An ESCA Study of Sorbed Metal Ions on Clay Minerals

11 An ESCA Study of Sorbed Metal Ions on Clay Minerals

Downloaded by NANYANG TECHNOLOGICAL UNIV on October 17, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch011

MITCHELL H. KOPPELMAN and JOHN G. DILLARD Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Va. 24061

In natural water systems, clay minerals comprise a large part of the suspended loads of rivers which empty into estuarine environments. A portion of this load ultimately enters the ocean domain. Numerous investigators have demonstrated that the clay mineral portion of this suspended load can act as a sponge for trace metals and serve as a taxi for the transport of the metals into the estuarine and ocean systems. 1,2 It is well known that the mode of interaction between the aqueous metal ion and the clay mineral is adsorption at the Stern layer. 3,4 The interaction is believed to occur as a result of the negative surface potential generated on the clay mineral by isomorphous substitution in either tetrahedral or octahedral sites, broken surface bonds or hydrolysis of exposed surface hydroxyl groups in the clay mineral. 5 Specifically it has been shown that various clay minerals adsorb iron species from solution.6-12 Although the degree and conditions under which iron is adsorbed were carefully documented in these studies, l i t t l e information was presented regarding the bonding interaction between the clay mineral and the metal ion. Information on the nature of the bonding between the metal ion and the clay mineral is important in providing insight into various ion exchange phenomena which are important to the maintenance of the chemical balance in the ocean system. A study of the bonding phenomena between the clay mineral and adsorbed metal ions Fe(III) and Cr(III), has been initiated using X-ray photoelectron spectroscopy (XPS or ESCA). ESCA would seem to be an ideal method for this study since it has been postulated that the adsorbed ions are located on the clay mineral surfaces. In X-ray photoelectron spectroscopy the binding energy of core and valence electrons can be 186 Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


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Sorbed Metal Ions on Chy

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measured from a s a m p l i n g depth o f up t o about 50Â.JL2. The measured b i n d i n g energy can be r e l a t e d t o t h e e l e c t r o n d e n s i t y o r c h a r g e on an element o f i n t e r e s t and can thus be used t o examine t h e o x i d a t i o n s t a t e ( b o n d i n g n a t u r e ) o f elements i n c l a y m i n e r a l s and m e t a l i o n s adsorbed a t c l a y m i n e r a l s u r f a c e s . To compliment t h e ESCA measurements the b o n d i n g n a t u r e of i r o n i n c l a y m i n e r a l systems has a l s o been exami n e d u s i n g Môssbauer s p e c t r o s c o p y . In t h i s p a p e r t h e r e s u l t s o f a s t u d y o f t h e b o n d i n g between t h e c l a y m i n e r a l s , c h l o r i t e , i l l i t e , and k a o l i n i t e and t h e m e t a l i o n s F e ( I I I ) and C r ( I I I ) w i l l be r e p o r t e d . Experimental The c l a y m i n e r a l s and compounds i n v e s t i g a t e d were c h l o r i t e ( f r o m Ishpeming, M i c h i g a n o b t a i n e d from Ward s N a t u r a l S c i e n c e E s t a b l i s h m e n t ) , i l l i t e ( F i t h i a n , I l l i n o i s , A.P.I, s t a n d a r d #35 o b t a i n e d from I l l i n o i s G e o l o g i c a l S u r v e y ) , k a o l i n i t e ( H y d r i t e RT, o b t a i n e d from G e o r g i a K a o l i n , I n c . ) , n o n t r o n i t e (Washington, o b t a i n e d from Dr. C. I . R i c h , VPI & SU Agronomy Department), FeoC>3 ( F i s h e r C h e m i c a l Company, r e a g e n t g r a d e ) , and a i r d r i e d amorphous f e r r i c h y d r o x i d e ( p r e c i p i t a t e d from 100 ppm F e ( N 0 ) 3 s o l u t i o n a t pH 3 ) . These c l a y s were s e l e c t e d because o f t h e i r n o n - e x p a n s i v e n a t u r e i n an e f f o r t t o d e c r e a s e i n t e r l a y e r s u b s t i t u t i o n and enhance s u r f a c e a d s o r p tion. The c l a y s were ground w i t h an a g a t e m o r t a r and p e s t l e and a b a l l m i l l ( u s i n g t u n g s t e n c a r b i d e b a l l s ) such t h a t a l l p a r t i c l e s were

