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Distribution of elements in geological processes
Chemical Geology - Elsevier Publishing Company, Amsterdam Printed in The Netherlands
DISTRIBUTION OF ELEMENTS IN GEOLOGICAL PROCESSES
R.G. BURNS and W.S. FYFE University of California, Berkeley, Calif. (U.S.A.); Manchester University, Manchester (Great Britain) (Received November
27, 1965)
SUMMARY The g e n e r a l l y a c c e p t e d e m p i r i c a l r u l e s of G o l d s c h m i d t , modified by Ringwood and A h r e n s , concerning e l e m e n t d i s t r i b u t i o n in g e o l o g i c a l p r o c e s s e s and the c o n t r o l l i n g a t o m i c p r o p e r t i e s a r e c o n s i d e r e d to lack g e n e r a l i t y and may fail to p r e d i c t actual behaviour. The b e h a v i o u r of t r a n s i t i o n e l e m e n t s frequently r e f l e c t s c r y s t a l - f i e l d influences which a r e a function of the c h e m i s t r y and s t r u c t u r a l s t a t e s of the s y s t e m s c o n s i d e r e d and in many c a s e s o b s e r v e d t r e n d s may be explained by c r y s t a l - f i e l d theory. These c o n s i d e r a t i o n s ind i c a t e that the magnitude and d i r e c t i o n of f r a c t i o n a t i o n of s o m e e l e m e n t p a i r s may be r a d i c a l l y changed in d i f f e r e n t c h e m i c a l s y s t e m s and e x i s t i n g r u l e s cannot d e s c r i b e such changes.
INTRODUCTION
One of the m a j o r p r o b l e m s of g e o c h e m i s t r y is to account for the o c c u r r e n c e and d i s t r i b u t i o n of e l e m e n t s between c o e x i s t i n g p h a s e s and r e g i o n s of the e a r t h during g e o l o g i c a l p r o c e s s e s . H i s t o r i c a l l y , the p r o b l e m has been divided into that of u n d e r s t a n d i n g the m a j o r p r i m a r y d i s t r i b u t i o n between an e s s e n t i a l l y m e t a l l i c c o r e and a s i l i c a t e m a n t l e and c r u s t which may contain m a s s i v e amounts of sulphides; and the s e c o n d a r y , but d i r e c t l y o b s e r v a b l e p r o b l e m , of u n d e r s t a n d i n g the r e d i s t r i b u t i o n of e l e m e n t s during p r o c e s s e s in the c r u s t and u p p e r mantle. The views of G o l d s c h m i d t (1937) on the p r i m a r y d i s t r i b u t i o n have not been c h a l l e n g e d s e r i o u s l y . G o l d s c h m i d t c o n s i d e r e d a model b a s e d on competition between an e x c e s s of m e t a l l i c e l e m e n t s and a d e f i c i e n c y of nonm e t a l s (oxygen and sulphur). On account of the g e n e r a l bulk c h e m i s t r y the dominant compounds a r e m e t a l l i c e l e m e n t s , s u l p h i d e s and s i l i c a t e s . E l e m e n t s w e r e c l a s s i f i e d as s i d e r o p h i l e if they w e r e r e l a t i v e l y i n e r t and e n t e r e d the m e t a l l i c phase, chalcophile if c o n c e n t r a t e d in s u l p h i d e s or lithophile if c o n c e n t r a t e d in s i l i c a t e s . It was a p p r e c i a t e d (Goldschmidt, 1958, pp. l l - 2 0 ) that by c o n s i d e r a t i o n of the r e l e v a n t data on f r e e e n e r g i e s of the compounds, t h i s b e h a v i o u r could be p r e d i c t e d with r e a s o n a b l e c e r t a i n t y . B r e w e r (1951) d i s c u s s e d the p r o b l e m in a m o r e fundamental m a n n e r by c o n s i d e r i n g the conditions of e q u i l i b r i u m in a s y s t e m s u b j e c t e d to v a r y i n g t e m p e r a t u r e , p r e s s u r e and position in the e a r t h ' s g r a v i t a t i o n a l field. While Chem. Geol., 1 (1966) 49-56
49
o n e m i g h t d o u b t o u r a b i l i t y t o p l a c e r e a s o n a b l e v a l u e s on a l l t h e p a r a m e t e r s involved in Brewer'~s treatment in many cases, there can be little doubt about t h e v a l i d i t y of s u c h a n a p p r o a c h if w e a s s u m e t h a t t h e g r o s s d i s t r i b u t i o n r e fleets a moderate approach to an equilibrium state. T h e a c c e p t e d c l a s s i f i c a t i o n of e l e m e n t s a s s i d e r o p h i l e , e t c . , ( G o l d s c h m i d t , 1 9 5 8 , p . 2 5 ) b a s e d on t e r r e s t r i a l and some meteoric data should not necess a r i l y b e t a k e n a s v a l i d f o r a l l p l a n e t a r y b o d i e s . T h e c o m p o u n d s i n w h i c h an e l e m e n t r e s i d e s , m a y b e q u i t e s e n s i t i v e t o t h e g r o s s b a l a n c e of e l e m e n t s a n d t h i s n e e d n o t a l w a y s b e e a r t h - l i k e . F o r e x a m p l e , c o n s i d e r t h e e l e m e n t s , Na, M g , F e , i n a n o x y g e n - d e f i c i e n t e n v i r o n m e n t . T h e r e l e v a n t f r e e e n e r g i e s of competing reactions are listed below: Na20+Fe
> FeO+2Na
LxG° = + 3 1 . 6 k c a l .
MgO +Fe
* FeO+Mg
AG ° = + 7 7 . 7 k c a l .
MgO +2Na
~ Na20 +Mg
AG ° = + 4 6 . 1 k e a l .
Thus given insufficient oxygen, magnesium dium and iron will appear in the metal phase. For sulphides we have:
will be oxyphile, while so-
Na2S + F e
* FeS
+2Na
AG ° = + 6 3 . 3 k c a l .
MgS +Fe
, FeS
~Mg
AG ° = + 6 0 . 3 k c a l .
MgS +2Na
>Na~+Mg
AG ° = -
Here, sodium will be chalcophile metal phase. For the competitive reactions: NazO + MgS
, N~eS + M g O
Na20 + FeS ~ MgO + FeS
Na2S + F e O ~FeO
3.0kcal.
and Mg and Fe will appear in the
AG ° = -
49
kcal.
AG ° = -
31.7 k c a l .
AG ° = + 17.3 k c a l .
+MgS
Thus Fe is more oxyphile than sodium, while magnesium is the most o x y p h i l e . D e p e n d i n g on O / S r a t i o s w e w o u l d h a v e e q u i l i b r i u m a s s e m b I a g e s : FeO, MgO, Na20
zero S
Na2S, F e O , M g O
1 S
Na2S , F e S , M g O
2 S
Na2S , F e S , M g S
zero 0
An
even
more
N a 2 0 + CuS
surprising
case
involves
~ CuO + Na~
the reaction:
AG ° = -- 15.3
For this reaction sodium is more chalcophile than copper but the situation is reversed in a reaction such as: Na~
+CuCO 3
Such simple
50
reactions
*CuS +Na2CO 3
AG ° = - 51.7
provide a warning against over generalization.
