cemen t in mud rocks o r ad jacent sands tone beds .Cher t i s a genera l descr ip t ion for f ine-gra ined s i li ceoussediments o f chemical , b iochem ical o r b iogenic or ig in .

I t is a dense , re la t ively hard mater ia l com posed of f ine lycrys ta l l ine quar tz crys ta l s o r f ib rous chalcedonic quar tz ,usual ly der ived f rom the d iagenet ic a l t era t ion of vo lcan-ogenic or b iogenic op al ine s i l ica (see Text Box below).Cher t s usual ly occur as e i ther bedde d or nodular forms .T h e b ed d ed fo rm s a re co m m o n l y a s s o c i a ted w i t h v o lcan i csequences whereas nodular cher t s are main ly hos tedin chalks or mudrocks . The nodular forms hos ted inchalks and o ther carbonates are sp eci f i ca l ly refer red to asf l in t s . Jasper i s a red co loured cher t tha t conta ins hem at i t eimpuri t ies .2.2.2. FeldsparsAs wi th quar tz , fe ldspars in c lay mater ia l s are predomi-nant ly det r i ta l in or ig in , bu t there i s pe t ro logical ev idenceto sugges t they ma y a l so form d iagenet ic cem ents aroundexis t ing fe ldspar gra ins .2.2.3. CarbonatesAcco rd i n g t o S h aw & W eav er (1 9 6 5 ) t h e re i s an av e rag eo f ab o u t 4 % b y we i g h t o f ca rb o n a t e m i n e ra l s i ns i l i c ic las t i c mudrocks bu t th i s can be h ighly var iab le .Calc i t e (CaCO3) and dolomi te (CaMg(CO3)2 are the pre-dom inant carbonate m inera l s in c lay mater ia l s , thoughs ider i t e (FeCO3) may a l so be local ly impor tan t . Meta-s tab l e p o l y m o rp h s o f C aC O 3 , a r ag o n i t e (o r t h o rh o m b ic )and , l ess commonly , va ter i t e (hexagonal ) a l so occuri n R ecen t c l ay m a t e r ia l s . T h e ca rb o n a te m a y b e p re s en tas c las t s der ived f rom organic skele ta l remains andthe weather ing of pre-ex i s t ing carbonate roc ks or asd iagenet ic cements and nodules .The source of carbonate for the format ion o f d iagenet iccarbonate c ements or nodules m ay be f rom the recrys ta l l i-za t ion of pr imary (skele ta l ) carbonate mater ia l and/ord i rec t p recip i t a t ion f rom alkal ine b icarbonate- r i ch porewaters . These proc esses are co ns idered to be of ten re la tedto the matura t ion of organic mat ter dur ing d iagenes i s(Curt is 1983).Dolomi te and s ider i t e format ion are favoured insu lphate-deple ted envi ronments (Baker & Kas tner 1981;Berner 1981) . The presence of au th igenic s ider i te i sind icat ive of anoxic su lphate-deple ted condi t ions a t thet ime o f it s format ion (Table 2 .3).


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24 THE COMPOSITIONOF CLAY MATERIALSTABLE 2.3. Classification of oxic-anoxic environments usingauthigenic Fe-mineral indicators (Bern er 1981)Environment Authigenic Fe-m ineralsOxic Hem atite, goethite(no org anic ma tter preserved)

Pyrite (organic matter preserved)noxic (sulphidic)Ano xic (non-sulphidic)(a) post-oxic(b) methanic

Glauconite, Fe2+/Fe + silicatessiderite (no su lphides some organicma tter preserved)Siderite after pyrite(organic matte r preserved)

