Crystal Chemistry, Electronic Structures, and Spectra of Fe Sites in

Chapter 15 Crystal Chemistry, Electronic Structures, a n d Spectra o f F e Sites i n C l a y M i n e r a l s Applications to Photochemistry and Electron Transport

Downloaded by UNIV LAVAL on July 9, 2014 | http://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch015

David M. Sherman U.S. Geological Survey, Denver, CO 80225 3+

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The electronic structures of Fe and Fe -Fe bearing clays and oxides are described with reference to SCFΧα-SW molecular orbital calculations on FeO, FeO and Fe O clusters. The results are used to interpret the optical spectra of iron bearing clays and iron oxides and provide some insight onto the possible mechanisms of photochemical redox reactions associated with those minerals. 6

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Iron bearing clays and oxide minerals, because of their partially occupied Fe(3d) orbitals, may participate in a number of redox processes in nature. For example, photochemically induced electron transfer between organic molecules and colloidal iron oxides is a significant process in natural waters (1-2). Clays are thought by many to have played a central role in the chemistry which led to the origin of life (3). Because mixed-valent iron bearing clays can both donate and accept electrons, one is tempted to speculate that such clays may have had a role in the early prebiotic pathways for electron transfer and metabolism. A basic understanding of the electronic structures of iron bearing clays and oxides is needed before one can understand the mechanisms of electron transfer and photochemical reactions associated with these minerals. This chapter will discuss the electronic structures of iron bearing clays and oxides (primarily from cluster molecular orbital calculations) and compare theoretical results with experiment. The discussion will be confined to states associated with simple Fe* and Fe* coordination sites in minerals and will not go into electronic states associated with defects. 3

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Crystal Chemistry of Fe-Bearing Clays Before describing the electronic structures of Fe coordination sites, it is worthwhile to first outline the crystal chemistry of clay minerals and describe the kinds of coordination environments that Fe * and Fe* cations may enter. This chapter not subject to U.S. copyright Published 1990 American Chemical Society 3

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In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

15.

SHERMAN

285

Fe Sites in Clay Minerals

2:1 C l a y S t r u c t u r e s . The s t r u c t u r e o f a 2:1 c l a y such as nontronite, ( C a / 2 , N a ) F e ( S i 7 Α1 )θ2ο(ΟΗ) ηΗ2θ, i s shown i n F i g u r e 1. One s h e e t o f (Fe ,A1 ,Mg)0 (OH) o c t a h e d r a i s sandwiched between two s h e e t s o f ( S i , A l ) 0 t e t r a h e d r a . Between t h e s e 2:1 l a y e r s i s an expandable i n t e r l a y e r r e g i o n w h i c h can accomodate H 0 molecules, c a t i o n s such as Na*, K*, and C a , o r g a n i c m o l e c u l e s and even t r a n s i t i o n m e t a l aquo complexes. Most F e * and Fe * i n 2:1 c l a y s occupies the s i t e s i n the o c t a h e d r a l s h e e t . Clays i n w h i c h the o c t a h e d r a l c a t i o n has a charge o f +3 are d e s c r i b e d as being d i o c t a h e d r a l s i n c e o n l y 2/3 o f the o c t a h e d r a l s i t e s are occupied. As can be seen i n F i g u r e 1, t h e r e a r e two kinds of o c t a h e d r a l s i t e s : c i s - M 0 ( O H ) and t r a n s - M 0 ( O H ) . I n the case o f the d i o c t a h e d r a l s m e c t i t e clays such as nontronite and montmorillonite, there i s some debate as t o whether the c a t i o n s are ordered w i t h i n the o c t a h e d r a l sheet. I n the disordered structure, 2/3 o f b o t h the c i s and t r a n s s i t e s a r e o c c u p i e d . In the o r d e r e d s t r u c t u r e , shown i n F i g u r e 2, a l l o f the c i s s i t e s are o c c u p i e d and a l l o f the t r a n s s i t e s a r e v a c a n t . Mossbauer s p e c t r a o f n o n t r o n i t e s , when f i t t o two quadrupole d o u b l e t s , s u g g e s t that i r o n i s d i s o r d e r e d i n n o n t r o n i t e ( 4 ) . As shown i n F i g u r e 3, the a r e a r a t i o s o f the i n n e r and outer Fe * d o u b l e s a r e 1/3:2/3 c o n s i s t e n t w i t h t h a t e x p e c t e d f o r i r o n d i s o r d e r e d o v e r the c i s and t r a n s s i t e s . The d i s o r d e r e d arrangement a l s o a c c o u n t s f o r the low N e e l temperature (1.3K) o f n o n t r o n i t e (5). Electron diffraction measurements o f the p l a n e group o f the o c t a h e d r a l s h e e t , however, s u g g e s t an o r d e r e d s t r u c t u r e ( 6 , 7 ) . A l t e r n a t i v e i n t e r p r e t a t i o n s o f the magnetic b e h a v i o r o f n o n t r o n i t e (8) a l s o s u g g e s t an o r d e r e d s t r u c t u r e . S e v e r a l i n v e s t i g a t o r s have a t t e m p t e d t o e x p l a i n how an o r d e r e d arrangement o f Fe * c a t i o n s can y i e l d two quadrupole d o u b l e t s by i n v o k i n g n e x t - n e a r e s t n e i g h b o r e f f e c t s ( 9 - 1 2 ) . A s m a l l f r a c t i o n o f i r o n may e n t e r the t e t r a h e d r a l s i t e s in 2:1 c l a y s . I n n o n t r o n i t e , t h e r e i s o f t e n enough t e t r a h e d r a l i r o n ( c a . 5 %) t o be d e t e c t a b l e u s i n g Mossbauer s p e c t r a . The Mossbauer s p e c t r a i n F i g u r e 3, f o r example, shows a weak q u a d r u p o l e d o u b l e t due t o t e t r a h e d r a l Fe *. O p t i c a l s p e c t r a o f n o n t r o n i t e s i n the v i s i b l e r e g i o n a l s o show an a b s o r p t i o n band t h a t i s a s s i g n e d t o t e t r a h e d r a l l y c o o r d i n a t e d Fe *. T h i s i s d i s c u s s e d i n more d e t a i l below. I r o n may a l s o o c c u r as an i n t e r l a y e r s p e c i e s . Mossbauer s p e c t r a show the p r e s e n c e o f an F e * aquo complex i n the i n t e r l a y e r o f m o n t m o r i l l o n i t e ( 1 3 ) . The l a b i l e nature of this Fe * i s s u g g e s t e d by the l a r g e temperature dependence o f i t s recoil-free-fraction. I n c o n t r a s t , Fe * aquo complexes are u n l i k e l y t o o c c u r as d i s c r e t e s p e c i e s i n a clay interlayer. I n s t e a d , F e ( O H ) ( H 0 ) complexes w i l l condense t o form ferric h y d r o x y polymers w h i c h , i n a c l a y i n t e r l a y e r , might form twodimensional sheets or three-dimensional p i l l a r s . Such p i l l a r s in n o n t r o n i t e have been c h a r a c t e r i z e d by Gangas e t a l . ( 1 4 ) . i66