Ch~'m. (;-,_'oL.. I (1966) 49-.,36
DISTRIBUTION IN CRUSTAL PROCESSES
P r e s e n t v i e w s r e g a r d i n g r u l e s which w i l l d e s c r i b e t h e b e h a v i o u r of e l e m e n t s in c r u s t a l p r o c e s s e s a g a i n s t e m l a r g e l y f r o m G o l d s c h m i d t . He a p p r e c i a t e d t h a t without d e t a i l e d k n o w l e d g e of the a t o m i c s t r u c t u r e of s o l i d s , t h e r e w a s l i t t l e hope of u n d e r s t a n d i n g e l e m e n t d i s t r i b u t i o n b e t w e e n m i n e r a l s . T h i s a p p r e c i a t i o n l e d to the f i r s t t a b l e s of i o n i c r a d i i f r o m which c e r t a i n e m p i r i c a l c o r r e l a t i o n s w e r e d e d u c e d . V e r y g e n e r a l l y the r u l e s p r o p o s e d by G o l d s c h m i d t were: Ca) F o r two e l e m e n t s to e n t e r into a s o l i d s o l u t i o n , t h e s i z e of the i o n s m u s t be c o m p a r a b l e . (b) In i o n i c c r y s t a l s , if s i z e s a r e s i m i l a r and c h a r g e s i d e n t i c a l then the s m a l l e r ion w i l l p r e f e r e n t i a l l y d i s p l a c e a l a r g e r ion. (c) If s i z e s of i o n s a r e s i m i l a r , a m o r e highly c h a r g e d ion w i l l d i s p l a c e an ion with s m a l l e r c h a r g e . T h e s e r u l e s s u g g e s t t h a t t h e l a t t i c e e n e r g y of a s o l i d m u s t p l a y a d o m i n a n t r o l e in the d i s t r i b u t i o n p r o c e s s and would a l l follow f r o m the B o r n e q u a tion r e l a t i n g e n e r g y V to c h a r g e s Z1Z 2 and d i s t a n c e R f o r i o n i c i n t e r a c t i o n s :
V =
Z1Z2e2+ ZIZ2 e2 R Rn
T h e r e a r e m a n y w e l l - d o c u m e n t e d e x c e p t i o n s to the r u l e s and t h e s e h a v e l e d to v a r i o u s m o d i f i c a t i o n s . In m o s t , a t t e n t i o n h a s b e e n f o c u s s e d on d e p a r t u r e f r o m s i m p l e i o n i c b e h a v i o u r . T h u s F y f e (1951) i n d i c a t e d s t e r e o c h e m i c a l c o n t r o l in c a s e s w h e r e b o n d i n g w a s m o r e c o v a l e n t . A h r e n s (1964) and R i n g w o o d (1955) s u g g e s t e d that m o r e e l e c t r o p o s i t i v e i o n s would be c o n c e n t r a t e d g i v e n r o u g h e q u a l i t y of c h a r g e and r a d i u s . T h e s e w r i t e r s h a v e i n d i c a t e d t h a t t h i s i s due to the m o r e i o n i c b o n d s b e i n g s t r O n g e r but t h e y do not d i s c u s s what i s m e a n t by s t r e n g t h . In f a c t , if l a t t i c e e n e r g i e s a r e u s e d a s a c r i t e r i a t h e o p p o s i t e r e s u l t i s g e n e r a l l y obtained. F u r t h e r m o r e one g e n e r a l l y a c c e p t e d e m p i r i c a l r u l e f o r bond s t r e n g t h ( W a l s h , 1951) s t a t e s t h a t bond e n e r g y i s p r o p o r t i o n a l to the p r o d u c t of the e l e c t r o n e g a t i v i t i e s of b o n d e d atoms. A s i g n i f i c a n t p a p e r by Shaw (1953) h a s b e e n l a r g e l y o v e r l o o k e d by w r i t e r s on t h i s s u b j e c t . Shaw p o i n t e d out t h a t no r u l e s b a s e d on r a d i i alone c o u l d a c c o u n t f o r f r a c t i o n a t i o n b e h a v i o u r in a s y s t e m e x h i b i t i n g c o n t i n u o u s s o l i d s o l u t i o n with a m i n i m u m o r m a x i m u m m e l t i n g point. Such s y s t e m s a r e not u n c o m m o n , a c a s e in p o i n t b e i n g the a l k a l i f e l d s p a r s . L e t u s look at t h e t y p e s of r e a c t i o n s which m a y be i n v o l v e d in d i s t r i b u tion p r o c e s s e s . S o m e a r e : Xmelt + YZsolid ~ XZsolid + Ymelt (1) w h e r e two s p e c i e s , X and Y, in the m e l t c o m p e t e f o r a l a t t i c e s i t e in a s o l i d which c r y s t a l l i z e s f r o m the m e l t : (X, Y)Zsoli d ~ iX, Y)Z'soli d
(2)
w h e r e the two s p e c i e s a r e d i s t r i b u t e d b e t w e e n two s o l i d p h a s e s : Xsolution + YZsolid ~ XZsolid + Ysolution (3) w h e r e the two s p e c i e s a r e f r a c t i o n a t e d b e t w e e n an a q u e o u s p h a s e and a s o l i d : Xgas
+ YZsolid ~- YZsolid + ~ a s
Chem. Geol., 1 (1966) 49-56
(4) 51
where
the two species are present in a low-pressure, low-density gas phase. A survey of the geochemical literature reveals that in almost every discussion of element distribution and fractionation in geological processes, thermodynamic properties of only the solid phase containing the major or trace element are considered. One would infer from these discussions that distribution coefficients for reactions I-4 could be predicted if chemical bonding energies in the solid phases were known. This implies that binding forces of relevant species in melts, solutions, and gases are small. Such a conclusion is generally invalid for all processes of mineral formation, except that involving a gas phase separation (reaction 4), which is the least significant process. Furthermore, if it were possible to make valid predictions of distribution coefficients, then accurate calculations could be made of melting points and solubilities of phases in polycomponent systems, and of relative lattice energies. The fact is, however, that at the present time such calculations are beyond us. When processes involving element distribution are examined in detail, it soon becomes obvious that discussions in which thermodynamic properties of only the solid phase are considered, are unsatisfactory. It is not enough to know that an atom or ion is strongly bound in a crystal lattice; we must assess how strongly it is bound in the phase from which it is separating. Therefore, most of the "rules" used in discussing the substitution of ions in c r y s t a l l a t t i c e a r e r e l e v a n t only to f o r m a t i o n p r o c e s s e s of the type: X ,'"~'l' } YZ~olid *
XZsolid + Y gus
(5)
As soon as we i n t r o d u c e any m e d i u m m o r e c o m p l e x than the gas p h a s e , such as an aqueous fluid o r s i l i c a t e m e l t , any a n a l y s i s of v a l u e m o s t i n v o l v e an e s t i m a t e of t h e r m o d y n a m i c f u n c t i o n s for the s t e p s of a cycle such as: X ' l l u i d + YZsotid --> XZsolid + Y~lluhl
t
AHso]v:~tion
X i g:/~ + YXs(H i({ '\
A Hsoh, atio n
'
X~
(6)
X g:ls ~ Z gas+ Y'gas The extent of r e a c t i o n (6) m u s t depend on: (a) The r e l a t i v e l a t t i c e e n e r g i e s , U, of the s o l i d s (b) The r e l a t i v e s o l v a t i o n energies,zXHsolvation , of the ions. (c) The o v e r a l l change in f r e e e n e r g y , A(;. In fact, the r e a c t i o n is c o n t r o l l e d by the r e l a t i v e m a g n i t u d e s of the l a t i c e e n e r g i e s and h e a t s of s o l v a t i o n of the r e a c t a n t s which m u s t be f i n e l y b a l a n c e d as the o v e r a l l /XG and AH of such r e a c t i o n s a r e s m a l l by c o m p a r i s o n . The following r e a c t i o n s i l l u s t r a t e how p r e c a r i o u s l y b a l a n c e d t h e s e eff e c t s a r e (data f r o m L a t i m e r , 1952).
52
('hem, (;c~)l.. I (I966) 4(]-56
Reaction A +
Rb aqueous
+
AHhy drati°n
KClcr5 stal < sylvite
RbClcrystal +
AH = 0.13 k c a l .
-- AHhydration
= - - 177.20 k c a l . Rb~gas
+
= 183.11 k c a l .
KClerysta 1
~RbClcrystal +
b o = - 166.8 k c a l . ~ ' x / IF +\ + C1 + b ÷ K gas - g a s R gas
-
K aqueous
K*gas
Uo= 162.0 k c a l .
Reaction B Rb aqueous
+
KIcrystal
_AHhydratio n
> RbIcry stal A G = - 0.76 k c a l . A H = - 1.33 k c a l .
+
K+aq ue°us ,~
AHhydration
= 177.20 k c a l . R b~ gas
+
= - 183.11 k c a l .
KIcrystal
/
R b l c r y stal + K+g as - Uo = 146.6 k c a l . 1
Uo = 151 k c a l .
~K+~g as +I-gas
+ Rb+gas
Reaction C N i2~aqueous
+ M g F 2 crystal "~ A G = 4.54 k c a l . AHhydration sellaite AH = 8.89 k c a l . = - 672.2 k c a l .
Ni2-gas
+ MgF2 crystal
-- /7o = - 695 k c a l .