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Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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AND DILLARD

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o f F e ( I I I ) and F e ( I I ) i n the i l l i t e sample were d e t e r m i n e d from r a t i o s o f peak areas o f L o r e n t z i a n peaks r e p r e s e n t a t i v e o f the s p e c i f i c f e r r i c and f e r rous i r o n s i t e s .


R e s u l t s and

Discussion

The c o r e e l e c t r o n b i n d i n g e n e r g i e s f o r the l a t t i c e elements i n c h l o r i t e , k a o l i n i t e , and i l l i t e are p r e s e n t e d i n T a b l e I . The s m a l l d i f f e r e n c e s i n b i n d i n g e n e r g i e s o f S i 2p^/2,3/2> Pl/2,3/2> 0 Is e l e c t r o n s between t h e s e ' t n r e e m i n e r a l s a r e not unexpected, s i n c e t h e r e a r e o n l y minor bonding d i f f e r e n c e s between the m i n e r a l s . The range o f b i n d i n g energy v a l u e s o f the S i 2p]/2 3/2 e l e c t r o n o b s e r v ed here (102.1 eV - 102.7 eV) i s i n good agreement w i t h those found by Adams e t . a l J L L (101. 8 - 102.4 f o r v a r i o u s s i l i c a t e s ) , H u n t r e s s e t . a l . J l 2 (103.0 for l u n a r m a t e r i a l s ) and Anderson e t . a l . (102.8 - 103.2 f o r a s e r i e s o f a l u m i n o s i l i c a t e s ) . The A l %V\/2,3/2 and 0 1ST / b i n d i n g e n e r g i e s r e p o r t e d h e r e are i n good agreement'with those p u b l i s h e d i n the l i t e r a t u r e . l ~ S c h u l t z et.al.-25. p r e s e n t e d b i n d i n g energy d a t a f o r the S i 2p /o Q/2 e l e c t r o n s i n k a o l i n i t e (107.4 eV) and i l l i t e (109.3 eV). D i f f e r e n c e s between t h e i r v a l u e s and the ones p r e s e n t e d i n t h i s r e p o r t p r o b a b l y a r i s e from f a i l u r e ^ ? t o c o m p l e t e l y c o r r e c t f o r the sample c h a r g i n g phenomena. The i n v e s t i g a t i o n o f t h e b i n d i n g energy o f the Fe P 3 / 9 e l e c t r o n s i n c h l o r i t e , i l l i t e , and k a o l i n i t e revealed rather i n t e r e s t i n g r e s u l t s . These b i n d i n g e n e r g i e s a r e t a b u l a t e d i n T a b l e I I . Adams e t . a l . - 2 I had p r e v i o u s l y r e p o r t e d d i f f i c u l t y i n d i s t i n g u i s h i n g between F e and F e s p e c i e s u s i n g ESCA. However, when comparing the b i n d i n g energy f o r the Fe P3/2 l e v e l i n n o n t r o n i t e where i r o n i s i n the +3 ( f e r r i c ) s t a t e ( s u b s t i t u t e d f o r some A l i n the o c t a h e d r a l l a y e r s ) and c h l o r i t e where the i r o n i s i n the +2 (ferrous) oxidation state (substituted for Mg i n the o c t a h e d r a l l a y e r s ) , a d i s t i n c t d i f f e r e n c e (1.9 eV) i s observed. The XPS spectrum o f Fe 2 p 3 / e l e c t r o n s i n i l l i t e showed a r a t h e r b r o a d peak w i t n a b i n d i n g energy o f 712.6 eV. Fe 2j>2/2 l always e x h i b i t b r o a d peaks i n the ESCA spectrum due t o m u l t i p l e t s p l i t t i n g phenomena,26-23 The f u l l w i d t h s at h a l f n o n t r o n i t e and c h l o r i t e (each c o n t a i n i n g o n l y one type o f i r o n ) are 4.9 eV and 5.2 w r e s p e c t i v e l y , w h i l e i l l i t e i s 6.4 eV. T h i s i n c r e a s e i n peak w i d t h i s s u g g e s t i v e t h a t i l l i t e c o n t a i n e d more than one type o f i r o n . A

