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ANALYSIS OF SOME OXIDIZED VIVIANITES
E. de Grave, R. Vochten, H. Desseyn, D. Chambaere
To cite this version:
E. de Grave, R. Vochten, H. Desseyn, D. Chambaere. ANALYSIS OF SOME OXIDIZED VIVIAN- ITES. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-407-C1-408. �10.1051/jphyscol:19801157�.
�jpa-00219643�
JOURNAL DE PHYSIQUE
Colloque Cl , supplgment au n " 1, Tome 41, janvier 1980, page C 1-407
ANALYSIS OF
SOME
OXIDIZED VIVIANITESE. De G r a v e , R. V o c h t e n
+ ,
H . ~ e s s e y n * a n d D. ChambaereLaboratory of Magnetism, University of Gent, Belgium.
+ Laboratory of Chemical and Physicai: Mineralogy, University of Antwerpen, Belgium.
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a o r a t o r y of Inorganic Chemistry, University of Antwerpen, Belgium.
T h e c o i o u r l e s s m i n e r a l v i v i a n i t e d r y s t a l l i z e s i n t h e s p a c e g r o u p C2/m w i t h c h e m i c a l f o r m u l a Fe3 (PO4I2.8H20. The s t r u c t u r e i s c o n s t r u c t e d f r o m 2+
PO4 t e t r a h e d r a a n d two t y p e s o f o c t a h e d r a , i . e . a Fe02LOH214 o c t a h e d r o n [ s i t e I ) w i t h t h e 0'- i o n s i n a t r a n s - p o s i t i o n a n d a d o u b l e FeZO6[0H2l4 g r o u p
[ s i t e s 1 1 ) i n which t h e two o c t a h e d r a s h a r e a common 0 - 0 e d g e . The two o t h e r 0 2- i o n s o f e a c h o c t a h e d r o n a r e a l s o i n a t r a n s - p o s i t i o n . T h e r e a r e t w i c e a s many s i t e s o f t y p e I1 t h a n o f t y p e I.
V i v i a n i t e i s n o t s t a b l e i n a i r . It i s e a s i l y 3 + o x i d i z e d , a n amount o f ~ e ' + b e i n g r e p l a c e d by Fe a n d v a l e n c y b e i n g c o m p e n s a t e d by r e p l a c e m e n t o f H20 by OH-. Its c o l o u r t h e n c h a n g e s t o b l u e and t h e r e s u l t i n g m i n e r a l c o u l d b e c a l l e d a n o x i d i z e d
3 + v i v i a n i t e w i t h g e n e r a l f o r m u l a ~ e z * ~ e ~ (PO4) Z.
(8-q)H20.qOH w h e r e p + q = 3 . Some q u e s t i o n s con- c e r n i n g t h i s o x i d a t i o n p r o c e s s , s u c h a s w h e t h e r o r n o t t h e s t r u c t u r e r e m a i n s e x a c t l y t h e same a f t e r o x i d a t i o n
.
w h e t h e r t h e o x i d a t i o n o c c u r s a t t h e same r a t e on b o t h s i t e s , e t c . , a r e n o t y e t f u l l y r e s o l v e d . The a i m o f t h e p r e s e n t work was t o i n v e s - t i g a t e w h e t h e r some new i n f o r m a t i o n i n t h a t c o n n e c - t i o n c o u l d b e o b t a i n e d f r o m t h e MGssbauer s p e c t r a .Two o x i d i z e d v i v i a n i t e s a m p l e s f r o m t h e p r o - v i n c e of Antwerp, Belgium. a s a m p l e f r o m Anloua, Cameroun, and a s a m p l e f r o m W a n n o n r i v e r , A u s t r a l i a , w e r e s t u d i e d w i t h 5 7 ~ e MGssbauer s p e c t r o s c o p y . C h a r a c t e r i s t i c s p e c t r a a t room t e m p e r a t u r e a r e shown i n F i g . 1 .
A l l r e c o r d e d s p e c t r a c o u l d b e decomposed i n t o f o u r q u a d r u p o l e d o u b l e t s . W i t h i n e x p e r i m e n t a l e r - r o r s , t h e h y p e r f i n e p a r a m e t e r s o f e a c h d o u b l e t are f o u n d t o be t h e s a n e f o r a l l i n v e s t i g a t e d s a m p l e s . T h e i r a v e r a g e v a l u e s a r e l i s t e d i n . T a b l e I w h e r e a s T a b l e I1 c o n t a i n s t h e r e l a t i v e d o u b l e t a r e a s f o r t h e f o u r d i f f e r e n t s a m p l e s . It i s f u r t h e r t o n o t e
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F i g . 1 . E x p e r i m e n t a l and c a l c u l a t e d N6ss- b a u e r s p e c t r a ( 3 0 0 K l o f two o x i - d i z e d v i v i a n i t e s a m p l e s f o u n d r e s - p e c t i v e l y i n ( a 1 Belgium and I b ) Cameroun.