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1:1 C l a y S t r u c t u r e s . The 1:1 s t r u c t u r e s a r e the s i m p l e s t since they contain only a single octahedral s h e e t and a single tetrahedral layer. The double l a y e r s a r e h e l d t o g e t h e r by h y d r o g e n b o n d i n g . The most n o t a b l e i r o n p h y l l o s i l i c a t e s w i t h the 1:1 s t r u c t u r e a r e b e r t h i e r i n e and c r o n s t e d i t e . V e r y l i t t l e iron



286

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES

F i g u r e 1. G e n e r a l i z e d s t r u c t u r e o f a 2:1 p h y l l o s i l i c a t e smectite clay). There a r e two d i f f e r e n t octahedral c o r r e s p o n d i n g t o c i s - M 0 ( O H ) and t r a n s - M 0 ( O H ) . ( M o d i f i e d Grimm, R.E. " C l a y M i n e r a l o g y " M c G r a w - H i l l , 1968). 4

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F i g u r e 2. S t r u c t u r e o f t h e o c t a h e d r a l s h e e t o f a d i o c t a h e d r a l c l a y showing the c o m p l e t e l y o r d e r e d ( c i s o n l y ) c o n f i g u r a t i o n . The d i s o r d e r e d s t r u c t u r e would have 2/3 o f the c i s - s i t e s a n d 2/3 o f the t r a n s s i t e s o c c u p i e d .


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o c c u r s i n t h e more common p h y l l o s i l i c a t e s w i t h t h e 1:1 s t r u c t u r e (e.g., k a o l i n i t e ) . E l e c t r o n i c S t r u c t u r e s o f Fe S i t e s i n C l a y s and Oxides


Theoretical Preliminaries. I t i s worthwile to f i r s t o u t l i n e some e s s e n t i a l concepts before d i s c u s s i n g the e l e c t r o n i c s t r u c t u r e s o f Fe s i t e s i n c l a y s and o x i d e s . The e l e c t r o n i c s t r u c t u r e o f an atom, m o l e c u l e o r s o l i d i s obtained by s o l v i n g the Schrodinger equation

where Ê i s t h e a p p r o p r i a t e H a m i l t o n i a n o p e r a t o r , Φ i s the w a v e f u n c t i o n w h i c h d e s c r i b e s t h e system, and Ε i s t h e t o t a l energy o f t h e system when i t i s i n t h e s t a t e Φ . F o r a n y t h i n g more com p l e x t h a n a hydrogen atom, o f c o u r s e , t h e S c h r o d i n g e r e q u a t i o n i s a many-body problem and cannot be s o l v e d e x a c t l y . We c a n , how e v e r , i n t r o d u c e a n i m p o r t a n t f o r m a l i s m known a s t h e independent e l e c t r o n a p p r o x i m a t i o n and e x p r e s s o u r many-body w a v e f u n c t i o n Φ (rj , . . . ,r ) i n terms of one-electron o r b i t a l s i>1 ( r ^ ) , φ ( 2 ) » · · · » Ψη ( n ) p r o v i d e d t h a t we obey t h e P a u l i e x c l u s i o n p r i n c i p l e b y h a v i n g Φ ( r - r« . .. , r ) t a k e the form o f a S l a t e r d e t e r m i n a n t : ' ' n

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(describing the electronHere, V , i s t h e coulomb p o t e n t i a l e l e c t r o n r e p u l s i o n and e l e c t r o n - n u c l e a r a t t r a c t i o n ) w h i l e V i s the exchange p o t e n t i a l w h i c h i s p u r e l y quantum m e c h a n i c a l i n o r i gin. F o r t h e case o f an atom, o u r o n e - e l e c t r o n o r b i t a l s a r e t h e I s , 2 s , 2p, 3 s , 3p, 3d e t c . atomic o r b i t a l s . F o r a m o l e c u l e , o u r o n e - e l e c t r o n φ 's a r e m o l e c u l a r o r b i t a l s w h i c h a r e l i n e a r combi n a t i o n s o f atomic o n e - e l e c t r o n o r b i t a l s , : cc