2 ~+ Mg
N i F 2 crystal + Mg 2+ aqueous - AHhydrati°n l = 715.8 k c a l . N i F 2 crystal + M g 2+aqueous /
U° = 728 kcal"
gas + 2 F - g a s + Ni2+gas
Reaction D Ni2~ aqueous .~
+ MgC12 crystal > NiC12crystal ~Mg2+aqueous AG = - 20.99 k c a l . c h l o r o m a g n e s i t e AH 17.21 k c a l . A Hhydratio n - A Hhydratio n | = 672.2 k c a l . = - 715.8 k c a l .
Ni2~ gas
+ MgC12 crystal
Uo = 632 k c a l .
NiC12 crystal+ Mg2+gas - 5 } ) = - 657.8 k c a l .
Mo-2+'x -.-~, gas + 2 Cl-gas + Ni2+gas R e a c t i o n A s h o w s t h a t Rb + (ionic r a d i u s 1.48 ~ ) d o e s not r e p l a c e K + (1.33 A) in s y l v i t e a s m i g h t h a v e b e e n p r e d i c t e d by the G o l d s c h m i d t r u l e s , w h e r e a s Rb + m a y r e p l a c e K + in KI ( r e a c t i o n B), c o n t r a r y to t h e s e r u l e s . R e a c t i o n s C and D, which i n v o l v e Mg 2+ (ionic r a d i u s 0.65 A; e l e c t r o n e g a t i v i t y Chem. Geol., 1 {1966) 49-56
53
] .2) and Ni 2~ (0.73 ~ ; 1.8), i n d i c a t e that the "Ringwood r u l e s " a r e obeyed by the t e n d e n c y for Mg 2~ to r e p l a c e Ni 2~ in NiF2, but not obeyed when Ni 2q r e p l a c e s Mg 2~ in c h l o r o m a g n e s i t e . The p r o b l e m is not s i m p l i f i e d by the knowledge that f a c t o r s that lead to l a r g e l a t t i c e e n e r g i e s also lead to l a r g e s o l v a t i o n e n e r g i e s , and the low h e a t s of fusion of m o s t s o l i d s , e s p e c i a l l y s i l i c a t e m i n e r a l s , i n d i c a t e that l i q u i d s t a t e b i n d i n g f o r c e s a r e v e r y s i m i l a r in m a g n i t u d e to b i n d i n g f o r c e s in s o l i d s . If we a r e to a c h i e v e a r e a l u n d e r s t a n d i n g of d i s t r i b u t i o n and f r a c t i o n a tion p r o c e s s e s , we m u s t obtain data for the c o m p l e t e p r o c e s s s i m i l a r to t h o s e c o m p i l e d for the r e a c t i o n s set out above. I m p r o v e m e n t of the e m p i r i c a l s h o r t cuts of the past cannot be s a t i s f y i n g u n t i l such data a r e c o m p i l e d for a l a r g e n u m b e r of p r o c e s s e s t a k i n g p l a c e in t y p i c a l geological r e a c t i o n m e d i a . T h e r e a r e s o m e c a s e s w h e r e , with l i m i t e d data, v a l i d c o m p a r i s o n s can be m a d e b e t w e e n t r a n s i t i o n - m e t a l ions and o t h e r i o n s of s i m i l a r size ano' c h a r g e (oxidation s t a t e s II and HI). It i s p o s s i b l e to a c c o u n t for the o r d e r of uptake of t r a n s i t i o n - m e t a l ions (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) in s i l i c a t e m i n e r a l s which c r y s t a l l i z e d f r o m a b a s i c m a g m a , by a p p l i c a t i o n of the c r y s t a l field t h e o r y ( W i l l i a m s , 1959: C u r t i s , 1964; B u r n s and Fyfe, 1964). The p r i n cipal f e a t u r e s of this t h e o r y have been d i s c u s s e d e a r l i e r ( B u r n s et al., 1964). In this p a p e r , we wish to i l l u s t r a t e how the c r y s t a l - f i e l d t h e o r y m a y be applied to solve a p r o b l e m i n v o l v i n g d i s t r i b u t i o n c o e f f i c i e n t s of t r a n s i t i o n m e t a l i o n s , p r e v i o u s l y m i s i n t e r p r e t e d (c.f. Ringwood, 1955, 1956; Mason, 1958; A h r e n s , 1964). In p a r t i c u l a r we wish to c o n s i d e r the m a g n e s i u m - n i c k e l association. The e n e r g y of t r a n s i t i o n - m e t a l ions i s a f u n c t i o n of the s y m m e t r y and p o s i t i o n of a n i o n s or l i g a n d s which s u r r o u H d an ion in l i q u i d s and s o l i d s . In such an e l e c t r o s t a t i c field the five "3d" o r b i t a l s a r e no l o n g e r d e g e n e r a t e and s o m e ions a r e s t a b i l i z e d r e l a t i v e to o t h e r s . The field i n d u c e d e n e r g y s e p a r a t i o n of ' 3 d " o r b i t a l s , A, is g r e a t e r for o c t a h e d r a l s y m m e t r y than t e t r a h e d r a l , and for a given value of 2x and s y m m e t r y it i s p o s s i b l e to e s t i m a t e c r y s t a l field s t a b i l i z a t i o n e n e r g i e s for each t r a n s i t i o n - m e t a l ion. It is now well e s t a b l i s h e d that the t h e r m o d y n a m i c p r o p e r t i e s of s e r i e s of c o m p o u n d s such as CaO .....-~ t r a n s i t i o n - m e t a l m o n o x i d e s . > ZnO a r e exp l i c a b l e in t e r m s of t r e n d s in r a d i i and s p e c i f i c c r y s t a l - f i e l d e n e r g y d i f f e r e n c e s (Orgel, 1960). T h e s e c r y s t a l - f i e l d e n e r g y t e r m s a r e l a r g e and d i f f e r e n c e s between m e m b e r s a r e l a r g e . It is also well e s t a b l i s h e d that when dif.f e r e n t c o o r d i n a t i o n s i t e s a r e a v a i l a b l e , s i t e p r e f e r e n c e is d o m i n a t e d by such c r y s t a l - f i e l d t e r m s , the s p i n e l s p r o v i d i n g an e l e g a n t e x a m p l e (Dunitz and O r g e l , 1957: M c C l u r e , 1957). In any d i s t r i b u t i o n p r o c e s s , if we can a n t i c i p a t e c h a n g e s in t h e s e s p e c i f i c i n t e r n a l e l e c t r o n i c effects, by c o n s i d e r a t i o n of c h a n g e s of bond l e n g t h s bond c h a r a c t e r (i.e., i o n i c - c o v a l e n t ) or site geom e t r y , then we may be able to p r e d i c t t r e n d s in d i s t r i b u t i o n c o e f f i c i e n t s . The s t r u c t u r e of a l i q u i d is q u a s i - c r y s t a l l i n e , and t h e r e a r e i n d i c a t i o n s that t r a n s i t i o n - m e t a l ions in s o l i d s and t h e i r d e r i v e d l i q u i d s o c c u r in s i m i l a r c o n f i g u r a t i o n s and c r y s t a l - f i e l d e n e r g i e s a r e s i m i l a r in the two e n v i r o n m e n t s . F o r e x a m p l e , the a b s o r p t i o n s p e c t r a of Ni(H20)62~ aq and c r y s t a l l i n e NiSO4" 6HL£) a r e a h n o s t i d e n t i c a l . In g e n e r a l , one a n t i c i p a t e s s l i g h t l y l a r g e r i n t e r a t o m i e d i s t a n c e s and l e s s r e g u l a r c o o r d i n a t i o n in l i q u i d s than in s o l i d s . H o w e v e r , t h e s e d i f f e r e n c e s a r e g e n e r a l l y s m a l l , as i n d i c a t e d by the c o m p a r a b l e h e a t s of f u s i o n of c o m p o u n d s of t r a n s i t i o n e l e m e n t s and o t h e r m e t a l s . But when ions a r e d i s s o I v e d in p o l y c o m p o n e n t I i q u i d s w h e r e eonfiguratiol~s 54