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Mossbauer d a t a ( T a b l e IV) c o n f i r m s t h a t n o n t r o n i t e c o n t a i n s o n l y F e ( I I I ) i n one environment and t h a t c h l o r i t e c o n t a i n s o n l y F e ( I I ) i n one environment. I l l i t e , on the o t h e r hand, was shown by Mossbauer s p e c t r a t o c o n t a i n b o t h F e ( I I ) and F e ( I I I ) , each i n o n l y one environment. S i n c e the Mossbauer r e s u l t s i n d i c a t e d t h a t F e ( I I ) and F e ( I I I ) i n i l l i t e were s i m i l a r t o F e ( I I ) and Fe(III) i n c h l o r i t e and n o n t r o n i t e , r e s p e c t i v e l y , i t was o f i n t e r e s t t o detemine t h e b i n d i n g e n e r g i e s o f F e ( I I ) and F e ( I I I ) i n i l l i t e u s i n g d e c o n v o l u t i o n . The i n p u t d a t a u s e d f o r the d e c o n v o l u t i o n were the b i n d i n g e n e r g i e s o f F e ( I I ) i n c h l o r i t e and F e ( I I I ) i n n o n t r o nite. The i n p u t d a t a a l s o i n c l u d e d s a t e l l i t e peak s t r u c t u r e d e t e r m i n e d f o r c h l o r i t e , ( F e ( I I ) ) and non t r o n i t e ( F e ( I I I ) ) , as shown i n F i g u r e 1. By s m a l l a l t e r a t i o n s o f the r e l a t i v e i n t e n s i t i e s o f the peaks and by s l i g h t changes i n peak h a l f w i d t h s and p o s i t i o n s , a good f i t o f the c a l c u l a t e d and e x p e r i m e n t a l peaks was obtained ( F i g . 2). Furthermore i t i s of i n t e r e s t to n o t e t h a t the p e r c e n t a g e o f F e ( I I I ) i n i l l i t e o b t a i n e d from XPS peak a r e a s i s 71%, which i s i n good agreement w i t h the p e r c e n t a g e (^80%) o b t a i n e d from Mossbauer measurements. B i n d i n g energy d a t a f o r b o t h e x p e r i m e n t a l and d e c o n v o l u t e d Fe 2ρο/ο l e v e l s a r e g i v e n i n Table I I . ' A l t h o u g h Mossbauer d a t a r e v e a l e d t h a t k a o l i n i t e c o n t a i n e d F e ( I I I ) ( a weak peak was o b s e r v e d , w i t h l o n g (>24 h o u r s ) c o u n t i n g times needed f o r r e s o l u t i o n ) , no ESCA Fe 2p3/2 peak was o b s e r v e d (12 hour s c a n n i n g t i m e ; t y p i c a l of a l l these s t u d i e s ) . T h i s probably i n d i c a t e s t h a t the s m a l l amount o f i r o n i n k a o l i n i t e i s p r e s e n t i n the l a t t i c e a t depths g r e a t e r than t h e escape depth f o r the p h o t o e j e c t e d e l e c t r o n s . S i n c e Mossbauer i s a b u l k t e c h n i q u e compared t o the ESCA s u r f a c e t e c h n i q u e , i t i s r e a s o n a b l e t h a t s m a l l amounts o f l a t t i c e i r o n p r e s e n t i n the b u l k would be detected. When kaolinite i s s t i r r e d w i t h 100 ppm F e ( N 0 3 ) s o l u t i o n (pH remained low enough t o p r e v e n t p r e c i p i t a t i o n o f any i r o n h y d r o x i d e s p e c i e s ) , t h e Fe 2P3/2 peak was o b s e r v e d i n t h e XPS spectrum ( F i g . I l l ) . Tne Mossbauer spectrum i n d i c a t e d approximately a three f o l d i n c r e a s e i n the amount o f F e ( I I I ) p r e s e n t . The b i n d i n g energy o f t h i s a d s o r b e d F e ( I I I ) s p e c i e s was found t o be 711.4 eV, 1.2 eV lower than t h a t f o r F e ( I I I ) i n a l a t t i c e s i t e i n the o c t a h e d r a l l a y e r o f a s i l i c a t e m i n e r a l . It i s s u g g e s t e d t h a t the l o w e r i n g o f the Fe 2 p / 2 b i n d i n g energy i s due t o the i n t e r a c t i o n o f F e ( I I l ) w i t h the c l a y s u r f a c e upon a d s o r p t i o n . Cations i n the c l a y s u r f a c e r e g i o n (Stern l a y e r ) are a t t r a c t e d to 3