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15. SHERMAN

Fe Sites in Clay Minerals i>

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where u ( r ) has the t r a n s l a t i o n a l p e r i o d i c i t y Τ o f the crystal l a t t i c e u ( r ) - u ( r + Τ ) . The o n e - e l e c t r o n o r b i t a l f o r m a l ism i s not o n l y the b a s i s f o r c o m p u t a t i o n a l quantum c h e m i s t r y but a l s o u n d e r l i e s our t h i n k i n g when a t t e m p t i n g t o u n d e r s t a n d c h e m i c a l phenomena i n terms o f quantum mechanics. For example, i n our o n e - e l e c t r o n o r b i t a l f o r m a l i s m we can t h i n k o f c h e m i c a l bonds as r e s u l t i n g from the o v e r l a p o f o n e - e l e c t r o n a t o m i c o r b i t a l s . How e v e r , we must u l t i m a t e l y r e t u r n t o the c o m p l i c a t e d n o t i o n o f mult i e l e c t r o n i c w a v e f u n c t i o n s i f we a r e t o c o r r e c t l y u n d e r s t a n d the e l e c t r o n i c s t a t e s o f an atom, m o l e c u l e o r s o l i d . Because c l a y m i n e r a l s and i r o n o x i d e s are s o l i d s , i t seems t h a t t h e i r e l e c t r o n i c s t r u c t u r e s s h o u l d be g i v e n i n terms o f B l o c h w a v e f u n c t i o n s . However, band s t r u c t u r e c a l c u l a t i o n s on something as complex as a t y p i c a l c l a y would be i m p r a c t i c a l . More fundamen t a l l y , however, i s t h a t the e l e c t r o n s o f i n t e r e s t i n s i l i c a t e s and o x i d e s , namely those i n the Fe(3d) o r b i t a l s , are q u i t e l o c a l i z e d . ( T h i s r e f l e c t s the p a r t i a l i o n i c c h a r a c t e r o f the b o n d i n g i n these m i n e r a l s . ) B l o c h w a v e f u n c t i o n s are awkward t o use when a t t e m p t i n g t o d e s c r i b e e l e c t r o n i c t r a n s i t i o n s between l o c a l i z e d s t a t e s . A d i f f e r e n t approach would be t o l o o k a t the e l e c t r o n i c s t r u c t u r e o f a s m a l l c l u s t e r o f atoms r e p r e s e n t i n g some s t r u c t u r a l u n i t i n the c r y s t a l by u s i n g m o l e c u l a r o r b i t a l t h e o r y . T h i s approach has i t s p r e c e d e n t i n l i g a n d f i e l d t h e o r y and s h o u l d be useful to the extent that the Fe(3d) e l e c t r o n s are l o c a l i z e d . W i t h i n c r e a s i n g c l u s t e r s i z e , the m o l e c u l a r o r b i t a l d e s c r i p t i o n w i l l converge to the band s t r u c t u r e o f the c r y s t a l . The m o l e c u l a r o r b i t a l v i e w can a c c o u n t f o r b o t h l o c a l i z e d ( i o n i c ) e l e c t r o n s and ( i f the cluster is s u f f i c i e n t l y large) delocalized (metallic) electrons. Some a p p l i c a t i o n s o f m o l e c u l a r o r b i t a l t h e o r y t o the c h e m i s t r y o f clay m i n e r a l s have been done. For example, A r o n o w i t z e t a l . (15) modeled the e l e c t r o n i c s t r u c t u r e s o f c l a y s u s i n g e x t e n d e d H u c k e l m o l e c u l a r o r b i t a l c a l c u l a t i o n s on a c l u s t e r a p p r o a c h i n g the f u l l u n i t c e l l . Those c a l c u l a t i o n s were a b l e t o p r e d i c t t r e n d s i n i s o morphic s u b s t i t u t i o n o f o c t a h e d r a l A l by F e , Mg , F e , Na , C a and K . We are p r i m a r i l y i n t e r e s t e d i n the Fe(3d) electrons. The u n p a i r e d e l e c t r o n s o f i r o n make the e l e c t r o n i c s t r u c t u r e s o f i r o n b e a r i n g c l a y s too complex t o t r e a t w i t h s i m p l e extended H u c k e l c a l c u l a t i o n s on l a r g e c l u s t e r s . More s o p h i s t i c a t e d approx i m a t i o n s are p o s s i b l e ( i n p a r t i c u l a r the Self-consistent field Χα-Scattered Wave method) but t h e s e w i l l r e q u i r e us t o l i m i t the c l u s t e r s i z e . However, the Fe(3d) e l e c t r o n s t e n d t o be almost e n t i r e l y l o c a l i z e d t o the Fe atom and i t s immediate c o o r d i n a t i o n s i t e . Hence we s h o u l d be a b l e t o d e s c r i b e the Fe(3d) e l e c t r o n i c states i n terms o f the m o l e c u l a r o r b i t a l s o f s i m p l e F e 0 , FeO (OH) , and F e 0 c l u s t e r s . There are situations (Fe -*Fe charge t r a n s f e r , i n p a r t i c u l a r ) i n w h i c h the Fe(3d) e l e c t r o n s are d e l o c a l i z e d o v e r more t h a n one F e 0 c o o r d i n a t i o n polyhedron. To u n d e r s t a n d s u c h d e l o c a l i z e d s t a t e s , we w i l l have t o use l a r g e r c l u s t e r s . C a l c u l a t i o n s on ( F e O ) " dimers have been done and w i l l be d e s c r i b e d h e r e . As computers become f a s t e r , s o p h i s t i c a t e d c a l c u l a t i o n s on even l a r g e r c l u s t e r s w i l l become p r a c t i c a b l e . A c c u r a t e m o l e c u l a r o r b i t a l c a l c u l a t i o n s on t r a n s i t i o n metal k