2.2.4 . I ron su lph idesPyri t e (FeS2) is the mos t c om mo n o f the su lphides foundin c lay mater ia l s occurr ing as veins , nodular aggregates ,sca t t ered crys ta l s and inf i l l ing / rep lacing/pseudom orphingskele ta l f ragments . Sedimentary au th igenic pyr i t e formsin anoxic su lphid ic envi ronmen ts (Table 2 .3) . W eather ingof pyr i t e produces i ron hydroxide s /ox ides and m obi l i zessu lphate which m ay be incorporated in to su lphate miner-a l s such as gypsu m and jaros i te . M arcas i t e i s an or thor-homb ic d im orph o f pyr i t e tha t has occas ional ly beenrepor ted in c lay mater ia l s . There i s some ev idence tosugges t tha t marcas i t e form s under more ac id ic condi t ionsthan pyr i t e . Metas tab le monosulphide (e .g . mackinawi teand gr ieg i te) are found in modem muds as precursors tola ter format ion of pyr i t e . Pyrrhot i t e (Fel_xS) i s a s t ab lemonosulphide minera l phase that occcas ional ly has beenrepor ted in c lay mater ia l s bu t s to ich iomet r ic FeS, theminera l t ro i li t e , i s on ly found in meteor i tes .2 .2 .5 . Ox id es an d h yd r o x id es2.2 .5 .1 . I ron ox ides an d hydrox ides . I ron oxides andhydroxides are presen t in var ious minera l forms inclay ma terials , nota bly hem ati te (c~-Fe203), ma ghe m ite (7-Fe203) , goeth i t e (~ -FeO.OH), l ep idocros i t e (7-FeO'OH).Limo ni te i s a genera l t e rm for a mix ture o f iron oxidesand hydroxides of a poor ly crys ta l l ine nature . The pres -ence of these iron oxides and hydroxides produc es thecharacter i s ti c red /brow n/yel low colours of many c laymater ia l s depending on which minera l i s p resen t . Goet -h i t e i s ye l low-brown, l ep idocros i t e orange, maghemi tebrown-black and hemat i t e red .2 . 2. 5 .2 . A lum in ium ox ide s and hy drox ide s . Gibbs i te(AI(OH)3) , boehmi te (y-AIO'OH) and d iaspore (~ -A1 0 "OH) a re t he m o s t co m m o n o f t h e a l u m i n i u m o x i d esand hydroxides and are typ ical ly s ign i f i can t cons t i tuen t sof la ter i tes and bauxi tes . Boehm i te i s i somorph ous wi thlep idocros i te and d iaspore w i th goeth it e .In addi t ion there are a luminium - s i l i con oxyhy-droxides , imo gol i t e and a l lophane, tha t cha racter i s t ica l lyoccur in so i l s der ived f rom the we ather ing o f vo lcanic"ocks.

Thei r no minal formu la i s SiA1203(OH)4 but bo th havepoor ly ordered crys ta l s t ructures and are of ten descr ibedas amorphous . Genera l ly a l lophane forms spheru les andappears to be l ess wel l o rdered than imogol i t e wi th i t stypic al ly cyl ind rical hab i t . The SiO2:A1203 rat io variesf rom 1-1 .2 for imogol i t e to 1 .3-2 .0 for a l lophane (Moore& Reyno lds 1997) .2 . 2 .5 . 3 . I on e x c hang e in ox ide s and hydrox ides . In theoxides and hydroxides the charge on the surfaces ar i seslargely f rom broken surface bonds producing unsharedsurface and edge O a nd O H ra ther than the ca t ion subs ti -tu t ions that are l argely respons ib le for impar t ing charge tothe c lay minera l s t ructures . A s descr ibed above, chargear i s ing f rom broken bonds i s pH dependent (Fig . 2 .14) .The pH at which the charge i s zero i s refer red to as thezero po in t o f charge or ZPC; w hen condi t ions are moreacid ic than the ZPC the ox ide/hydroxide wi l l be pos i -t ive ly charged and negat ively charged for condi t ionsmore a lkal ine than the ZPC. For example , the ZPC forboehm i te i s 8 .2 and 6 to 7 for goeth i te (Es l inger & Pevea r1988).2 .2 .6 . S u lp h atesSulphates may form nodules or veins in c lay mater ia l s ,wi th gypsum (CaSO4"2H20) , fo l lowed by anhydr i t e(CaSO4) being the mos t common, bu t ce les t i t e (SrSO4) ,bar i t e (BaSO4) and the more water-so luble sodium andmagnes ium su lphates may a l so occur . Thei r occurrencem ay sugges t hypersa l ine con di t ions o f format ion , thoughgypsu m and var ious i ron su lphates ma y a l so form dur ingweather ing of shales conta in ing pyr i t e , the ox idat ion ofthe su lphide being the source o f the su lphate .Jarosi te (KFe3(SO4)dOH)6) is a dis t inct ively yel lowminera l tha t character i s t i ca l ly forms as surface coat ingsand ag gregates f rom the ox idat ion of pyr i t e to i ronsu lphate , fo l low ed by fur ther react ion w i th i l li t e /mica orK-fe ldspars :