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CHEMISTRY


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A N D DiLLARD

KAOLINITE Fe

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Binding Energy (ev) Figure 3. Photoelectron spectra for the Fe 2p level in untreated kaolinite and kaolinite treated with 100 ppm Fe(NO ) s/t

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n e g a t i v e l y c h a r g e d s u r f a c e bonding s i t e s where they experience a negative s i t e p o t e n t i a l . i L The e f f e c t o f a n e g a t i v e b o n d i n g s i t e p o t e n t i a l on i r o n would be t o lower t h e b i n d i n g energy o f t h e a d s o r b e d c a t i o n . In s i m i l a r a d s o r p t i o n s t u d i e s o f F e ( I I I ) on c h l o r i t e * t h e b i n d i n g energy measured f o r i r o n from t h e e x p e r i m e n t a l photopeak i s 711.4 ( s e e T a b l e I I ) . T h i s v a l u e i s g r e a t e r than t h a t measured f o r i r o n i n n a t i v e c h l o r i t e and s i m i l a r t o t h a t d e t e r m i n e d f o r F e ( I I I ) a d s o r b e d on kaolinite. I t was d e t e r m i n e d t h a t t h e composite e x p e r i m e n t a l photopeak c o u l d be d e c o n v o l u t e d i n t o two i r o n peaks ( i n c l u d i n g s a t e l l i t e s t r u c t u r e f o r each peak) w i t h b i n d i n g e n e r g i e s o f 711.4 and 710.3 eV. The i r o n s p e c i e s r e p r e s e n t e d by t h e peak a t a b i n d i n g energy o f 711.4 i s a t t r i b u t e d t o a d s o r b e d F e ( I I I ) w h i l e t h a t a t 710.3 c o r r e s p o n d s t o F e ( I I ) i n t h e chlorite lattice. S i m i l a r l y , t h e composite e x p e r i m e n t a l Fe 2p^/2 peak f o r i l l i t e , o b t a i n e d when t h e c l a y m i n e r a l was s t i r r e d a t pH 1 i n a 100 ppm F e ( N 0 ) s o l u t i o n , c o u l d be d e c o n v o l u t e d i n t o t h r e e i r o n peaks ( i n c l u d i n g s a t e l l i t e s t r u c t u r e f o r each peak) w i t h b i n d i n g e n e r g i e s o f 712.6 eV, 711.5 eV, and 710.4 eV. The i r o n s p e c i e s r e p r e s e n t e d by t h e peak a t 712.6 eV i s a t t r i b u t e d t o l a t t i c e F e ( I I I ) , t h a t a t 711.5 eV t o a d s o r b e d F e ( I I I ) , and t h e peak a t 710.4 eV i s c h a r a c t e r i s t i c of l a t t i c e Fe(II). In a f u r t h e r e f f o r t t o i n v e s t i g a t e t h e e f f e c t o f the n e g a t i v e s u r f a c e p o t e n t i a l upon a d s o r b e d c a t i o n s , C r ( I I I ) a d s o r p t i o n e x p e r i m e n t s onto k a o l i n i t e , c h l o r i t e and i l l i t e were p e r f o r m e d . Uvarovite, i n which C r ( I I I ) i s i n an o c t a h e d r a l l a y e r s i t e , was i n v e s t i g a t e d w i t h XPS t o determine t h e b i n d i n g energy f o r Cr 2p>g/2 e l e c t r o n s f o r t h e l a t t i c e s u b s t i t u e n t Cr( I I I ) . XPS i n v e s t i g a t i o n o f t h e t h r e e u n t r e a t e d c l a y s gave no evidence f o r a Cr 2 p « peak. Upon a d s o r p t i o n o f C r ( I I I ) from 100 ppm C r ( N 0 ) (pH = 1 . 0 ) s o l u t i o n s , the XPS spectrum o f each o f t h e s e c l a y s r e v e a l e d a Cr 2p #2 P©ak, i n d i c a t i n g t h a t chromium had i n d e e d been adsorbed. The b i n d i n g energy d a t a f o r t h e Cr 2 p / e l e c t r o n s f o r a l a t t i c e s u b s t i t u t e d C r ( I I I ) as w e l l as t h e d a t a f o r t h e a d s o r b e d C r s p e c i e s can be f o u n d i n T a b l e I I I . The b i n d i n g energy r e s u l t s f o r t h e a d s o r b e d C r ( I I I ) species r e f l e c t e d a s i m i l a r decrease i n b i n d i n g energy f o r t h e a d s o r b e d c a t i o n , as was found i n the F e ( I I I ) a d s o r p t i o n s t u d y . The n e g a t i v e s u r f a c e p o t e n t i a l a t t h e a d s o r p t i o n s i t e m a n i f e s t s i t s e l f by i n c r e a s i n g t h e e l e c t r o n d e n s i t y on t h e a d s o r b e d m e t a l ion thus lowering the binding energy of the metal. 3