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290


o x i d e c l u s t e r s cannot be e a s i l y done u s i n g c u r r e n t H a r t r e e - F o c k type ab i n i t i o methods b a s e d on the l i n e a r c o m b i n a t i o n o f atomic orbital (LCAO) f o r m a l i s m . A v e r y s u c c e s f u l approach, however, i s the Χα-scattered wave method. Here, t h e c o m p l i c a t e d exchange p o t e n t i a l i s s i m p l i f i e d b y u s i n g the Χα a p p r o x i m a t i o n (16) w h i l e the m o l e c u l a r o r b i t a l s a r e o b t a i n e d u s i n g the s c a t t e r e d wave f o r malism ( 1 7 ) . 3

E l e c t r o n i c S t r u c t u r e o f Fe * i n O c t a h e d r a l C o o r d i n a t i o n . F i g u r e 4 shows t h e m o l e c u l a r o r b i t a l diagram f o r a ( F e 0 ) " c l u s t e r w i t h and Fe-0 bond l e n g t h o f 200 pm. The e l e c t r o n i c s t r u c t u r e o f t h e same c l u s t e r w i t h a somewhat l o n g e r Fe-0 bond l e n g t h i s d i s c u s s e d i n d e t a i l elswhere ( 1 8 ) . The 0(2p) l i k e m o l e c u l a r orbitals are Fe-0 b o n d i n g w h i l e t h e Fe(3d) l i k e m o l e c u l a r o r b i t a l s a r e Fe-0 a n t i b o n d i n g . Some o f the o r b i t a l s a r e l a b e l e d a c c o r d i n g t o t h e i r a s s o c i a t e d i r r e d u c i b l e r e p r e s e n t a t i o n o f the 0 p o i n t group. The orbitals ofe symmetry a r e σ-bonding ( 3 e ) o r σ-antibonding (4e.). The o r b i t a l s w i t h t symmetry a r e π-bonding ( l t ) o r πantibonding ( 2 t ) . The c a l c u l a t i o n was done u s i n g a s p i n u n r e s t r i c t e d exchange p o t e n t i a l . T h i s g i v e s s e p a r a t e exchange p o t e n t i a l f o r spin-up (α-spin) and spin-down (0-spin) e l e c t r o n s and i s essential f o r describing the electronic structures o f o p e n - s h e l l c o n f i g u r a t i o n s . Note t h a t the s p i n - u n r e s t r i c t e d molec u l a r o r b i t a l r e s u l t s c o r r e c t l y show t h a t Fe * w i l l be i n the h i g h s p i n c o n f i g u r a t i o n i n i t s ground s t a t e . Low s p i n Fe * o r Fe** has n o t been observed i n any s i l i c a t e o r o x i d e m i n e r a l . Presumea b l y , one must a t t a i n p r e s s u r e s comparable t o t o those i n the lower mantle f o r the s p i n p a i r i n g t r a n s i t i o n t o o c c u r . Two k i n d s o f e l e c t r o n i c t r a n s i t i o n s can be d e s c r i b e d u s i n g t h i s s i m p l e c l u s t e r : d-d o r " l i g a n d f i e l d " t r a n s i t i o n s and l i g a n d - t o - m e t a l charge t r a n s f e r (LMCT) t r a n s i t i o n s . These a r e also the only e l e c t r o n i c t r a n s i t i o n s o f i r o n ( I I I ) clays that can be i n d u c e d by s u n l i g h t on the E a r t h ' s s u r f a c e . As d i s c u s s e d i n (18), the o n e - e l e c t r o n o r b i t a l s g i v e o n l y a p a r t i a l d e s c r i p t i o n o f the d i f f e r e n t l i g a n d f i e l d s t a t e s o f the Fe * c a t i o n . The l i g a n d to m e t a l charge t r a n s f e r t r a n s i t i o n s , however, seem t o be w e l l d e s c r i b e d i n terms o f the o n e - e l e c t r o n o r b i t a l s o f t h e ( F e 0 ) ' cluster. The l o w e s t energy LMCT t r a n s i t i o n , from the 0(2p) band to the 2 t ( 0 ) o r b i t a l , i s c a l c u l a t e d t o l i e n e a r 38,000 cm" . T h i s energy agrees w r i l w i t h t h a t found i n t h e s p e c t r a o f i r o n ( I I I ) - b e a r i n g c l a y s (4,19,20). Spectra o f several clay m i n e r a l s w i t h d i l u t e c o n c e n t r a t i o n s o f l a t t i c e Fe * show an 0 " -•Fe * band and a r e g i v e n i n F i g u r e 5. The e n e r g i e s o f the s t a t e s a r i s i n g from the d i f f e r e n t confi g u r a t i o n s over the Fe(3d) o n e - e l e c t r o n o r b i t a l s ( 4 e and 2 t ^ ) c a n n o t be d i r e c t l y c a l c u l a t e d from t h e m o l e c u l a r o r b i t a l diagram. This i s because a g i v e n o n e - e l e c t r o n o r b i t a l c o n f i g u r a t i o n corresponds t o s e v e r a l m u l t i e l e c t r o n i c s t a t e s . F o r example, t h e o n e - e l e c t r o n o r b i t a l t r a n s i t i o n 4 e ( a ) -» 2 t ( a ) corresponds t o b o t h the A -* T and A -» T spectroscopic transitions that a r e observed i n the v i s i b l e and near i n f r a r e d s p e c t r a o f i r o n ( I I I ) c l a y s . We can, however, u n d e r s t a n d the s t a t e s a r i s i n g from t h e d - o r b i t a l e l e c t r o n i c c o n f i g u r a t i o n s using l i g a n d f i e l d theory. L i g a n d f i e l d t h e o r y e x p r e s s e s the s t a t e e n e r g i e s i n terms o f t h e 9