12FeSO4 + 4KAlaSi3Os(OH)2 + 48H 20 + 0 2= 4KFe3(SO4)2(OU)6 + 8AI(OH)3 +12Si(OH)4 + 4H2SO4.2.2 .6 .1 . Et tr ing i te group . The e t t r ing i te group ofminerals , mixed calcium carbonate sulphate s i l icates , andespecial ly thaumasi te (Ca3(SO4)(CO3)(Si(OH)6) '12H20),has a t t rac ted a t t en t ion rec ent ly because of the observat ionof the growth o f such minera l s a t concre te-c lay bound-ar ies , which can cause s ign i f i can t damage to concre tes t ructures (Thaum as i te Exper t Group 1999) .2.2.7 . Zeo l i tesZeol i t es are hydrous f ramework s i l i ca tes and those thatoccur in c lay mater ia l s are pred om inant ly a lkal i var ie t i es(Table 2 .4) , They are commonly associa ted wi th a lumi-nous sm ect i tes forme d by the weather ing and a l t era tion of


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THE COMPOSITION OF CLAY MATERIALS 2 5TABLE 2.4 . Common zeo l i tes occurring in c lay materia lsNatroliteAnalcitePhillipsiteErioniteHeulanditeClinoptiloliteMordenite

Na2A12Si3010"2H20NaA1Si2Or.H20(Ca ,Na ,K) 3A13SisOI6.6H20(Na2K2CaMg)4_sA19Si27072"27H20(Na,K)CaaAIgSi27072.24H20(K,Na)6 AI6Si30072-20H20(Na ,K, Ca)A12Si10024.7H20