Chem. C;c(~I.. I (1!}66) 4 ~ 5 6
of s e v e r a l t y p e s a r e p r e s e n t , a d i s t r i b u t i o n of i o n s b e t w e e n the d i f f e r e n t s i t e s will o c c u r if t h e r m a l e n e r g i e s a r e c o m p a r a b l e with s i t e e n e r g y d i f f e r e n c e s . In s i l i c a t e m e l t s t e t r a h e d r a l and o c t a h e d r a l s i t e s p r e d o m i n a t e and t r a n s i t i o n m e t a l i o n s m a y e n t e r e i t h e r . H o w e v e r , t r a n s i t i o n - m e t a l i o n s a r e r a r e l y found in t e t r a h e d r a l s i t e s in c r y s t a l s which s e p a r a t e f r o m a s i l i c a t e liquid. The b e h a v i o u r of n i c k e l and m a g n e s i u m d u r i n g m a g m a t i c c r y s t a l l i z a t i o n cannot be e x p l a i n e d on the b a s i s of e x i s t i n g r u l e s . In the s i m p l e b i n a r y s o l i d s o l u t i o n s e r i e s , Mg2SiO4 - Ni2SiO4, on a c c o u n t of the h i g h e r m e l t i n g point of Mg2SiO4, c r y s t a l s a r e e n r i c h e d in the m a g n e s i u m c o m p o n e n t (Ringwood, 1956). But d u r i n g the c r y s t a l l i z a t i o n of b a s i c m a g m a s the s i t u a t i o n i s r e v e r s e d . O b v i o u s l y no s i m p l e r u l e will c o v e r both t h e s e c a s e s , p a r t i c u l a r l y a r u l e which t a k e s into a c c o u n t p r o p e r t i e s of the s o l i d p h a s e alone. To e x p l a i n such an i n v e r s i o n one m u s t c o n s i d e r c h a n g e s in the liquid state. In the s y s t e m Mg2SiO 4 - Ni2SiO4, both liquid and s o l i d s t a t e s a r e d o m i n a t e d by c a t i o n s in six c o o r d i n a t e s i t e s . S t u d i e s of the s p e c t r a of g l a s s e s ( B u r n s and Fyfe, 1964) s u p p o r t t h i s view. It is a l s o well known that a l k a l i s i l i c a t e l i q u i d s p r o d u c e a l a r g e a r r a y of 4 c o o r d i n a t e s i t e s and in t e r m s of c r y s t a l - f i e l d t h e o r y t r a n s i t i o n - m e t a l i o n s , except t h o s e with 5 and 10 "d" e l e c t r o n s , p r e f e r the o c t a h e d r a l s i t e s . If the c h a r a c t e r of s i t e d i s t r i b u t i o n in a l i q u i d c h a n g e s with the c o m p o s i t i o n of the liquid s p e c i f i c d i f f e r e n c e s b e t w e e n i o n s with and without u n f i l l e d "d" e l e c t r o n s h e l l s a r e a n t i c i p a t e d . C o n s i d e r the two r e a c t i o n s : Mg2SiO 4 + 6:4 liquid
~s o l u t i o n
and: Ni2SiO 4 + 6:4 liquid
>s o l u t i o n
w h e r e we u s e a s o l v e n t known to have s o m e d i s t r i b u t i o n of six and four coo r d i n a t e s i t e s fixed by i t s own b i n d i n g f o r c e s . The n i c k e l and m a g n e s i u m i o n s have i d e n t i c a l c h a r g e s and a l m o s t i d e n t i c a l r a d i i . If we c o m p a r e , and only c o m p a r e , the i n t e r a c t i o n s of Ni 2+ and Mg 2+ with the s o l v e n t two f e a t u r e s a r i s e . B e c a u s e of s t r o n g s i t e p r e f e r e n c e , n i c k e l will occupy o c t a h e d r a l s i t e s if a v a i l able and will e n t e r t e t r a h e d r a l s i t e s a p p r e c i a b l y only if the t h e r m a l e n e r g y i s c o m p a r a b l e with s i t e p r e f e r e n c e e n e r g y . M a g n e s i u m m a y a l s o f a v o r o c t a h e d r a l s i t e s t h r o u g h a c o m p a r a b l e " l a t t i c e e n e r g y " effect but an a d d i t i o n a l e l e c t r o n i c p r e f e r e n c e will not o p e r a t e . T h u s in the s o l v e n t , m a g n e s i u m ions will be s p r e a d m o r e e v e n l y o v e r a v a i l a b l e s i t e s . The s o l u b i l i t y of the two c o m p o u n d s (and h e n c e t h e i r m e l t i n g b e h a v i o u r ) will depend on the AH and AS of solution. O b v i o u s l y if m a g n e s i u m i s s p r e a d o v e r m o r e s i t e s then for the s o l u t i o n p r o c e s s ASMgZ+ ~ ASNiZ+ F u r t h e r , as s o m e n i c k e l i o n s will e n t e r or be f o r c e d to e n t e r t e t r a h e d r a l s i t e s : AHNi2+ ~ AHMg2~ Both t h e s e t e r m s will lead to: AGNi2+ ~ AGMg2+ an:l a l o w e r s o l u b i l i t y and h e n c e h i g h e r m e l t i n g t e m p e r a t u r e f o r the n i c k e l o l i v i n e . R e s e a r c h in p r o g r e s s i n d i c a t e s that the s y s t e m Mg2SiO4 - Ni2SiO4 Na2Si205 p r o b a b l y shows such an i n v e r s i o n of s o l i d - s o l u t i o n r e l a t i o n s . In o l i v i n e - r i c h t e r n a r y m i x t u r e s m a g n e s i u m e n r i c h m e n t o c c u r s while n e a r the a l k a l i s i l i c a t e c o r n e r o l i v i n e s a r e e n r i c h e d in n i c k e l . Chem. Geol.. 1 (1966) 49-56
55
Many other c a s e s could be c o n s i d e r e d where c r y s t a l - f i e l d t e r m s dominate in distribution phenomena. But the present case i s sufficient to indicate the importance of considering all parts of the distribution c y c l e including the liquid state and c l e a r l y shows why the existing e m p i r i c a l rules lack generality.
ACKNOW LEDGEM E NT
The w r i t e r s gratefully acknowledge support of this work by grants f r o m t h e R e s e a r c h Fund of t h e A m e r i c a n
National Science Foundation and the Petroleum Chemical Society.
