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C hemi ca l Geo l ogy , 5 4 ( 1 9 8 6 ) 2 7 1 - - 2 7 8 2 7 1El s ev ie r S c i ence P ub l i she r s B .V . , Am s t e rd am - - P r i n t ed i n The N e t he r l ands[3 ]

G E O C H E M IC A L B EH A V IO R O F A N T IM O N Y A R S E N ICC A D M IU M A N D T H A L LIU M :E h -p H D I A G R A M S F O R 2 5C 1 -B A R P R E S S U R E

D O U G L A S G . B R O O K I N SD e p a r t m e n t o f G e o l o g y, U n i v e r s it y o f N e w M e x i c o , A l b u q u e r q u e , N M 8 7 1 3 1 U . S .A . )

( R e c e i v e d S e p t e m b e r 5 , 1 9 8 5 ; a c c e p t e d f o r p u b l i c a ti o n O c t o b e r 1 , 1 9 8 5 )

A b s t r a c tB r o o k i n s, D . G . , 1 9 8 6 . G e o c h e m i c a l b e h a v i o r o f a n t i m o n y , a r s en i c, c a d m i u m a n d t h a l li u m : E h - - p H d i a g r a m sfo r 25 C , 1 -ba r p res s u re . C hem . Geo l . , 54 : 271 - -278 .

New Eh - -pH d i ag ram s have b een de r i ved fo r S b , As , C d and T1. Thes e d i ag ram s a re u s e fu l i n d i scus s ingt he com m on as s oc i a t i on o f s eve ra l o r a l l o f t hes e e l em en t s a round ep i t he rm al Au depos i t s . S l i gh t d i f f e ren -c e s in p H , E h o r b o t h p H - - E h a r e s u f f ic i e n t t o a c c o u n t f o r p a r t it i o n in g o f t h e s e e l e m e n t s f r o m d e p o s it todepos i t . I n add i t i on , ch l o r i de co nce n t r a t i on - -pH d i ag ram s fo r Au , Hg and T1 a re u s e fu l t o fu r t h e r d i s cus st h e r o l e o f c h l o r id e ( a n d p o s s i b ly c h lo r i d e c o m p l e x e s ) in t h e m i g r a t i o n o f s o m e o f t h e s e e l e m e n t s u n d e rl o w T ,P cond i t i ons .

1 . n t r o d u c t i o nM a n y e l e m e n t s h a v e b e e n p r o p o s e d a s

p a t h f i n d e r e l e m e n t s f o r t e l e t h e r m a l - - x e n o -t h e r m a l - - e p i t h e r m a l o r e d e p o s i t s o f g o l d .T h e s e i n c l u d e S b , A s , C d , T 1 , M n , H g a n d A g ,a m o n g o t h e r s . T h a l l i u m , i n p a r t i c u l a r , h a sa t t r a c t e d m u c h i n t e r e s t a s a g o l d p a t h f i n d e r( I k r a m u d d i n e t al . , 1 9 8 3 ) . L e s s e r b u t n o n e -t h e l e s s i m p o r t a n t t r a c e s o f S b , A s a n d C d( a n d o t h e r s ) a r e a ls o c o m m o n a r o u n d t h e s el o w - p r e s s u r e g o l d d e p o s i t s .

T h i s n o t e w i ll p r e s e n t n e w E h - - p H d i a g r a m sf o r t h e e l e m e n t s S b , A s , C d a n d T 1; a s w e l la s p u b l i s h e d C l - - p H d i a g ra m s f o r T 1 - - H g - - A u( B a es a n d M e s m e r , 1 9 7 6 ) a n d a t t e m p t t o ex -p l a i n w h y c e r t a i n e l e m e n t s s u c h a s T1 a r eu s e fu l p a t h f i n d e r s f o r A u a n d o t h e r m e t a l s .