volcanics . A nalc i t e i s a lso character i s t i ca l ly found as anauth igenic minera l in hypersa l ine m udd y sed iments .Zeol i t es can be used as ind icators of d iagenes i s andlow grade metamo rphism of arg i l l aceous sed iments . Themixed a lkal i (K, Na and some Ca) zeo l i t es are progres -s ively rep laced by more sodic , l ess s i l i ceous var ie t i eswh ich wi th increas ing metam orphism are in tu rn rep lacedby a lb i t e and eventual ly K-fe ldspars . The ca lc ic formspers i s t to h igher t emperatures .Zeol i t es have open f ramework s i l i ca te s t ructures wi ththe capabi l i ty of ac t ing as 'molecular s i eves ' b y absorb ingcat ions , an ions and polar o rganic m olecules in to the openchannel s wi th in thei r s t ructures.2.2 .8 . PhosphatesT h e m o s t co m m o n p h o s p h a t e m i n e ra l s o ccu r r i n g i nclay mater ia l s are the carbonate apat i t e g roup(C as (PO4 , CO3)3(OH,F, C1) . Phospha tes m ay occur in acryptocrys ta l l ine form refer red to sensu la to as 'co l l -ophane ' . The i ron-r ich phosphate minera l , v iv ian i te ,which has a very d i s t inc t ive b lue co lour , may form inlacustrine environments and soi ls .Phosphates may be b ioclas t i c (e .g . accumulat ions ofver tebra te skele ta l f ragments ) , faecal o r d iagenet ic inor ig in . Auth igenic phosphates can precip i t a te in mud-rocks and sands tones as nodules , oo l i ths , p i so l i ths ,ceme nts or rep lacem ents o f ca lcareous skele ta l c las tsin envi ronments wi th low clas t i c sed imenta t ion and h ighorganic produc t iv i ty enr iched in phosphate .In addi t ion to being presen t in phosphate minera l s ,phosphorus i s sorbed onto c lay and o ther co l lo idalpar t i c les and th i s mechanism can be a major fac tor incont ro l l ing phophorus mobi l i ty in so i l and aqueousenvi ronments .2.2 .9 . Ha l idesHal i t e (NaC 1) and , to a m uch less ex ten t , o ther hal idessuch as sy lv i t e (KC1) and ca mal l i t e (KMgC13"6H20) arecharacter i s t i ca lly found in c lay mater ia l s associa ted w i thevapor i t i c sed imentary and so i l envi ronments . Evapora-t ion of sea water and ch lor ide-r i ch br ines can create asequence of evapor i t ic minera l s tha t are of ten cycl ic dueto rep len i shment o f the evapo rat ing br ines or in terbeddedwi th f ine-gra ined s i l ic ic las ti c or carbona te sed iments .

2 .3 . O r g a n i c m a t t e rOrganic mat ter i s p resen t in a var ie ty of forms in c laymater ia l s inc luding d i scre te organic par t i c les , absorbedonto c lay an d o ther associa ted co l lo idal par ti c les (e .g . i ronoxides ) and micro-organisms . The nature , abundanceand d i s t r ibu t ion of organic mat ter can inf luence thep h y s i co -ch em i ca l b eh av i o u r o f c l ay m a t e r ia l s .Gen era l ly the organic mater ia l s can be subdiv ided in tolab i le and ref rac tory organic f rac t ions . Labi le organicmat ter genera l ly does no t surv ive bur ia l whereas ref rac-tory organics are non-metabol izab le , genera l ly surv ivebur ia l and can be preserve d in ancien t sed iments as kero-g en . Kero g en m ak es u p 9 5 % o r m o re o f th e o rg an i cm a t t e r p re s e rv ed i n s ed i m en t a ry ro ck s . Kero g en h as b eendef ined as the organic cons t i tuen t s tha t are inso luble inaqueous a lkal ine and com mo n organic so lvent s (Ti sso t &Wel te 1984) which i s the equivalen t o f 'humin ' in so i lsc ience (Tyso n 1995).