HEFERENCES A h r e n s , i,.tl.. 1953. T h e u s e o[ ionization p o t e n t i a l s . P a r t 2. Anion affinity and g e o c h e m i s t r y . G e o c h i m . C o s m o e h i m , Acta. 3: 1-29. A h r e n s . L.H., 1964. The s i g n i f i c a n c e of the c h e m i c a ! bond for c o n t r o l l i n g the g e o c h e m ical d i s t r i b u t i o n of" the e l e m e n t s . P h y s . C h e m . E a r t h , 5: 1-54. B:'ewer. L.. 1951. The e q u i l i b r i u m di~'.cib~',~ io>~ :)1" the eh~rnt, nts in the e a r t h ' s g r a v i t a tional lield. J. Geol., 59: 490-497. I{.trms. I{.G. :~nd Fb'~:-. W.S,. 196,i. Site P r e f e r e n c e e n e r g y and s e l e c t i v e uptake of t r a n s i t i o n - m , ' t a l i:)ns d u r i n g m a g m a t i e c r y s t a l l i z a t i o n . Science, 144: 1001-1003. B u r n s , I{.G., Clark. R.II. and Fyfe, W.S., 1964. C r y s t a l - f i e l d t h e o r y and a p p l i c a t i o n s to p r o b l e m s in g e o c h e m i s t r y . In: C h e m i s t r y of the E a r t h ' s C r u s t . V e r n a d s k y Centennial Symp.. 2: 8s 11!6. C u r t i s , C.D,. 1961. Application of the c r y s t a l - f i e l d t h e o r y to the i n c l u s i o n of t r a c e t r a n s i t i o n e l e m e n t s in m i n e r a l s (luring m a g m a t i e d i f f e r e n t i a t i o n . G e o e h i m . C o s m o e h i m . Aeta, 28: 389-403. Dunitz. J.D. and O r g e l , L.E.. 1957. E l e c t r o n i c p r o p e r t i e s of t r a n s i t i o n e l e m e n t o x i d e s . II. Cation d i s t r i b u t i o n a m o n g s t o e t a h e d r a l and t e t r a h e d r a l s i t e s . P h y s . C h e m . Solids, 3: 318-333. Fyfe, W.S., 1951. I s o m o r p h i s m and bond type. Am. M i n e r a l o g i s t , 36: 538-542. G o l d s c h m i d t . V.M., 1937. ' t h e p r i n c i p l e s of d i s t r i b u t i o n of c h e m i e a l e l e m e n t s in m i n e r a l s and r o c k s . J. C h e m . So(.. 1937: 65;~-672. G o l d s c h m i d t , V.M.. 1958. G e o c h e m i s t r y . Oxford Univ. P r e s s , London, 730 pp. ~l~atimer, W.M.. 1(2t52. Oxidation P o t e n t i a l s , 2nd ed., P r e n t i c e - H a l l , Englewood Cliffs, N.J., 392 pp, M a s o n . B.. 1.(t58. P r i n c i p l e s o17 G e o c h e m i s t r y . Wiley, New York, N.Y., 308 pp. M e C l u r e , D.S.. 1957, The d i s t r i b u t i o n ol t r a n s i t i o n - m e t a l ions in s p i n e l s . P h y s . C h e m . Solids. 3: 311-517. O r g e t , L.I:]., 1960. An Introduction to T r m ~ s i t i o n - M e t a l C h e m i s t r y : L i g a n d - F i e l d T h e o r y . Methuen. London. 180 DD. l{ingwood, A.E., 1955. The p r i n c i p l e s g o v e r n i n g t r a e e c l e m e n t d i s t r i b u t i o n d u r i n g m a g m a t i e c r y s t a l l i z a t i o n . P a r t I. The influence of e l e e t r o n e g a t i v i t y . G e o c h i m . Cosmoet~im. Acta, 7: 18f>202. Ilingwood, A.E.. 1956. Melting r e l a t i o n s m the s y s t e m Mg2SiO4 - Ni2SiO4. G e o c h i m . C o s m o e h i m . Aeta, 10: 297-303. Shaw, D.M., 1953. T h e c a m o u f l a g e p r i n e i p l e and t r a c e e l e m e n t d i s t r i b u t i o n in m a g m a t i c m i n e r a l s . Geol.. 61: 142--151. W a l s h , A.D., 1951. F a c t o r s a f f e c t i n g bond s t r e n g t h s . P r o e . Roy. Soe. (London), S e r . A . . 207: 13. W i l l i a m s , R . J . P . . 1959. Deposition oF t r a c e e l e m e n t s in b a s l e m a g m a . N a t u r e , 184: 44.
56
C h e m . Geol.. 1 (19661 49-56
DISTRIBUTION OF ELEMENTS IN GEOLOGICAL PROCESSES
R.G. BURNS and W.S. FYFE University of California, Berkeley, Calif. (U.S.A.); Manchester University, Manchester (Great Britain) (Received November
27, 1965)
SUMMARY The g e n e r a l l y a c c e p t e d e m p i r i c a l r u l e s of G o l d s c h m i d t , modified by Ringwood and A h r e n s , concerning e l e m e n t d i s t r i b u t i o n in g e o l o g i c a l p r o c e s s e s and the c o n t r o l l i n g a t o m i c p r o p e r t i e s a r e c o n s i d e r e d to lack g e n e r a l i t y and may fail to p r e d i c t actual behaviour. The b e h a v i o u r of t r a n s i t i o n e l e m e n t s frequently r e f l e c t s c r y s t a l - f i e l d influences which a r e a function of the c h e m i s t r y and s t r u c t u r a l s t a t e s of the s y s t e m s c o n s i d e r e d and in many c a s e s o b s e r v e d t r e n d s may be explained by c r y s t a l - f i e l d theory. These c o n s i d e r a t i o n s ind i c a t e that the magnitude and d i r e c t i o n of f r a c t i o n a t i o n of s o m e e l e m e n t p a i r s may be r a d i c a l l y changed in d i f f e r e n t c h e m i c a l s y s t e m s and e x i s t i n g r u l e s cannot d e s c r i b e such changes.
INTRODUCTION
One of the m a j o r p r o b l e m s of g e o c h e m i s t r y is to account for the o c c u r r e n c e and d i s t r i b u t i o n of e l e m e n t s between c o e x i s t i n g p h a s e s and r e g i o n s of the e a r t h during g e o l o g i c a l p r o c e s s e s . H i s t o r i c a l l y , the p r o b l e m has been divided into that of u n d e r s t a n d i n g the m a j o r p r i m a r y d i s t r i b u t i o n between an e s s e n t i a l l y m e t a l l i c c o r e and a s i l i c a t e m a n t l e and c r u s t which may contain m a s s i v e amounts of sulphides; and the s e c o n d a r y , but d i r e c t l y o b s e r v a b l e p r o b l e m , of u n d e r s t a n d i n g the r e d i s t r i b u t i o n of e l e m e n t s during p r o c e s s e s in the c r u s t and u p p e r mantle. The views of G o l d s c h m i d t (1937) on the p r i m a r y d i s t r i b u t i o n have not been c h a l l e n g e d s e r i o u s l y . G o l d s c h m i d t c o n s i d e r e d a model b a s e d on competition between an e x c e s s of m e t a l l i c e l e m e n t s and a d e f i c i e n c y of nonm e t a l s (oxygen and sulphur). On account of the g e n e r a l bulk c h e m i s t r y the dominant compounds a r e m e t a l l i c e l e m e n t s , s u l p h i d e s and s i l i c a t e s . E l e m e n t s w e r e c l a s s i f i e d as s i d e r o p h i l e if they w e r e r e l a t i v e l y i n e r t and e n t e r e d the m e t a l l i c phase, chalcophile if c o n c e n t r a t e d in s u l p h i d e s or lithophile if c o n c e n t r a t e d in s i l i c a t e s . It was a p p r e c i a t e d (Goldschmidt, 1958, pp. l l - 2 0 ) that by c o n s i d e r a t i o n of the r e l e v a n t data on f r e e e n e r g i e s of the compounds, t h i s b e h a v i o u r could be p r e d i c t e d with r e a s o n a b l e c e r t a i n t y . B r e w e r (1951) d i s c u s s e d the p r o b l e m in a m o r e fundamental m a n n e r by c o n s i d e r i n g the conditions of e q u i l i b r i u m in a s y s t e m s u b j e c t e d to v a r y i n g t e m p e r a t u r e , p r e s s u r e and position in the e a r t h ' s g r a v i t a t i o n a l field. While Chem. Geol., 1 (1966) 49-56
49
o n e m i g h t d o u b t o u r a b i l i t y t o p l a c e r e a s o n a b l e v a l u e s on a l l t h e p a r a m e t e r s involved in Brewer'~s treatment in many cases, there can be little doubt about t h e v a l i d i t y of s u c h a n a p p r o a c h if w e a s s u m e t h a t t h e g r o s s d i s t r i b u t i o n r e fleets a moderate approach to an equilibrium state. T h e a c c e p t e d c l a s s i f i c a t i o n of e l e m e n t s a s s i d e r o p h i l e , e t c . , ( G o l d s c h m i d t , 1 9 5 8 , p . 2 5 ) b a s e d on t e r r e s t r i a l and some meteoric data should not necess a r i l y b e t a k e n a s v a l i d f o r a l l p l a n e t a r y b o d i e s . T h e c o m p o u n d s i n w h i c h an e l e m e n t r e s i d e s , m a y b e q u i t e s e n s i t i v e t o t h e g r o s s b a l a n c e of e l e m e n t s a n d t h i s n e e d n o t a l w a y s b e e a r t h - l i k e . F o r e x a m p l e , c o n s i d e r t h e e l e m e n t s , Na, M g , F e , i n a n o x y g e n - d e f i c i e n t e n v i r o n m e n t . T h e r e l e v a n t f r e e e n e r g i e s of competing reactions are listed below: Na20+Fe
> FeO+2Na
LxG° = + 3 1 . 6 k c a l .
MgO +Fe
* FeO+Mg
AG ° = + 7 7 . 7 k c a l .
MgO +2Na
~ Na20 +Mg
AG ° = + 4 6 . 1 k e a l .
Thus given insufficient oxygen, magnesium dium and iron will appear in the metal phase. For sulphides we have:
will be oxyphile, while so-
Na2S + F e
* FeS
+2Na
AG ° = + 6 3 . 3 k c a l .
MgS +Fe
, FeS
~Mg
AG ° = + 6 0 . 3 k c a l .