T h i s n o t e d o e s n o t p r e t e n d t o c o v e r t h eb a c k g r o u n d g e o c h e m i s t r y o f t h e e l e m e n t sS b , A s , C d a n d T 1 , a n d t h e r e a d e r i s r e f e r r e dt o g e n e r a l g e o c h e m i c a l l i t e r a t u r e f o r s u c hi n f o r m a t i o n . S o m e k e y r e f e r e n c e s w il l b ei d e n t i f i e d a s t h e y p e r t a i n t o th i s w o r k , h o w -e v e r .2 . P r e v i o u s w o r k

T h e u s e o f E h - - p H d i a g r a m s f o r a s s i s t a n c ein d e a l in g w i t h g e o c h e m i c a l p r o b l e m s h a sb e e n d e s c r i b e d , f o r e x a m p l e , b y G a r re l s a n dC h r is t ( 1 9 6 5 ) , K r a u s k o p f ( 1 9 7 9 ) , V e r i n k( 1 9 7 9 ) a n d B r o o k i n s ( 1 9 8 4 ) . P r e v i o u s E h - - p Hd i a g r a m s f o r S b a n d C d h a v e b e e n g i v e n b yB r o o k i n s ( 1 9 7 8 ) , a n d f o r A s b y B r o o k i n s( 1 9 7 9 ) a n d L o n g m i r e ( 1 9 8 3 ) . T h e d a t a u s e d

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272in these earlier works were subject to fairlylarge errors, however, and these three di-agrams have been recalculated here. No prev-ious diagram, using modern data, has beenatte mpt ed for T1, however.

The role of total dissolved C1 in affectingthe geochemical behavior of the elementsSb, As, Cd and T1 is not well known, butBaes and Mesmer (1976) have calculated thetota l Cl--pH diagrams for the stability fieldsof Hg, Au and T1. Some of their work, withpermission, are presented herein to helpinterpret the Eh--pH diagrams. The authorassumes, however, responsibility for all dis-cussion and conclusio ns drawn involving theirwork.3. Ther modyn amic data

In order to be as consistent as possible, Ihave restr ic ted my choice of thermodynamicdata to that presented by Wagman et al.(1982). The data are presented in Table I.All Gibbs free energies of f orm ati on (A G}) forthe species of interest have been recalculatedto kilocalories per mole (kcal. mo1-1) andonly 298.15 K, and 1-bar pressure data havebeen employed. Further, since I am mostinterested in trace amounts of the elementsSb, As, Cd and T1 in waters, activities fordissolved amounts of these elements aretak en at 10 -8 and 10 -6 . Th ese values arechosen in accord with the approaches of theearlier workers (i.e. Garrels and Christ, 1965;Brookins , 1978 , 1984) and, since the di-agrams are for generic purposes only, theseactivities are assumed to be equal to concen-tration (molalit ies) and unaffected by ionicstrength. While an over-simplification, thiswill not drastically change the Eh--pH di-agrams presented. Finally, the errors for thethermodynamic data are discussed in detailby Wagman et al. (1982) and the readeris referred to this source for detail . The readeris also reminded that not shown fo r theEh--pH diagrams are stability fields for mostsulfide complexes, etc., due to a lack oftherm odynam ic data .

TABLE IGibbs free energies of formation for speciesSpecies AG~(kcal. mol -~ )

A r s e n i c :AsS --16.80As:S 3 --40.30As203 --137.66H3AsO4 --183.08H~AsO~ --180.01HAsO:~ --170.69AsO~- --154.97

C a d m i u m :Cd 2 --18.55CdS --37.40CdO --54.59CdO~- --67.97CdOH+ --62.40HCdO --86.88Cd(OH)2 --113.19CdCO: --159.99

A n t i m o n y :SbO --42.33SbO~ --81.31Sb2S ~ --41.49S b 2 S ~ - - 2 3 . 7 8H S b O : - - 9 7 . 3 9S b O H ) 3 - - 1 6 3 . 7 7S b 2 0 4 - - 1 9 0 . 1 8Sb:O~ --198.18

T h a l l i u m :Tl --7.74TI(OH)~ --58.48TIOH~ --3.80T ~O --35.20T120 ~ --74.50T120 --82.98TI(OH)~ --121.18TI2S --22.39

O t h e r s :H2S --6.65HS- +2.89SOt --177.95H20 --56.69H:CO~ --148.94HCO3 --140.26C O ~ - - - 1 2 6 . 1 7All data from Wagman et al. (1982).