The l ab i l e organic components degrade v ia l eachingand decomp os i t ion processes . The l eaching process invo-lves the breakdown of ce l lu lar mater ia l by in t racel lu laren zy m es p ro d u c i n g s o lu b l e co m p o u n d s. T h e d eco m p o s i -t ion process re la tes to the bacter ia l co lonizat ion and thefermen t ing ac t iv i ty of the bacter ia which i s g reates t underwarm , ox ic condi t ions .In addi t ion to bacter ia , o ther micro-organ isms m ay a l sobe presen t such as pro tozoa and smal l a lgae , fungi andviruses. In soi ls there are 107 to 108 bac teria per gra m o fdry mass, 105 to 106 fungi and 104 protozoa (Campbell1992) . These micro-organ isms show a wide range of s izesand morphologies and can a l so ex i s t in var ious dormants ta tes such as spores and cys t s tha t are res i s tan t todess ica t ion and temperature changes . Micro-organismscan f lour i sh even under apparen t ly very adverse condi -t ions such as hydro thermal vent s on mid-ocean r idgeswi th t emperatures above 200~ hypersa l ine soda l akeswi th a pH > 10 or in ac id mine dra inage w aters wi th a pHo f < 2 .Some micro-organisms are aerobic , acqui r ing thei ren e rg y f ro m reac t i o n s w i t h o x y g en , wh i ch i s co n s e -quent ly consumed, l ead ing to oxygen-def ic ien t condi -t ions. Others are anaerobic , gen era t ing thei r energy byreducing inorganic chemical species (su lphate , n i t ra te ,carbon d ioxide) or o rganic species ( fermenta t ion reac-t ions) and/or ox id iz ing o ther inorganic species (su lphide ,fer rous ions) in the absence of oxygen . The bacter iaThiobaci l lus denatr i f i cans uses su lphide , carbon d ioxideand n i t ra te as energy sources and produces su lphate ,s u g a r s an d r ed u ced n i t ro g en co m p o u n d s (C am p b e l l1992) . This i s one o f ma ny th iobaci l lus bacter ia tha t ex i s ti n c l ay m a t e r i a l s wh i ch d e r i v e en e rg y f ro m o x i d i z i n ginorganic su lphur usual ly under aerobic condi t ions . Onevar ie ty , Thiobaci l lus f errooxidans al so ox id izes fer rousions to fer r ic in ac id ic envi ronments and i s a key inf luencein the natura l degradat ion of pyr i t e (Hawkins & Pinches1992).T h e n a t u re an d ab u n d an ce o f m i c ro -o rg an i s m s i n c l aymater ia l s and the inorganic compounds avai lab le for


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2 6 THE COMPOSITION OF CLAY MATERIALSoxidat ion and/or reduct ion wi l l have a major in f luence indetermining thei r chemical behaviour .2.3.1. Organ ic-clay com plexes and interactionsOrg an i c -c l ay co m p l ex es r ep res en t p a r t o f t h e h u m i n o rkeroge n organic f rac t ion . Organ ic molecules are absorbedonto the charged c lay minera l surfaces and in ter layer s i t esb y:9 com pet ing wi th and rep lacing the water molecu les andforming com plexes w i th in ter layer ca t ions ;9 be ing bon ded to the ca t ions by br idg ing w atermolecules ;9 bonding of organic ca tions , molecules wi th a s trongt en d en cy t o fo rm h y d ro g en b o n d s an d p o l a r o rg an i cmolecu les on to the charged c lay surfaces .M os t organic compou nds o f in teres t as envi ronmen ta lpol lu tan t s are smal l non- ion ic molecules and thei r sorp-t ion onto c lay minera l s involves we ak in teract ions v ia vand e r Waa l fo rces an d h y d ro g en b o n d i n g (Sa wh n ey 1 9 96 ).Such complexes and organo-clay react ions can haves igni f i can t in f luences on chem ical mo bi l i ty and cata lys isof chemical react ions. These can af fec t , fo r example , thechem ical beha viour of po l lu tan ts , fer t il i zers, pes t i c idesin so i l s and the phys ica l p roper t i es of f loccula t ion anddispers ion in aqueous en vi ronments (Johns ton 1996) .The ca ta ly t i c behaviour i s re la ted to the l arge surfaceareas of c lays and o ther co l lo ids , which i s a funct ion ofminera logy , crys ta l s i ze and habi t . C lay surfaces a l sohave ac id ic proper t i es due to the presence of A1 subs t itu t -ing for Si in the tetrahedral s i tes . The resul tant netnegat ive charge can be offse t by the presence o f chargebalancing H3O corresponding to a pro ton-donat ingBrons ted ac id s i te . In addi t ion , AP + ions on the ed ges o fc lay par t i c le are capable o f being e lec t ron pa i r acceptorsand thus can ac t as Lewis ac id s i t es (Ruper t e t a l . 1987).The degree to which th i s surface ac id i ty i s p roduced i san essen t ia l fac tor in crea t ing the condi t ions for c layminera l s to ac t as ef fec t ive ca ta lys t s o f var ious organicreact ions.