6

h

g

g

2 g

2 g

2 g

3

3

3

9

6

1

2g

3

2

3

g

g

6

4

1g

6

1g

g

2 g

4

1g

2g


15.

Fe Sties in day Minerah

SHERMAN

(Fe0 ) ~ 6

9

R(Fe-O)»200


Spin Up

-25

291

pm

^

L

F i g u r e 4. Spin-unrestricted m o l e c u l a r o r b i t a l diagram f o r an ( F e 0 ) " c l u s t e r w i t h R(Fe-0)-200 pm. The o r b i t a l s i n d i c a t e d by dashed l i n e s are u n o c c u p i e d . Note t h a t o r b i t a l energy d i f f e r e n c e s do not c o r r e s p o n d e x a c t l y t o the e n e r g i e s r e q u i r e d f o r e l e c t r o n i c t r a n s i t i o n s between t h e s e o r b i t a l s . S i n c e an electronic transi t i o n w i l l change the e l e c t r o n i c c o n f i g u r a t i o n , i t w i l l a l s o change the i n t e r e l e c t r o n i c p o t e n t i a l . To d e t e r m i n e e l e c t r o n i c t r a n s i t i o n e n e r g i e s , one must p e r f o r m a " t r a n s i t i o n s t a t e " c a l c u l a t i o n ( 1 6 ) . The " t r a n s i t i o n s t a t e " approach w i l l a c c o u n t f o r the orbital energy r e l a x a t i o n about the new e l e c t r o n i c c o n f i g u r a t i o n . For sim p l i c i t y , o n l y the most i m p o r t a n t Fe-0 b o n d i n g and antibonding o r b i t a l s are l a b e l l e d . The r e m a i n i n g o r b i t a l s ( 6 a , 5 t , 6 t , l t , and l i ) are m o s t l y 0(2p) non-bonding. 9

6

1 g

2 u

t

1 u

g


1 u

292



J τ

I

I

7

•

1

,

F i g u r e 5. Near u l t r a v i o l e t t o v i s i b l e r e g i o n s p e c t r a o f k a o l i n i t e , p y r o p h y l l i t e , and s a p o n i t e showing a n a b s o r p t i o n band n e a r 40,000 cm" due t o 0 "-»Fe charge t r a n s f e r . (Data from r e f e r e n c e 2 5 ) . 1

2

3+


293

Fe Sites in day Minerals

15. SHERMAN

t h r e e p a r a m e t e r s lODq, Β and C. lODq i s a measure o f the s p l i t t i n g between t h e 4 e and 2 t o r b i t a l s ; Β and C a r e r e l a t e d t o the coulomb and exchange i n t e g r a l s . I n p r i n c i p l e , we c a n c a l c u l a t e t h e s e parameters from the o n e - e l e c t r o n o r b i t a l s . However t h e y a r e u s u a l l y d e t e r m i n e d from o p t i c a l s p e c t r a . The e n e r g i e s o f the Fe(3d) s t a t e s i n terms o f the l i g a n d f i e l d p a r a m e t e r s a r e g i v e n i n T a b l e 1. g

2 g

3

T a b l e 1.

E n e r g i e s f o r the Fe * L i g a n d F i e l d S t a t e s (21,22)


State

Energy

2

Fe t r a n s i t i o n , i t f o l l o w s t h a t i r o n o x i d e s and s i l i c a t e s s h o u l d n o t be h i g h l y p h o t o r e a c t i v e under s u n l i g h t a t the E a r t h ' s s u r f a c e . As d i s c u s s e d below, some p h o t o c h e m i c a l r e a c t i v i t y i s o b s e r v e d w i t h h e m a t i t e (a-Fe 03) a t e n e r g i e s b e g i n i n g n e a r 20,000 cm" ( e . g . , 34-35). P h o t o e l e c t r o c h e m i c a l r e a c t i o n s i n d u c e d by v i s i b l e l i g h t a r e m o s t - l i k e l y a s s o c i a t e d w i t h the tail o f the 0 "-*Fe charge t r a n s f e r band t h a t extends i n t o the v i s i b l e r e g i o n o f the spectrum. The quantum e f f i c i e n c y o f these r e a c t i o n s 3

1

3


3 +

+

4 +

3 +

3 +

3

3

3 +

2

2

3

3+

2

1

2

3+


296


increase r a p i d l y ultraviolet.