MgS +2Na
>Na~+Mg
AG ° = -
Here, sodium will be chalcophile metal phase. For the competitive reactions: NazO + MgS
, N~eS + M g O
Na20 + FeS ~ MgO + FeS
Na2S + F e O ~FeO
3.0kcal.
and Mg and Fe will appear in the
AG ° = -
49
kcal.
AG ° = -
31.7 k c a l .
AG ° = + 17.3 k c a l .
+MgS
Thus Fe is more oxyphile than sodium, while magnesium is the most o x y p h i l e . D e p e n d i n g on O / S r a t i o s w e w o u l d h a v e e q u i l i b r i u m a s s e m b I a g e s : FeO, MgO, Na20
zero S
Na2S, F e O , M g O
1 S
Na2S , F e S , M g O
2 S
Na2S , F e S , M g S
zero 0
An
even
more
N a 2 0 + CuS
surprising
case
involves
~ CuO + Na~
the reaction:
AG ° = -- 15.3
For this reaction sodium is more chalcophile than copper but the situation is reversed in a reaction such as: Na~
+CuCO 3
Such simple
50
reactions
*CuS +Na2CO 3
AG ° = - 51.7
provide a warning against over generalization.
Ch~'m. (;-,_'oL.. I (1966) 49-.,36
DISTRIBUTION IN CRUSTAL PROCESSES
P r e s e n t v i e w s r e g a r d i n g r u l e s which w i l l d e s c r i b e t h e b e h a v i o u r of e l e m e n t s in c r u s t a l p r o c e s s e s a g a i n s t e m l a r g e l y f r o m G o l d s c h m i d t . He a p p r e c i a t e d t h a t without d e t a i l e d k n o w l e d g e of the a t o m i c s t r u c t u r e of s o l i d s , t h e r e w a s l i t t l e hope of u n d e r s t a n d i n g e l e m e n t d i s t r i b u t i o n b e t w e e n m i n e r a l s . T h i s a p p r e c i a t i o n l e d to the f i r s t t a b l e s of i o n i c r a d i i f r o m which c e r t a i n e m p i r i c a l c o r r e l a t i o n s w e r e d e d u c e d . V e r y g e n e r a l l y the r u l e s p r o p o s e d by G o l d s c h m i d t were: Ca) F o r two e l e m e n t s to e n t e r into a s o l i d s o l u t i o n , t h e s i z e of the i o n s m u s t be c o m p a r a b l e . (b) In i o n i c c r y s t a l s , if s i z e s a r e s i m i l a r and c h a r g e s i d e n t i c a l then the s m a l l e r ion w i l l p r e f e r e n t i a l l y d i s p l a c e a l a r g e r ion. (c) If s i z e s of i o n s a r e s i m i l a r , a m o r e highly c h a r g e d ion w i l l d i s p l a c e an ion with s m a l l e r c h a r g e . T h e s e r u l e s s u g g e s t t h a t t h e l a t t i c e e n e r g y of a s o l i d m u s t p l a y a d o m i n a n t r o l e in the d i s t r i b u t i o n p r o c e s s and would a l l follow f r o m the B o r n e q u a tion r e l a t i n g e n e r g y V to c h a r g e s Z1Z 2 and d i s t a n c e R f o r i o n i c i n t e r a c t i o n s :
V =
Z1Z2e2+ ZIZ2 e2 R Rn
T h e r e a r e m a n y w e l l - d o c u m e n t e d e x c e p t i o n s to the r u l e s and t h e s e h a v e l e d to v a r i o u s m o d i f i c a t i o n s . In m o s t , a t t e n t i o n h a s b e e n f o c u s s e d on d e p a r t u r e f r o m s i m p l e i o n i c b e h a v i o u r . T h u s F y f e (1951) i n d i c a t e d s t e r e o c h e m i c a l c o n t r o l in c a s e s w h e r e b o n d i n g w a s m o r e c o v a l e n t . A h r e n s (1964) and R i n g w o o d (1955) s u g g e s t e d that m o r e e l e c t r o p o s i t i v e i o n s would be c o n c e n t r a t e d g i v e n r o u g h e q u a l i t y of c h a r g e and r a d i u s . T h e s e w r i t e r s h a v e i n d i c a t e d t h a t t h i s i s due to the m o r e i o n i c b o n d s b e i n g s t r O n g e r but t h e y do not d i s c u s s what i s m e a n t by s t r e n g t h . In f a c t , if l a t t i c e e n e r g i e s a r e u s e d a s a c r i t e r i a t h e o p p o s i t e r e s u l t i s g e n e r a l l y obtained. F u r t h e r m o r e one g e n e r a l l y a c c e p t e d e m p i r i c a l r u l e f o r bond s t r e n g t h ( W a l s h , 1951) s t a t e s t h a t bond e n e r g y i s p r o p o r t i o n a l to the p r o d u c t of the e l e c t r o n e g a t i v i t i e s of b o n d e d atoms. A s i g n i f i c a n t p a p e r by Shaw (1953) h a s b e e n l a r g e l y o v e r l o o k e d by w r i t e r s on t h i s s u b j e c t . Shaw p o i n t e d out t h a t no r u l e s b a s e d on r a d i i alone c o u l d a c c o u n t f o r f r a c t i o n a t i o n b e h a v i o u r in a s y s t e m e x h i b i t i n g c o n t i n u o u s s o l i d s o l u t i o n with a m i n i m u m o r m a x i m u m m e l t i n g point. Such s y s t e m s a r e not u n c o m m o n , a c a s e in p o i n t b e i n g the a l k a l i f e l d s p a r s . L e t u s look at t h e t y p e s of r e a c t i o n s which m a y be i n v o l v e d in d i s t r i b u tion p r o c e s s e s . S o m e a r e : Xmelt + YZsolid ~ XZsolid + Ymelt (1) w h e r e two s p e c i e s , X and Y, in the m e l t c o m p e t e f o r a l a t t i c e s i t e in a s o l i d which c r y s t a l l i z e s f r o m the m e l t : (X, Y)Zsoli d ~ iX, Y)Z'soli d
(2)
w h e r e the two s p e c i e s a r e d i s t r i b u t e d b e t w e e n two s o l i d p h a s e s : Xsolution + YZsolid ~ XZsolid + Ysolution (3) w h e r e the two s p e c i e s a r e f r a c t i o n a t e d b e t w e e n an a q u e o u s p h a s e and a s o l i d : Xgas
+ YZsolid ~- YZsolid + ~ a s
Chem. Geol., 1 (1966) 49-56
(4) 51
where
the two species are present in a low-pressure, low-density gas phase. A survey of the geochemical literature reveals that in almost every discussion of element distribution and fractionation in geological processes, thermodynamic properties of only the solid phase containing the major or trace element are considered. One would infer from these discussions that distribution coefficients for reactions I-4 could be predicted if chemical bonding energies in the solid phases were known. This implies that binding forces of relevant species in melts, solutions, and gases are small. Such a conclusion is generally invalid for all processes of mineral formation, except that involving a gas phase separation (reaction 4), which is the least significant process. Furthermore, if it were possible to make valid predictions of distribution coefficients, then accurate calculations could be made of melting points and solubilities of phases in polycomponent systems, and of relative lattice energies. The fact is, however, that at the present time such calculations are beyond us. When processes involving element distribution are examined in detail, it soon becomes obvious that discussions in which thermodynamic properties of only the solid phase are considered, are unsatisfactory. It is not enough to know that an atom or ion is strongly bound in a crystal lattice; we must assess how strongly it is bound in the phase from which it is separating. Therefore, most of the "rules" used in discussing the substitution of ions in c r y s t a l l a t t i c e a r e r e l e v a n t only to f o r m a t i o n p r o c e s s e s of the type: X ,'"~'l' } YZ~olid *
XZsolid + Y gus
(5)
As soon as we i n t r o d u c e any m e d i u m m o r e c o m p l e x than the gas p h a s e , such as an aqueous fluid o r s i l i c a t e m e l t , any a n a l y s i s of v a l u e m o s t i n v o l v e an e s t i m a t e of t h e r m o d y n a m i c f u n c t i o n s for the s t e p s of a cycle such as: X ' l l u i d + YZsotid --> XZsolid + Y~lluhl
t
AHso]v:~tion
X i g:/~ + YXs(H i({ '\
A Hsoh, atio n
'
X~
(6)
X g:ls ~ Z gas+ Y'gas The extent of r e a c t i o n (6) m u s t depend on: (a) The r e l a t i v e l a t t i c e e n e r g i e s , U, of the s o l i d s (b) The r e l a t i v e s o l v a t i o n energies,zXHsolvation , of the ions. (c) The o v e r a l l change in f r e e e n e r g y , A(;. In fact, the r e a c t i o n is c o n t r o l l e d by the r e l a t i v e m a g n i t u d e s of the l a t i c e e n e r g i e s and h e a t s of s o l v a t i o n of the r e a c t a n t s which m u s t be f i n e l y b a l a n c e d as the o v e r a l l /XG and AH of such r e a c t i o n s a r e s m a l l by c o m p a r i s o n . The following r e a c t i o n s i l l u s t r a t e how p r e c a r i o u s l y b a l a n c e d t h e s e eff e c t s a r e (data f r o m L a t i m e r , 1952).