2 .4 . W a t e rIn c lay mater ia l s w ater i s p resen t in the in ter -par t i c le po respaces , adsorbed onto or absorbed in to c lay minera l s ,o ther minera l s and organic mat ter . Guven (1993) def inedthree forms of hydrat ion in c lay mater ia l s:9 in ter lamel lar hydrat ion involv ing adsorp t ion of wateronto the in ternal surfaces of c lay minera l par t i c les ;9 cont inuous osmot ic hyd rat ion involv ing unl imi tedadsorp t ion o f water on to the in ternal and ex ternalsurfaces of p r imary minera l and organic par t i c les;9 cap i l l ary condensat ion o f wa ter in to micro-pores .I t needs to be emphas ized that a l l minera l surfaces arecapable of adsorb ing water on to thei r surface . How ever ,

th i s i s mos t s ign i f i can t for the c lay minera l s bec ause thei rf ine gra in s ize and p la ty habi t p roduce v ery l arge surfaceareas and thus g ive a re la t ively greater capaci ty for wateradsorp t ion .The presence of adsorbed w ater l ayers cover ing theclay par t i c les produces the character i s ti c c ohes ive p las t i cbehav iour o f c lay mater ia ls . The w ater adsorbed onto theclay minera l surfaces has proper t i es midway betweenbulk l iqu id water and i ce and forms a s t ructured waterlayer and a mo re d i f fuse l ess s t ructured water l ayer . Theth ickness o f the adsorbed surface water l ayer var iesdepending on the c lay surface charge , the exchangecat ions and cat ion hydrat ion and the sa l in i ty of the aque-ous solut ion. With increasing ionic s t rength there isa l esser t endency for ca t ions to d i f fuse away f rom thenegat ively charged c lay par t i c les and the absorbed waterlayer becomes com pressed an d less d i f fuse than a t lowerionic s t rengths.The nature and beh aviour o f the water absorbed in to thein ter layer s i t es of c lay minera l s i s a funct ion o f :9 the po lar nature of the water molecules ;9 the s ize and charge o f the in ter layer ca tions and thei rhydrat ion s ta te ;9 the locat ion and value of the charge on the s i l ica telayers o f the c lay minera l s t ructures.In the in ter layer s i tes of expandable c lay m inera ls thewater forms coord inated shel ls around in ter layer cat ions.In the smect i t es the water molecules form s ing le , doubleor t r ip le water l ayers , depending on the humidi ty o f thesurrounding envi ronment . The vermicul i t es can exhib i ts ing le or double water l ayers depending on the humidi tycondit ions.There are s ign i f i can t d i f ferences in the in ter layer waterpresen t in hal loys i t es compared wi th the smect i t es andvermicu l i t es tha t he lp expla in why rew et t ing of dehy-drated hal loysi tes does not cause the s t ructure torehydrate . In the hydrated s ta te there appears to be nopreferen t ia l o rder ing of the water molecules and onlyweak in teract ions wi th the hal loys i t e surfaces . Therei s a lso ev idence that deh ydrat ion o f the hal loys i t e isaccom panied by delaminat ion of the hal loys i te l ayers thusinhib i t ing hal loys i te rehydrat ion (New man 1987) .The absorp t ion of water in to sep io l i tes and paly-gorsk i tes involves adsorp t ion onto the ex ternal surfacesand absorpt ion into the channel s i tes within the crystals t ructure , w hich i s analogous to the absopt ion o f waterin to the channel s i tes of zeo l i t e s t ructures. The pack ing ofthe water molecules absorbed onto the ou ter surfaces ofthe sep io l i t es and palygorsk i tes i s much less dense thanthan that o f a surface water mono layer on o ther sheets i l i ca tes (Newman 1987) .The dens i ty of the water in the in ter layer s it es of swel l -ing c lay minera l s i s cons idered to be greater than thatof l iqu id water and to increase wi th increas ing pressure(Sk i p p e r e t a l . 1993) . However , Moore & Reynolds(1997) argu ed that wi th increas ing pressure the in ter layerwa ter should ha ve a s imi lar dens i ty to tha t o f l iqu id porewaters .