as the p h o t o n energy i s moved towards t h e near-

3

E l e c t r o n i c S t r u c t u r e o f Fe * i n T e t r a h e d r a l C o o r d i n a t i o n . As men tionedearlier, some Fe * i n c l a y s may s u b s t i t u t e f o r S i i n t h e t e t r a h e d r a l sheet. I n t h e case o f n o n t r o n i t e , t h e f r a c t i o n o f t e t r a h e d r a l l y c o o r d i n a t e d i r o n i s o n l y a few p e r c e n t . Other p h y l l o s i l i c a t e s (such as c r o n s t e d i t e ) c a n have a c o n s i d e r a b l e f r a c t i o n of i r o n i n t e t r a h e d r a l coordination. The e l e c t r o n i c s t r u c t u r e o f Fe * i n t e t r a h e d r a l c o o r d i n a t i o n (18) i s g i v e n b y t h e m o l e c u l a r o r b i t a l diagram f o r an ( F e 0 ) c l u s t e r i n F i g u r e 7. As b e f o r e , t h e Fe(3d) o r b i t a l a r e Fe-0 a n t i b o n d i n g w h i l e t h e 0(2p) o r b i t a l s a r e Fe-0 b o n d i n g and 0 nonbonding. The l o w e s t energy LMCT t r a n s i t i o n i s c a l c u l a t e d t o o c c u r a t 40,400 cm" when t h e Fe-0 bond l e n g t h i s 186.5 pm. The l i g a n d f i e l d t r a n s i t i o n s o f t e t r a h e d r a l l y coordinated Fe * are Laporte-allowed. C o n s e q u e n t l y , s m a l l amounts o f t e t r a h e d r a l l y c o o r d i n a t e d Fe * may have a l a r g e e f f e c t on t h e s p e c t r a o f i r o n - b e a r i n g c l a y s . I n t h e o p t i c a l spectrum o f non t r o n i t e ( 4 ) , t h e s m a l l amount o f t e t r a h e d r a l Fe * g i v e s an i n t e n s e a b s o r p t i o n band n e a r 23,000 cm' t h a t i s a s s i g n e d t o the A ^ A^E transition. 3

3

4


1

3

3

3

1

6

2

E l e c t r o n i c S t r u c t u r e o f Fe * i n O c t a h e d r a l C o o r d i n a t i o n . F i g u r e 8 gives a molecular o r b i t a l diagram f o r an ( F e 0 ) " c l u s t e r w i t h R(Fe-O) - 216 pm. The o v e r a l l t o p o l o g y o f t h e o r b i t a l s i s the same as i n t h e analogous Fe * c l u s t e r . The o n e - e l e c t r o n o r b i t a l t r a n s i t i o n 2t (0)-»4e (0) c o r r e s p o n d s t o t h e T -* E absorption band o b s e r v e d i n t h e o p t i c a l s p e c t r a o f Fe * s i l i c a t e s n e a r 10,00 " dimer shown i n F i g u r e 11. T h i s dimer c o n s i s t s o f Fe ana F e c a t i o n s occupying octahedral coordination s i t e s s h a r i n g a common edge. The dimer h a s C symmetry (Fe s i t e s i n e q u i v a l e n t ) . I f we c o n s i d e r t h e two s i t e s A and B, we c a n c o n s i d e r two z e r o t h - o r d e r s t a t e s c o r r e s p o n d i n g t o t h e two p o s s i b l e charge configurations : 3

2

2 +

3 +

2 v

W

W

What we a r e i n t e r e s t e d i n i s t h e e n e r g i e s o f these s t a t e s a l o n g a c o n f i g u r a t i o n a l c o o r d i n a t e q. Here, q i s a normal mode o f A " c l u s t e r , the Fe *(2t (0)) o r b i t a l s are Fe *-Fe bonding w h i l e the Fe *(2t (0)) o r b i t a l s are Fe -Fe * antibonding ( a l l o f the F e ( 3 d ) - l i k e molecular orbitals a r e Fe-0 a n t i b o n d i n g , o f c o u r s e ) . To d e t e r m i n e t h e k i n d o f a c t i v a t i o n energies a s s o c i a t e d w i t h thermally induced IVCT wcjuld r e q u i r e t h a t we c a l c u l a t e t h e t o t a l energy o f t h e (Fe 0- > 1 5

2

2

3+

2

2g

1 0

3

2g

2+

3

2


0

304


Φ >%

O) Φ

c

-20

~2 Έ k_ Ο

(-

-25

F i g u r e 12. M o l e c u l a r o r b i t a l diagram f o r an Fe^O^ c l u s t e r used t o u n d e r s t a n d t h e o r b i t a l s i n v o l v e d i n Fe *-*Fe^ charge t r a n s f e r . The a b s o r p t i o n band o b s e r v e d n e a r 13,000 cm" i n t h e s p e c t r a o f mixed-valence silicates i s due t o t h e t r a n s i t i o n from t h e Fe (t g)->Fe (t g) o r b i t a l s . A t r a n s i t i o n state c a l c u l a t i o n f o r t h a t energy i n t h e c l u s t e r p r e s e n t e d h e r e g i v e s 10,570 cm" i n f a i r agreement w i t h e x p e r i m e n t . 2