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Sorbed Metal Ions on Cfoy Minerals

The differences in binding energy for Cr 2p 3 /2 electrons for a lattice substituted Cr(III) as compared to an adsorbed Cr(III) ranged from 0.8 eV (Cr(III)) on kaolinite) to 1.1 eV (Cr(III) on chlorite).


Conclusions The technique of X-ray photoelectron spectroscopy has been shown to be a useful tool in the investigation of adsorbed metal ions on solid surfaces. Not only may i t prove to be an analytical tool for detecting small (ppm level) quantities of adsorbed cations, but may be instrumental in determining the bonding nature of an adsorbed cations. Through further investigations of the adsorption of cations of different oxidation states, i t is hoped that a site-bonding model for cations adsorbed on mineral surfaces can be developed. Acknowledgements This work has been supported by the NSF (Grant GP-29178 ESCA Spectrometer) and by the VPI Small Projects Fund. We would also like to thank Mrs. N. Crews for assistance with the compilation and interpretation of the Mossbauer data and Drs. J. Craig and C. I. Rich for help in obtaining the minerals used in this study.

Abstract The bonding nature of metal ions sorbed from aqueous solutions by the marine clay minerals kaolinite, i l l i t e , and chlorite has been examined using X-ray Photoelectron Spectroscopy (XPS). Binding energies for lattice elements Si, Al, O, Mg, K, and Ca are reported, and are in good agreement with published values. Significant differences (1.9 eV) in binding energies for the Fe 2 p 3 / 2 level for iron in nontronite and chlorite were observed. This difference in binding energy was attributed to the two different oxidation states of iron, Fe(II) in chlorite and Fe(III) in nontronite. I l l i t e , which contains both Fe(II) and Fe(III) in lattice positions was found to have an unusually broad Fe 2 p 3 / 2 photopeak. This broad peak could be deconvolutea into two peaks whose binding energies were indicative of both Fe(II) and Fe(III) lattice constituents. Fe(III) adsorbed onto kaolinite had an Fe 2p3/2 level binding energy 1.2 eV lower than lattice Fe(III). This lowering of binding energy for the adsorbed iron species as compared to