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THE COMPO SITION OF CLAY MATER IALS 2/2.5. Conclusions

C l a y m a t e r i a l s a r e c o m p o s e d o f a v a r y i n g m i x o f s o l id ,l i q u i d a n d v a p o u r p h a s e s , w i t h t h e c l a y m i n e r a l s u s u a l l yt h e m o s t a b u n d a n t m i n e r a l p h a s e s . T h e v a r y i n g p r o p e r t i e so f th e i n d i v i d u a l c l a y m i n e r a l s a n d t h e i r o f te n c o m p l e xi n t e r ac t i o n s w i t h f l ui d p h a s e s u n d e r d i f f e r e n t p h y s i c a l a n dc h e m i c a l c o n d i t i o n s c a n p l a y a m a j o r r o l e i n c o n t r o ll i n gt h e b e h a v i o u r o f c l a y m a t e r i a ls . H o w e v e r , i t i s s im p l i s t ica n d m i s l e a d i n g t o e q u a t e c l a y m a t e r ia l s w i t h c l a y m i n e r -a ls . T h e n a t u r e a n d a b u n d a n c e o f o th e r m i n e r a l s a n d a l s oo r g a n i c m a t t e r , a n d t h e i r v a r y i n g i n t e r a c t i o n s w i t h f l u i dp h a s e s , c a n a l s o h a v e a s i g n i f i c a n t e f fe c t o n t h e b e h a v i o u ro f c l a y m a t e r i a ls .

C l a y m a t e r i a l s a r e u s e d i n a v a r i e t y o f a p p l i c a ti o n s a sc o n s t r u c t i o n m a t e r i a l s a n d a d e t a i l e d k n o w l e d g e o f th e i rc o m p o s i t i o n s i s o f f u n d a m e n t a l i m p o r t a n c e i f w e a r e t om a x i m i z e t h e e f f ic i e n t u s e o f s u c h m a t e r i a ls f o r a g i v e na p p l i c a t i o n .

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Sedimentary Rocks. Prent i ce-Hal l , Ne w York .BURST, J . F. 195 8. Glauco nite pellets: their minera l nature an dapplications to stratigraphic interpretations. AAPG Bulletin,42 (2) , 310-327.CAMPBELL, R. 199 2. Microb iology of soi ls . In: HAWKINS, A. B.(ed.) Proc. IGCC 92 Conference on The implications ofground chemistry and microbiology or construction, Bristol,1 - 1 7 .CURTIS, C. D. 1983. Geochem is t ry of poros i ty enhance ment andreduct ion in clast ic sediments. In: BROOKS, J. (ed .) PetroleumGeochemistry and Exploration of Europe. Geologica l Soc i -ety, London, 113-126.DREVER, J. I. 19 82 . The Geochemistry of Natural Waters.Prent ice-Ha l l, Ne w Yo r k .ESL1NGER, E. & PEVEAR, D. 19 88 . Clay Minerals for PetroleumGeologists and Engineers. SEPM Short Course , 22 .GUVEN, N. 1993 . Mo lecular aspects of clay-w ater interact ions. In:GUVEN, N. & PALLASTRO, R. M . (eds) Clay Water Interfaceand its Rheological Properties. CMS Workshop Lectures, 4,Clay M inera ls S oc ie ty , 2-79 .GUVEN, N . & PALLASTRO,R. M. (eds) 1993. Clay Water Interfaceand its Rheological Properties. CMS Workshop Lectures, 4.Clay M inera l s Soc ie ty .HAWKINS, A. B. & PINCHES, G. M. 1992 . Understand ing sulphategenera ted heave f rom pyr i t e degrada t ion . In: HAWKINS,A. B.(ed.) Proc. 1GCC 92 Conference on The Implications ofGround Chemistry and Microbiology for Construction.Bristol .

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Composition Clay Minerals