1

2+

3+

2

2

1


15. SHERMAN

305


c l u s t e r a l o n g t o q c o o r d i n a t e . Such c a l c u l a t i o n s cannot be accu r a t e l y done u s i n g t h e SCF-Xa-SW method b u t might be p o s s i b l e u s i n g the D i s c r e t e V a r i a t i o n a l Χα approach. I t appears, however, t h a t most i r o n p h y l l o s i l i c a t e s a r e b e s t d e s c r i b e d by t h e s i t u a t i o n i n F i g u r e l i b where t h e two Fe s i t e s a r e e n e r g e t i c a l l y d i f f e r e n t . I n a d d i t i o n t o t h e a c t i v a t i o n b a r r i e r , e l e c t r o n s must a l s o overcome the s i t e p o t e n t i a l energy d i f f e r e n c e (Ε ) f o r e l e c t r o n t r a n s f e r t o o c c u r . F o r t h i s r e a s o n , t h e r m a l l y i n d u c e d IVCT i s seldom observed i n these m i n e r a l s . We c a n c o n s i d e r t h e Fe(3d) e l e c t r o n s t o be t r a p p e d a t t h e i r p a r e n t Fe s i t e . λ


A p p l i c a t i o n s t o Photochemistry,

E l e c t r o n t r a n s p o r t and R e a c t i v i t y

The t r a p p e d and l o c a l i z e d n a t u r e o f t h e Fe(3d) e l e c t r o n s i n c l a y s and i r o n o x i d e s would suggest t h a t t h e Fe(3d) o r b i t a l s i n l a t t i c e i r o n p l a y no d i r e c t r o l e i n heterogeneous ( i . e . , m i n e r a l - s o l u t i o n ) e l e c t r o n t r a n s f e r processes. The o n l y i r o n t h a t may a c t as an e l e c t r o n a c c e p t o r o r donor i s t h a t i r o n on t h e m i n e r a l s u r f a c e . On t h e o t h e r hand, e l e c t r o n s e x c i t e d from t h e 0(2p) v a l e n c e band i n t o t h e Fe(3d) o r b i t a l s by u l t r a v i o l e t r a d i a t i o n w o u l d r e s u l t i n (presumably m o b i l e ) h o l e s i n t h e 0(2p) band. To u n d e r s t a n d t h e o r b i t a l pathways b y w h i c h such h o l e s may be t r a n s p o r t e d would require molecular o r b i t a l c a l c u l a t i o n s on much l a r g e r c l u s t e r s . I n p a s s i n g i t s h o u l d be mentioned t h a t Marusak e t a l . (34) o b t a i n e d an i n t e r e s t i n g r e s u l t when measuring t h e photoconduc t i v i t y o f h e m a t i t e ( a - F e 0 ) . A peak i n t h e p h o t o c o n d u c t i v i t y spectrum was found a t an energy c o r r e s p o n d i n g t o the A -» T2 t r a n s i t i o n n e a r 15,000 cm" . T h i s t r a n s i t i o n r e s u l t s i n a onêe l e c t r o n o r b i t a l c o n f i g u r a t i o n ( t ~ > ( e ) w h i c h means t h a t b o t h the t ( 0 ) and e ( a ) Fe(3d) bands are p a r t i a l l y occupied. The c o n d u c t i v i t y i m p l i e s t h a t e l e c t r o n s i n one o f these bands must be m o b i l e ( p r o b a b l y t h e one i n t h e t ( 0 ) band) and t h a t e l e c t r o n t r a n s p o r t can occur v i a the charge-transfer t r a n s i t i o n 2

3

4

A

1g

1

1

g

2 g

g

2 g

3

4

2+

+

Fe3+(6 ) + F e * ( T ) - Fe (5T ) + F e * ( E ) Alg

2g

2g

g

R e g a r d l e s s o f t h e e l e c t r o n t r a n s f e r mechanism, l i g h t - i n d u c e d e l e c t r o n t r a n s f e r processes on i r o n ( I I I ) c l a y s must i n v o l v e t h e 0 "->Fe charge t r a n s f e r t r a n s i t i o n i n t h e u l t r a v i o l e t . On t h e E a r t h ' s s u r f a c e , the s o l a r s p e c t r a l i r r a d i a n c e c u r v e ( F i g u r e 13) c u t s o f f a t a maximum energy o f -33,000 cm' (0.3 m i c r o n s ) . This l i m i t s t h e k i n d s o f p h o t o c h e m i c a l p r o c e s s e s we may e x p e c t i n t h e n a t u r a l environment. The energy o f t h e 0 '->Fe charge t r a n s f e r t r a n s i t i o n (38,000 cm' ) i s o u t s i d e t h e p r e s e n t spectrum o f s o l a r r a d i a t i o n i n c i d e n t on t h e E a r t h ' s s u r f a c e . Nevertheless, the w i d t h o f t h e charge t r a n s f e r a b s o r p t i o n band i s s u f f i c i e n t l y b r o a d (-10,000 cm" h a l f - w i d t h ) t o a l l o w some p h o t o c h e m i c a l a c t i v i t y due t o s u n l i g h t on t h e E a r t h ' s s u r f a c e (1,2,35,36). For F e i n o c t a h e d r a l c o o r d i n a t i o n , we do n o t e x p e c t photoo x i d a t i o n o f o r g a n i c m o l e c u l e s v i a 0 "-»Fe * charge t r a n s f e r . How ever, the p o s s i b i l i t y o f photoreduction o f organics v i a t r a n s i t i o n s from t h e Fe(3d) o r b i t a l s t o t h e F e ( 4 s ) o r b i t a l s o r F e ( 4 s ) c o n d u c t i o n band s h o u l d be c o n s i d e r e d . The lowest energy Fe(3d)-»Fe(4s) t r a n s i t i o n i s c a l c u l a t e d t o be 58600 cm" . Although 2

3+

1

2

3+

1

1

2 +

2

2

1



p2 S

WAVELENGTH (μ)

F i g u r e 13. S o l a r s p e c t r a l i r r a d i a n c e c u r v e (From r e f e r e n c e 33).