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273

4 . E h - - p H d i a gr a m sT h e E h - - p H d i a g r a m s , c a l c u l a t e d f o l l o w i n g

t h e m e t h o d s d e s c r i b e d i n B r o o k i n s ( 1 9 8 4 ) ,a r e s h o w n f o r S b , A s , C d a n d T 1 a s F i g . 1 A ,B , C a n d D , r e s p e c t i v e l y . T h e s e a r e d i s c u s s e di n t u r n .T r a n s p o r t o f S b i n h y d r o t h e r m a l w a t e r sm a y b e a s ac i d ic o r b a s i c o x y i o n s , S b O o rS b O ~ , r e s p e c t i v e l y , u n d e r m i l d l y o x i d i z i n gc o n d i t i o n s ( i. e. j u s t a b o v e t h e s u l f i d e - - s u l f a t ef e n c e ) a s s h o w n i n F i g. 1A . U n d e r re d u c i n g ,n e a r - n e u t r a l t o m o r e a lk a l in e c o n d i t i o n s ,t r a n s p o r t a s a s u lf i d e c o m p l e x s u c h a s S b : S ~ -i s p o s s i b l e ( n o t e : a c t i v i t y o f t o t a l d i ss o l v e dS = 1 0 -3 a s s u m e d ) . B y f a r th e b u l k o f t h es t a b i l i t y f i e ld o f w a t e r i s c o v e r e d b y s o l idS b s p e c i e s ( S b 2 S 3 , S b ( O H ) 3 , S b 2 0 4 , S b 2 0 5 ) ,t h u s s u g g e st in g t h a t S b t r a n s p o r t m u s t t a k ep l a c e a t m o d e r a t e l y l o w E h - v a lu e s . N o d a t aa r e a v a i l a b l e f o r c h l o r i d e c o m p l e x e s o f S b .T r a n s p o r t o f S b s p e c i e s a s a c h l o r i d e c o m -p l e x is d i f fi c u l t t o a s se ss . B o y l e a n d J o n a s s o n( 1 9 8 4 ) s u g g e s t t h a t s p e c i e s s u c h a s S bC136 ,S bC 13 , Sb O C 1 , e t c . , m a y b e l o c a l l y i m p o r t a n ti n c h l o r i d e - r i c h a c i d i c h o t s p r i n g w a t e r s . N od a t a a r e a v a i la b l e f o r t h e a q u e o u s s p ec i e s,h o w e v e r .T h e E h - - p H d i a g r a m f o r A s is s h o w n a sF i g. l B . O n l y o n e a c t i v i t y ( 1 0 -6 ) h as b e e na s s u m e d f o r d i s s o lv e d A s s p e c i es f o r c o n -v e n i e n c e . W i d e t r a n s p o r t b y A s ( V ) s p e c ie sa b o v e t h e s u l f i d e - - s u l f a t e f e n c e i s i n d i c a t e d( H 3 A s O 4 , H 2 A s O ~ , H A s O2 4 , A s O ~ - ) . A s m a l lf i e l d o f A s 2 0 3 m a y s e r v e a s a p a r t i a l b a r r i e rt o d i s s o l v e d A s m i g r a t i o n u n d e r a c i di c m i l d -l y - o x i d i z i n g c o n d i t i o n s . U n d e r s u l f i d e - s t a b l ec o n d i t i o n s , A s m o b i l i t y s h o u l d b e f ai r lyl i m i t e d ( i gn o r i ng s u l f id e c o m p l e x e s ) . A g a i n,A s -- C 1 c o m p l e x e s a r e n o t w e l l d e f i n e d a n dn o t h e r m o d y n a m i c d a t a a re a v a il ab l e f o rs u c h s p e c ie s . B o y l e a n d J o n a s s o n ( 1 9 8 4 )p o i n t o u t t h a t S b is a g o o d p a t h f i n d e r e le -m e n t f o r a w i d e v a r i e t y o f m i n e r a l d e p o s i t s ,i n c l u d in g A u . F u r t h e r , s o m e s i m i la r b e h a v i o ro f S b w i t h A s i s a l so n o t e d , p o s s i b l y d u e t oz o n e o f f o r m a t i o n o f S b ( O H ) 3 a n d A s :O 3 .T h e C d E h - - p H d i a g r a m i s s h o w n a s F i g.