52
('hem, (;c~)l.. I (I966) 4(]-56
Reaction A +
Rb aqueous
+
AHhy drati°n
KClcr5 stal < sylvite
RbClcrystal +
AH = 0.13 k c a l .
-- AHhydration
= - - 177.20 k c a l . Rb~gas
+
= 183.11 k c a l .
KClerysta 1
~RbClcrystal +
b o = - 166.8 k c a l . ~ ' x / IF +\ + C1 + b ÷ K gas - g a s R gas
-
K aqueous
K*gas
Uo= 162.0 k c a l .
Reaction B Rb aqueous
+
KIcrystal
_AHhydratio n
> RbIcry stal A G = - 0.76 k c a l . A H = - 1.33 k c a l .
+
K+aq ue°us ,~
AHhydration
= 177.20 k c a l . R b~ gas
+
= - 183.11 k c a l .
KIcrystal
/
R b l c r y stal + K+g as - Uo = 146.6 k c a l . 1
Uo = 151 k c a l .
~K+~g as +I-gas
+ Rb+gas
Reaction C N i2~aqueous
+ M g F 2 crystal "~ A G = 4.54 k c a l . AHhydration sellaite AH = 8.89 k c a l . = - 672.2 k c a l .
Ni2-gas
+ MgF2 crystal
-- /7o = - 695 k c a l .
2 ~+ Mg
N i F 2 crystal + Mg 2+ aqueous - AHhydrati°n l = 715.8 k c a l . N i F 2 crystal + M g 2+aqueous /
U° = 728 kcal"
gas + 2 F - g a s + Ni2+gas
Reaction D Ni2~ aqueous .~
+ MgC12 crystal > NiC12crystal ~Mg2+aqueous AG = - 20.99 k c a l . c h l o r o m a g n e s i t e AH 17.21 k c a l . A Hhydratio n - A Hhydratio n | = 672.2 k c a l . = - 715.8 k c a l .
Ni2~ gas
+ MgC12 crystal
Uo = 632 k c a l .
NiC12 crystal+ Mg2+gas - 5 } ) = - 657.8 k c a l .
Mo-2+'x -.-~, gas + 2 Cl-gas + Ni2+gas R e a c t i o n A s h o w s t h a t Rb + (ionic r a d i u s 1.48 ~ ) d o e s not r e p l a c e K + (1.33 A) in s y l v i t e a s m i g h t h a v e b e e n p r e d i c t e d by the G o l d s c h m i d t r u l e s , w h e r e a s Rb + m a y r e p l a c e K + in KI ( r e a c t i o n B), c o n t r a r y to t h e s e r u l e s . R e a c t i o n s C and D, which i n v o l v e Mg 2+ (ionic r a d i u s 0.65 A; e l e c t r o n e g a t i v i t y Chem. Geol., 1 {1966) 49-56
53
] .2) and Ni 2~ (0.73 ~ ; 1.8), i n d i c a t e that the "Ringwood r u l e s " a r e obeyed by the t e n d e n c y for Mg 2~ to r e p l a c e Ni 2~ in NiF2, but not obeyed when Ni 2q r e p l a c e s Mg 2~ in c h l o r o m a g n e s i t e . The p r o b l e m is not s i m p l i f i e d by the knowledge that f a c t o r s that lead to l a r g e l a t t i c e e n e r g i e s also lead to l a r g e s o l v a t i o n e n e r g i e s , and the low h e a t s of fusion of m o s t s o l i d s , e s p e c i a l l y s i l i c a t e m i n e r a l s , i n d i c a t e that l i q u i d s t a t e b i n d i n g f o r c e s a r e v e r y s i m i l a r in m a g n i t u d e to b i n d i n g f o r c e s in s o l i d s . If we a r e to a c h i e v e a r e a l u n d e r s t a n d i n g of d i s t r i b u t i o n and f r a c t i o n a tion p r o c e s s e s , we m u s t obtain data for the c o m p l e t e p r o c e s s s i m i l a r to t h o s e c o m p i l e d for the r e a c t i o n s set out above. I m p r o v e m e n t of the e m p i r i c a l s h o r t cuts of the past cannot be s a t i s f y i n g u n t i l such data a r e c o m p i l e d for a l a r g e n u m b e r of p r o c e s s e s t a k i n g p l a c e in t y p i c a l geological r e a c t i o n m e d i a . T h e r e a r e s o m e c a s e s w h e r e , with l i m i t e d data, v a l i d c o m p a r i s o n s can be m a d e b e t w e e n t r a n s i t i o n - m e t a l ions and o t h e r i o n s of s i m i l a r size ano' c h a r g e (oxidation s t a t e s II and HI). It i s p o s s i b l e to a c c o u n t for the o r d e r of uptake of t r a n s i t i o n - m e t a l ions (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) in s i l i c a t e m i n e r a l s which c r y s t a l l i z e d f r o m a b a s i c m a g m a , by a p p l i c a t i o n of the c r y s t a l field t h e o r y ( W i l l i a m s , 1959: C u r t i s , 1964; B u r n s and Fyfe, 1964). The p r i n cipal f e a t u r e s of this t h e o r y have been d i s c u s s e d e a r l i e r ( B u r n s et al., 1964). In this p a p e r , we wish to i l l u s t r a t e how the c r y s t a l - f i e l d t h e o r y m a y be applied to solve a p r o b l e m i n v o l v i n g d i s t r i b u t i o n c o e f f i c i e n t s of t r a n s i t i o n m e t a l i o n s , p r e v i o u s l y m i s i n t e r p r e t e d (c.f. Ringwood, 1955, 1956; Mason, 1958; A h r e n s , 1964). In p a r t i c u l a r we wish to c o n s i d e r the m a g n e s i u m - n i c k e l association. The e n e r g y of t r a n s i t i o n - m e t a l ions i s a f u n c t i o n of the s y m m e t r y and p o s i t i o n of a n i o n s or l i g a n d s which s u r r o u H d an ion in l i q u i d s and s o l i d s . In such an e l e c t r o s t a t i c field the five "3d" o r b i t a l s a r e no l o n g e r d e g e n e r a t e and s o m e ions a r e s t a b i l i z e d r e l a t i v e to o t h e r s . The field i n d u c e d e n e r g y s e p a r a t i o n of ' 3 d " o r b i t a l s , A, is g r e a t e r for o c t a h e d r a l s y m m e t r y than t e t r a h e d r a l , and for a given value of 2x and s y m m e t r y it i s p o s s i b l e to e s t i m a t e c r y s t a l field s t a b i l i z a t i o n e n e r g i e s for each t r a n s i t i o n - m e t a l ion. It is now well e s t a b l i s h e d that the t h e r m o d y n a m i c p r o p e r t i e s of s e r i e s of c o m p o u n d s such as CaO .....-~ t r a n s i t i o n - m e t a l m o n o x i d e s . > ZnO a r e exp l i c a b l e in t e r m s of t r e n d s in r a d i i and s p e c i f i c c r y s t a l - f i e l d e n e r g y d i f f e r e n c e s (Orgel, 1960). T h e s e c r y s t a l - f i e l d e n e r g y t e r m s a r e l a r g e and d i f f e r e n c e s between m e m b e r s a r e l a r g e . It is also well e s t a b l i s h e d that when dif.f e r e n t c o o r d i n a t i o n s i t e s a r e a v a i l a b l e , s i t e p r e f e r e n c e is d o m i n a t e d by such c r y s t a l - f i e l d t e r m s , the s p i n e l s p r o v i d i n g an e l e g a n t e x a m p l e (Dunitz and O r g e l , 1957: M c C l u r e , 1957). In any d i s t r i b u t i o n p r o c e s s , if we can a n t i c i p a t e c h a n g e s in t h e s e s p e c i f i c i n t e r n a l e l e c t r o n i c effects, by c o n s i d e r a t i o n of c h a n g e s of bond l e n g t h s bond c h a r a c t e r (i.e., i o n i c - c o v a l e n t ) or site geom e t r y , then we may be able to p r e d i c t t r e n d s in d i s t r i b u t i o n c o e f f i c i e n t s . The s t r u c t u r e of a l i q u i d is q u a s i - c r y s t a l l i n e , and t h e r e a r e i n