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lattice Fe(in) may arise from the negative potential of the electrical double layer at the mineral surface when the cation is adsorbed. Comparison of binding energies of sorbed ions ( F e + 3 , C r + 3 ) and their stoichiometric or mineral lattice counterparts i n d i cate a decrease in binding energy for the sorbed ions. Extent of sorption was determined by monitoring sorbed metal ion species concentrations as well as examining changes in solution pH, dissolved SiOo and concentra tion of stoichiometric lattice elements ( A l + , K + , Mg + 2 , F e + 2 ' + 3 ) . Literature Cited 1. Krauskopf, Κ. Β . , Geochim. Cosmochim. Acta, (1956) 9, 1. 2. Kharkar, D. P., Κ. K. Turekian, and Κ. K. Bertine, Geochim. Cosmochim. Acta, (1968) 32, 285. 3. Stumm, W., and Morgan, J. J . , Aquatic Chemistry, Wiley-Interscience, New York, 1970. 4. Van Olphen, Η., An Introduction to Clay Colloid Chemistry, Wiley-Interscience, New York, 1963. 5. Grim, R. Ε . , Clay Mineralogy, 2nd ed., McGraw-Hill, New York, 1968. 6. Whittig, L. D., Page, A. L . , Soil Sci. Soc. Amer. Proc., (1961), 278. 7. Follett, E. A. C . , J. of Soil Science, (1965) 16, 334 8. Fordham, A. W., Aust. J. Of S o i l . Res., (1969) 7 185. 9. Fordham, A. W., Aust. J. of Soil Res., (1969) 7, 199. 10. Fordham, A. W., Aust. J. of Soil Res., (1969) 8, 107. 11. Blackmore, Α. V . , Aust. J. of Soil Res., (1973) 11, 75. 12. Fordham, A. W., Clays and Clay Minerals, (1973) 21, 175. 13. Lindau, I., Spicer, W. Ε., J. Of Electron Spectros copy and Related Phenomena, (1974) 3, 409. 14. Singleton, J. Η., Vac. Symp. Trans., (1963) 15,267. 15. Personal communication with Dr. Peter Orenski of Union Carbide Corporation. 16. Mullin, J. Β . , Riley, J. P . , Analytica Chimica Acta (1955) 12, 162-176. 17. Fanning, Κ. A . , Pilson, M.E.A., Anal. Chem., (1973) 45, 136. 18. Written by G. W. Dulaney, VPI & SU, 1969 (Presently at Digital Equipment, Corp., Maynard, Mass.)



11.

KOPPELMAN AND

DiLLARD


19. Seals, R. D., Alexander, R., Taylor, L. T . , Dillard, J. G., Inorg. Chem., (1973) 12, 2485. 20. Burness, J. H . , Dillard, J. G., Taylor, L. T . , Inorg. Nucl. Chem. Lett., (1974) 10, 387. 21. Adams, I., Thomas, J. M., Bancroft, G. Μ., Earth and Planetary Science Letters, (1972) 16, 429. 22. Huntress, W. T. J r . , Wilson, L . , Earth and Plane tary Science Letters, (1972) 15, 59. 23. Anderson, P. R., Swartz, W. E. J r . , Inorg. Chem. (1974) 13, 2293. 24. Yin, L. T . , Ghose, S., Adler, I., Science, (1971) 173, 633-635. 25. Schultz, H. D., Vesely, C. J . , Langer, D. W., Appl Spect., (1974) 28, 374. 26. Carver, J. C . , Schweitzer, G. Κ., Carlson, T . A . , J. of Chem. Phys., (1972) 57, 973. 27. Fadley, C. S., Shirley, D. Α., Phys. Rev., A. (1970) 2, 1109. 28. Fadley, C. S., Shirley, D. Α . , Freeman, A. J . , Bagus, P. S., Mallow, J. V . , Phys. Rev. Lett., (1969) 23, 1397. 29. Uvarovite - C a 3 C r 2 ( S i O 4 ) 3 (garnet group) - from Outokumpo, Finlana, sample provided by Dr. J. Craig, Dept. of Geological Sciences, VPI & SU, Chemical composition data may be found in Rock Forming Minerals by W. A. Deer, R. A. Howie, and J. Zussman, Longmans Press, London, 1962.


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An ESCA Study of Sorbed Metal Ions on Clay Minerals

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