1 C . W h i l e a l a r g e s t a b i l i t y f i e l d f o r C d S i sp r e s e n t u n d e r s u l f id e - s t a b le E h - - p H , a b o v et h e s u l f i d e - - s u l f a te f e n c e C d s h o u l d b e v e r ym o b i l e a t n e a r -n e u t r a l a n d a c i d p H - v a l u e . T h isi s c o n s i s t e n t , f o r e x a m p l e , w i t h o b s e r v e dC d m o b i l i t y a t t h e O k l o n a t u r a l r e a c t o r ,G a b o n ( B r o o k i n s , 1 9 7 8 , 1 9 8 4 ) , a s o n l y s m a l lf i e l d s o f C d C O 3 a n d C d ( O H ) 2 , a n d a t h i g hp H , a r e p r e s e n t u n d e r o x i d i z i n g c o n d i t i o n s .M o b i l i t y a s C d 2 ( a s s u m e d t o b e m o r e a b u n -d a n t t h a n C d O H ) a n d / o r C d O ~ u n d e r o x i d iz -i n g c o n d i t i o n s i s a s s u m e d . H o w e v e r , i n t h ep H r a n ge o f 8 - -1 0 , C d C O 3 - - C d ( O H ) 3 m a yp r e v e n t C d m o b i l i t y .

T h e E h - - p H d i a g r a m f o r T 1 i s s h o w n i nF i g. 1 D . W h i le m o s t o f t h e E h - - p H f i e ld isc o v e r e d b y s o l i d s p e c i e s , f r o m m i l d l y t os t r o n g l y o x i d i z i n g c o n d i t i o n s , T 1 2 0 , T 1 2 0 3 ,T1 2 0 4, t h e r e i s a s i g n i f i c a n t l y l a r g e f i e l d f o rT 1 t h a t e x i s t s f r o m a c i d i c t o s l i g h t ly a l k a l i n ep H u n d e r r e d u c i n g c o n d i t i o n s t o , a s a f u n c -t i o n o f d e c r e a s in g p H , s o m e w h a t o x id i z i n g,a c i di c c o n d i t i o n s . T h i s is i m p o r t a n t f o r s e v e ra lr e a s o n s ( d i s c u s s e d b e l o w ) . S o m e T 1 - - C l - - p Hd a t a h a v e b e e n d e t e r m i n e d o r c a l c u l a t e d ( B a e sa n d M e s m e r , 1 9 7 6 ) , t h e i m p o r t a n c e o f w h i c hi s d i s c u s s e d l a t e r i n th i s p a p e r .5 . D i s c u s s i o n

T h e r o l e o f s p e c ie s o t h e r t h a n S o n t h em e t a l - - w a t e r s y s t e m E h - - p H d i ag r a m s f o r S b ,A s , C d a n d T 1 is n o t w e l l k n o w n . Y e t i n t h ec a se o f T 1, B a e s a n d M e s m e r ( 1 9 7 6 ) h a v ec a l c u l a t e d t h e C l - - p H d i a g r a m f o r T 1 s p e c ie s .T h i s i s s h o w n a s F i g. 2 A . I n a d d i t i o n , a l s of r o m t h e i r w o r k , F i g. 2 B a n d C s h o w s t h eC l - - p H d i a g r a m s f o r H g a n d A u s p e ci e s. T h e s ew i ll b e d i s c u s s e d b e l o w .I t i s a p p a r e n t f r o m i n s p e c t i o n o f F ig . 1t h a t t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s in t h eE h - - p H d i ag r a m s f o r t h e f o u r e l e m e n t s inq u e s t i o n . A l t h o u g h a ll c a n b e t r a n s p o r t e di n s o l u t i o n t o g e t h e r u n d e r v e r y a c i d i c m i l d l y -o x i d i z i n g c o n d i t i o n s , a t o t h e r E h - - p H c o n d i -t i o n s t h e y e x h i b i t q u i t e d i f f e r e n t b e h a v i o r .A t i n t e r m e d i a t e - p H m i l d l y - o x i d i z i n g c o n d i -t i o n s , T 1 - - C d - -A s c a n s t i ll b e t r a n s p o r t e d

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5/24/2018 Pourbaix Diagram of Cadmium