15. SHERMAN

307

t h i s i s o u t s i d e t h e range o f p r e s e n t l y a v a i l a b l e s o l a r energy a t the E a r t h ' s s u r f a c e , t h e t a i l o f t h e a b s o r p t i o n band may a l l o w t r a n s i t i o n s i n d u c e d by n e a r - u l t r a v i o l e t r a d i a t i o n ( i . e . , below 50000 cm" . Indeed, as shown b y G e t o f f ( 3 7 ) , Fe * c a t i o n s c a n reduce w a t e r m o l e c u l e s when i r r a d i a t e d b y l i g h t a t 254 nm (40,000 cm" ) 1

2

1

2

3

Fe * + H 0 + hv - Fe *0H' + 1/2H 2

2

I n t h e p r e c a m b r i a n ( o r on p r e s e n t day M a r s ) , t h e absence, o f an ozone l a y e r a l l o w e d s o l a r UV r a d i a t i o n w i t h e n e r g i e s as h i g h as -40,000 cm' (0.25 m i c r o n s ) . P h o t o r e d u c t i o n t r a n s i t i o n s v i a t h e Fe(3d) t o F e ( 4 s ) t r a n s i t i o n may have been v e r y s i g n i f i c a n t . The p h o t o c h e m i c a l o x i d a t i o n o f Fe *, and t h e p r e c i p i t a t i o n o f FeOOH, may be t h e o r i g i n o f t h e e x t e n s i v e p r e c a m b r i a n banded i r o n forma t i o n s (38-40). Moreover, t h e p h o t o o x i d a t i o n o f Fe * may have reduced C0 t o o r g a n i c molecules (37.41): 1


2

2

2

2

3

2Fe * + H 0 + C 0 - 2Fe * + HCOOH + 20H2

2

I t would be a w o r t h w h i l e experiment t o i n v e s t i g a t e t h e photoredox p r o p e r t i e s o f those Fe * b e a r i n g c l a y s ( e . g . , g l a u c o n i t e and f e r r o s a p o n i t e ) t h a t were abundant i n t h e p r e c a m b r i a n . 2

Areas f o r Future I n v e s t i g a t i o n The t h e o r e t i c a l r e s u l t s d e s c r i b e d h e r e g i v e o n l y a z e r o t h - o r d e r d e s c r i p t i o n o f the e l e c t r o n i c s t r u c t u r e s o f i r o n b e a r i n g c l a y m i n e r a l s . These r e s u l t s c o r r e l a t e w e l l , however, w i t h t h e e x p e r i m e n t a l l y d e t e r m i n e d o p t i c a l s p e c t r a and p h o t o c h e m i c a l r e a c t i v i t i e s of t h e s e m i n e r a l s . S t i l l , we would l i k e t o go beyond the simple approach p r e s e n t e d h e r e and p e r f o r m m o l e c u l a r o r b i t a l c a l c u l a t i o n s ( u s i n g t h e Χα-Scattered wave o r D i s c r e t e V a r i a t i o n a l method) w h i c h a d d r e s s t h e e l e c t r o n i c s t r u c t u r e s o f much l a r g e r c l u s t e r s . Clus t e r s w h i c h accomodate s e v e r a l u n i t c e l l s o f t h e c r y s t a l would be of g r e a t i n t e r e s t s i n c e t h e r e s u l t s would be a v e r y c l o s e a p p r o x i m a t i o n t o t h e f u l l band s t r u c t u r e o f t h e c r y s t a l . The r e s u l t s o f such c a l c u l a t i o n s may a l l o w us t o a d d r e s s s e v e r a l major problems: 1. What a r e t h e l o n g - r a n g e superexchange pathways w h i c h g i v e rise to t h e magnetic s t r u c t u r e s o f i r o n b e a r i n g p h y l l o s i l i c a t e s ? 2. What p r e v e n t s t h e r m a l l y a c t i v a t e d e l e c t r o n h o p p i n g I n mixed v a l e n t c l a y s ? What i s t h e n a t u r e o f t h e d-band s t r u c t u r e ? 3. When e l e c t r o n s a r e e x c i t e d from t h e 0(2p) band t o t h e Fe(3d) band, what k i n d o f o r b i t a l pathways may a l l o w h o l e s i n t h e 0 ( 2 p ) band t o m i g r a t e t o t h e s u r f a c e o r i n t e r l a y e r r e g i o n and f a c i l i t a t e e l e c t r o n t r a n s f e r processes? 4. Assuming l a r g e c l u s t e r s may approximate t h e band s t r u c t u r e o f the c r y s t a l , what energy i s r e q u i r e d t o e x c i t e e l e c t r o n s i n t o t h e F e ( 4 s ) c o n d u c t i o n band i n Fe * b e a r i n g c l a y s ? Can such e l e c t r o n s reduce o r g a n i c m o l e c u l e s adsorbed i n t h e s u r f a c e o r t r a p p e d i n t h e interlayer? 2



308


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Crystal Chemistry, Electronic Structures, and Spectra of Fe Sites in

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