d i c a t i o n s that t r a n s i t i o n - m e t a l ions in s o l i d s and t h e i r d e r i v e d l i q u i d s o c c u r in s i m i l a r c o n f i g u r a t i o n s and c r y s t a l - f i e l d e n e r g i e s a r e s i m i l a r in the two e n v i r o n m e n t s . F o r e x a m p l e , the a b s o r p t i o n s p e c t r a of Ni(H20)62~ aq and c r y s t a l l i n e NiSO4" 6HL£) a r e a h n o s t i d e n t i c a l . In g e n e r a l , one a n t i c i p a t e s s l i g h t l y l a r g e r i n t e r a t o m i e d i s t a n c e s and l e s s r e g u l a r c o o r d i n a t i o n in l i q u i d s than in s o l i d s . H o w e v e r , t h e s e d i f f e r e n c e s a r e g e n e r a l l y s m a l l , as i n d i c a t e d by the c o m p a r a b l e h e a t s of f u s i o n of c o m p o u n d s of t r a n s i t i o n e l e m e n t s and o t h e r m e t a l s . But when ions a r e d i s s o I v e d in p o l y c o m p o n e n t I i q u i d s w h e r e eonfiguratiol~s 54
Chem. C;c(~I.. I (1!}66) 4 ~ 5 6
of s e v e r a l t y p e s a r e p r e s e n t , a d i s t r i b u t i o n of i o n s b e t w e e n the d i f f e r e n t s i t e s will o c c u r if t h e r m a l e n e r g i e s a r e c o m p a r a b l e with s i t e e n e r g y d i f f e r e n c e s . In s i l i c a t e m e l t s t e t r a h e d r a l and o c t a h e d r a l s i t e s p r e d o m i n a t e and t r a n s i t i o n m e t a l i o n s m a y e n t e r e i t h e r . H o w e v e r , t r a n s i t i o n - m e t a l i o n s a r e r a r e l y found in t e t r a h e d r a l s i t e s in c r y s t a l s which s e p a r a t e f r o m a s i l i c a t e liquid. The b e h a v i o u r of n i c k e l and m a g n e s i u m d u r i n g m a g m a t i c c r y s t a l l i z a t i o n cannot be e x p l a i n e d on the b a s i s of e x i s t i n g r u l e s . In the s i m p l e b i n a r y s o l i d s o l u t i o n s e r i e s , Mg2SiO4 - Ni2SiO4, on a c c o u n t of the h i g h e r m e l t i n g point of Mg2SiO4, c r y s t a l s a r e e n r i c h e d in the m a g n e s i u m c o m p o n e n t (Ringwood, 1956). But d u r i n g the c r y s t a l l i z a t i o n of b a s i c m a g m a s the s i t u a t i o n i s r e v e r s e d . O b v i o u s l y no s i m p l e r u l e will c o v e r both t h e s e c a s e s , p a r t i c u l a r l y a r u l e which t a k e s into a c c o u n t p r o p e r t i e s of the s o l i d p h a s e alone. To e x p l a i n such an i n v e r s i o n one m u s t c o n s i d e r c h a n g e s in the liquid state. In the s y s t e m Mg2SiO 4 - Ni2SiO4, both liquid and s o l i d s t a t e s a r e d o m i n a t e d by c a t i o n s in six c o o r d i n a t e s i t e s . S t u d i e s of the s p e c t r a of g l a s s e s ( B u r n s and Fyfe, 1964) s u p p o r t t h i s view. It is a l s o well known that a l k a l i s i l i c a t e l i q u i d s p r o d u c e a l a r g e a r r a y of 4 c o o r d i n a t e s i t e s and in t e r m s of c r y s t a l - f i e l d t h e o r y t r a n s i t i o n - m e t a l i o n s , except t h o s e with 5 and 10 "d" e l e c t r o n s , p r e f e r the o c t a h e d r a l s i t e s . If the c h a r a c t e r of s i t e d i s t r i b u t i o n in a l i q u i d c h a n g e s with the c o m p o s i t i o n of the liquid s p e c i f i c d i f f e r e n c e s b e t w e e n i o n s with and without u n f i l l e d "d" e l e c t r o n s h e l l s a r e a n t i c i p a t e d . C o n s i d e r the two r e a c t i o n s : Mg2SiO 4 + 6:4 liquid
~s o l u t i o n
and: Ni2SiO 4 + 6:4 liquid
>s o l u t i o n
w h e r e we u s e a s o l v e n t known to have s o m e d i s t r i b u t i o n of six and four coo r d i n a t e s i t e s fixed by i t s own b i n d i n g f o r c e s . The n i c k e l and m a g n e s i u m i o n s have i d e n t i c a l c h a r g e s and a l m o s t i d e n t i c a l r a d i i . If we c o m p a r e , and only c o m p a r e , the i n t e r a c t i o n s of Ni 2+ and Mg 2+ with the s o l v e n t two f e a t u r e s a r i s e . B e c a u s e of s t r o n g s i t e p r e f e r e n c e , n i c k e l will occupy o c t a h e d r a l s i t e s if a v a i l able and will e n t e r t e t r a h e d r a l s i t e s a p p r e c i a b l y only if the t h e r m a l e n e r g y i s c o m p a r a b l e with s i t e p r e f e r e n c e e n e r g y . M a g n e s i u m m a y a l s o f a v o r o c t a h e d r a l s i t e s t h r o u g h a c o m p a r a b l e " l a t t i c e e n e r g y " effect but an a d d i t i o n a l e l e c t r o n i c p r e f e r e n c e will not o p e r a t e . T h u s in the s o l v e n t , m a g n e s i u m ions will be s p r e a d m o r e e v e n l y o v e r a v a i l a b l e s i t e s . The s o l u b i l i t y of the two c o m p o u n d s (and h e n c e t h e i r m e l t i n g b e h a v i o u r ) will depend on the AH and AS of solution. O b v i o u s l y if m a g n e s i u m i s s p r e a d o v e r m o r e s i t e s then for the s o l u t i o n p r o c e s s ASMgZ+ ~ ASNiZ+ F u r t h e r , as s o m e n i c k e l i o n s will e n t e r or be f o r c e d to e n t e r t e t r a h e d r a l s i t e s : AHNi2+ ~ AHMg2~ Both t h e s e t e r m s will lead to: AGNi2+ ~ AGMg2+ an:l a l o w e r s o l u b i l i t y and h e n c e h i g h e r m e l t i n g t e m p e r a t u r e f o r the n i c k e l o l i v i n e . R e s e a r c h in p r o g r e s s i n d i c a t e s that the s y s t e m Mg2SiO4 - Ni2SiO4 Na2Si205 p r o b a b l y shows such an i n v e r s i o n of s o l i d - s o l u t i o n r e l a t i o n s . In o l i v i n e - r i c h t e r n a r y m i x t u r e s m a g n e s i u m e n r i c h m e n t o c c u r s while n e a r the a l k a l i s i l i c a t e c o r n e r o l i v i n e s a r e e n r i c h e d in n i c k e l . Chem. Geol.. 1 (1966) 49-56
55
Many other c a s e s could be c o n s i d e r e d where c r y s t a l - f i e l d t e r m s dominate in distribution phenomena. But the present case i s sufficient to indicate the importance of considering all parts of the distribution c y c l e including the liquid state and c l e a r l y shows why the existing e m p i r i c a l rules lack generality.
ACKNOW LEDGEM E NT
The w r i t e r s gratefully acknowledge support of this work by grants f r o m t h e R e s e a r c h Fund of t h e A m e r i c a n
National Science Foundation and the Petroleum Chemical Society.
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C h e m . Geol.. 1 (19661 49-56