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t o g e t h e r , b u t n o t S b . U n d e r h ig h e r- p H c o n -d i t i o n s C d - - A s c a n s t il l b e m o b i l e , b u t n o tT 1 - -S b . F r o m t h e E h - - p H d i a g r a m s a l o n e ,o n e w o u l d h a v e t o a r g u e f o r a c i d ic o x i d i z in g( E h = 0 . 4 5 V o r s o ) f o r t h e t r a n s p o r t o f t h eg r o u p S b - - A s - - C d - -T 1 . A n y s h i f t in e i t h e r p Ho r E h w i ll r e m o v e o n e o r m o r e s p e c ie s f r o ms o l u t i o n . I n a c t u a l i t y , t h i s m a y a c c o u n t f o rh a l o e s o f , f r o m A u o r e o u t w a r d , o f A s, t h e nS b , t h e n T 1 w i t h C d f ix e d i n f a r t h e s t r e m o v e ds e c o n d a r y s u l fi d e s ( n o t e : t h i s k i n d o f z o n a -t i o n i s f o r a g e n e r i c c a s e o n l y ) .

T h e E h - - p H d i a g r a m f o r T1 ( F ig . 1 D )is a l so u s e f u l t o e x p l a i n t h e c o m m o n f i xa -t i o n o f T1 i n K - b e a r i n g m i n e r a l s f o u n d i na l t e ra t i o n h a l o e s a r o u n d s o m e o r e d e p o s i ts .T h e i o n i c r a d i u s o f T1 i s 1 . 4 9 A , w h i c h i sv e r y c l o s e t o t h a t o f K o f 1 . 3 3 A ( S h a w ,1 9 5 2 ) , a n d s u b s t i t u t i o n o f T1 f o r K is b o t hp r o b a b l e a n d o b s e r v e d ( s e e a l so V o s k r e s e n s -k a y a , 1 9 5 9 ) . I n ca s e s w h e r e T 1 - -K d i a d o c h yi s p o s s i b l e , t h i s w i l l a l l o w i t t o b e s e g r e g a t e df r o m t h e r e m a i n i ng S b - - A s - -C d . A n y o x y -h y d r o x i d e p h a s e s w i ll p r e s u m a b l y r e m o v e S ba n d A s in t h e s a m e r o c k s w h e r e T 1 - - K f i xa -t i o n h a s o c c u r r e d . O n l y C d w o u l d b e t r an s -p o r t e d f u r t h e r i n t h i s c as e .T h e C l - - p H d i a g r a m s ( F ig . 2 ) f o r T 1 - - H g - -A u a r e u s e f u l i n t h a t t h e r e la t i v e i m p o r t a n c eo f c h l o r i d e c o m p l e x e s o f t h e s e m e t a l s c a nb e e x a m i n e d . W h i le E h c o n d i t i o n s a re n o tg i v en i n B a e s a n d M e s m e r ( 1 9 7 6 ) , t h e E hm u s t b e a s s u m e d t o b e q u i t e h i g h , i .e . h i ghe n o u g h t h a t t h e s p e c i e s T l ( I I I) , H g ( I I ) a n dA u ( I I I ) p r e d o m i n a t e . F r o m t h e E h - - p H di -a g r a m f o r T 1 ( F ig . 1 D ) , i t w o u l d a p p e a r t h a tT l ( I V ) s h o u l d b e t h e s p e c i e s c o e x i s t i n g w i t hH g ( I I ) a n d A u ( I I I ) , b u t d a t a a r e n o t a v a il -a b l e .B a e s a n d M e s m e r ( 1 9 7 6 ) p o i n t t o t h e im -p o r t a n c e o f c h lo r i d e - -h y d r o x i d e c o m p l e x e sf o r t h e m e t a l s T 1 - - H g - - A u . I n d e e d , t h e s p e c i e sT 1O H C1 + i s c o n s i d e r e d b y B a e s a n d M e s m e r( 1 9 7 6 ) t o b e i m p o r t a n t , a s a r e H g O H C 1 a n dA u O H ( C 1 ) : -- A u ( O H ) 2 C 1 . W h a t t h ei r w o r ki n d i c a t e s i s t h a t a ll t h r e e m e t a l s , T 1 - - H g - - A um a y b e t r a n s p o r t e d a s c h l o r i d e - - h y d r o x i d ec o m p l e x e s f o r t h e c o n d i t i o n s g i v e n . I n n a t u r a l

277s e t ti n g s , s i m i l ar c o m p l e x e s m a y b e a s i m p o r -t a n t i f n o t m o r e s o . T h e p r o b l e m o f A us p e c i e s is c o m p l i c a t e d b y t h e f a c t t h a t w h i l eA u ( I ) is k n o w n , t h e r e a r e v er y f e w t h e r m o -d y n a m i c d a t a f o r A u ( I ) s p e c i es . T h e l i m i t e dd a t a ( W a g m a n e t al ., 1 9 8 2 ) d o , h o w e v e r , s ug -g e s t t h a t A u ( 0 ) w i ll o x i d i z e t o A u ( I ) inA u C l~ a t a n E h o f r o u g h l y 1 . 2 5 V , a p p r o x i -m a t e l y r i g h t a t t h e u p p e r s t a b i l i t y l i m i t o fw a t e r u n d e r p H = 0. T h e E h f o r A u ( I ) t oA u ( I I I ) i s h i g h e r t h a n t h i s f o r a ll s p e c i e s.F u r t h e r , w h i l e t h e b o u n d a r i e s f o r A u ( 0 ) - -A u ( I I I ) s p e c i e s s u c h a s H 2 A u O ~ a n d H A u O ~ -f a ll a b o v e t h e w a t e r s t a b i l i t y fi e ld , in t h e o r yt h e A u ( I ) s p e c i e s s h o u l d f a l l b e l o w t h e m .T h i s c o u l d b r i n g s o m e a q u e o u s A u ( I ) s p ec i e si n t o t h e s t a b i l i t y fi e ld f o r w a t e r . T h e r e v i se dE h - - p H d i ag r am f o r A u - - C I - - S - - H - O b y t h ew r i t e r w i ll b e p u b l i s h e d s e p a r a t e l y ( D . G .B r o o k i n s , i n p r e p . ) .6 . C o n c l u s i o n s

T h e c o n c l u s i o n s f r o m t h is w o r k a r e a sf o l l o w s :

( 1 ) T h e g r o u p T 1 - - C d - - A s - - S b c a n b e t r an s -p o r t e d t o g e t h e r u n d e r a c i d i c o x i d i z i n g c o n d i -t i o n s .( 2 ) U n d e r h i gh p H b u t s t il l o x i d i z i n g c o n -d i t io n s , A s - - C d m a y r e m a i n m o b i l e , b u tT 1 - -S b m a y b e i n c o r p o r a t e d i n t o s o li d s.

( 3 ) U n d e r i n t e r m e d i a t e - p H m o d e r a t e - E hc o n d i t i o n s , T 1 - - C d - -A s c a n b e t r a n s p o r t e dt o g e t h e r , b u t n o t S b .

( 4 ) H a l o e s o f t h e e l e m e n t s S b - - T 1 - - C d - - A sa b o u t e p i t h e r m a l A u d e p o s i ts ar e c o m m o n ;t h e E h - - p H d i ag r a m s h e l p e x p l a in s o m e , b u tc e r t a i n l y n o t a ll , o f t h e h a l o e s n o t e d .

( 5 ) T h e r o l e o f c h l o r i d e c o m p l e x e s i s n o tw e l l u n d e r s t o o d , y e t t h e C l - - p H d i ag r a m sf o r H g - - A u - - T 1 s p e c i e s ( f r o m B a e s a n dM e s m e r , 1 9 7 6 ) s t r o n g l y s u g g e s t t h a t s u c hc o m p l e x e s a r e v e r y i m p o r t a n t f o r t r a n s p o r t o fn o t o n l y H g - -A u - -T 1 , b u t f o r o t h e r e l e m e n t sa s w e l l .cknowledgementsT h a n k s a r e d u e t o D r . C h a r l e s B a e s , a n d

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J o h n W i l e y S o n s , P u b l i s h e rs , f o r p e r m i s s i o nf o r u s e o f t h e d i a g r a m s s h o w n a s F i g. 2 .B . K a l m a n a s s i s t e d i n t h e c a l c u l a t i o n s , a n dE . S t e w a r t d i d t h e d r a f t i n g o f F i g . 1 .

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