TABLE I : i s o m e r s h i f t
6
( r e l a t i v e t o P d ) , q u a d r u - p o l e s p l i t t i n g AEQ and l i n e w i d t hr
, a l l i n m m / s . a t 3 0 0 K and 8 0 K f o r t h e f o u r d o u b l e t s 01. 02, 0 3 and 0 4 f i t t e d t o t h e M 6 s s b a u e r s p e c t r a .TABLE I1 : r e l a t i v e d o u b l e t a r e a s S f o r t h e d i f f e - r e n t o x i d i z e d v i v i a n i t e s a m p l e s .
o r i g i n S1 5 2 53 5 4
B e l g i u m I 0 . 5 8 0 . 1 1 0 . 2 8 0 . 0 3
Belgium I1 0 . 5 5 0 . 1 3 0.27 0 . 0 5
Cameroun 0 . 5 5 0.28 0 . 1 5 0 . 0 2
A u s t r a l i a 0.58 0 . 2 8 0.11 0 . 0 4
-
t h a t t h e s e p a r a m e t e r s w e r e n o t a f f e c t e d by t h e s i z e o f t h e v i v i a n i t e g r a i n s u s e d f o r t h e a b s o r b e r .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801157
C 1-408 JOURNAL DE PHYSIQUE
A s i n z i e c r y s t a l o f n o n - o x i d i z e d v i v i a n i t e h a s b e e n i n v e s t i g a t e d w i t h M 6 s s b a u e r s p e c t r o s c o p y by G o n s e r e t a l . ( 1 1 . T h e s e a u t h o r s o b s e r v e d two q u a d r u p o l e d o u b l e t s which c o u l d b e a s s i g n e d t o t h e two f e r r o u s s i t e s I and 11. T h e i r h y p e r f i n e d a t a , a l l i n mm/s and w i t h a n e r r o r o f 0 . 0 2 mm/s, w e r e f o u n d t o b e f o r F e I 2+ : 6 = 1 . 1 2 , 5E = 2 . 5 9 a t 8 0 K
Q 2+
and 6 = 1 . 0 3 , AE = 2 . 5 0 a t 295 K : f o r F e I I :
Q
8 = 1 . 1 6 , 5EQ = 3 . 1 6 a t 8 0 K and 6 = 1 . 0 5 , 5E = 2 . 9 8 4 a t 3 0 0 K , i n which 6 i s r e l a t i v e t o Pd. I n v i e w of t h e s e d a t a , we may a s s o c i a t e d o u b l e t Dl t o F e 2+
on s i t e s I1 and 0 2 t o F e 2+ on s i t e s I. The s m a l l , b u t s i g n i f i c a n t d i f f e r e n c e s i n m a g n i t u d e and tem- p e r a t u r e d e p e n d e n c e o f 5EQ o n s i t e s I, a s compared t o n o n - o x i d i z e d v i v i a n i t e , s u g g e s t t h a t t h e sym- m e t r y o f t h e s i n g l e o c t a h e d r a i n v i v i a n i t e c h a n - g e s s l i g h t l y on o x i d a t l o n . T h i s c o n c l u s i o n i s i n a g r e e m e n t w i t h t h e a s s e r t i o n s o f T i e n e t a l . ( 2 1 who m e n t i o n t h e e x i s t e n c e of s m a l l symmetry d i s t o r - t i o n s , n o t d e t e c t a b l e by X-ray d i f f r a c t i o n , i n o x i - d i z e d v i v i a n i t e s a m p l e s . It i s however s u r p r i s i n g t h a t t h e s e d i s t o r t i o n s seem n o t t o b e d e p e n d e n t on t h e o x i d a t i o n d e g r e e o f t h e s a m p l e .
3 + The two d o u b l e t s 0 3 and 0 4 a r i s e f r o m F e on s i t e s I and I1 r e s p e c t i v e l y . The r a t i o [S2+S31/
(S'l+S41, t h e n f o u n d t o b e 0 . 6 6 _t 0 . 0 6 , d e v i a t e s f r o m t h e e x p e c t e d v a l u e o f 0 . 5 0 . T h i s d i f f e r e n c e c a n b e a c c o u n t e d f o r by t h e a s s u m p t i o n t h a t t h e r e c o i l l e s s f r a c t i o n on t h e s i n g l e o c t a h e d r a i s a - b o u t 1 0 B 20 % s m a l l e r t h a n on t h e d o u b l e o c t a h e d r a .
P s s e e n f r o m T a b l e I, t h e a v e r a g e w i d t h o f e a c h a 2 s o r p t i o n l i n e i s s m a l l , i n d i c a t i n g a h i g h d e g r e e o f h o m o g e n e i t y f o r b o t h c r y s t a l l o g r a p h i c and c h e m i c a l s t r u c t u r e . T h i s i m p l i e s t h a t t h e OH-
2+ 3 +
g r o u p , r e p l a c i n g H20 o n o x i d i z i n g F e I t o F e I must b e l o c a l i z e d on t h a t p a r t i c u l a r o c t a h e d r o n i n which o x i d a t i o n t a k e s p l a c e . A s a r e s u l t . a s t r o n g - l y a s y m m e t r i c c h a r g e d i s t r i b u t i o n a r o u n d t h e f i n a l
3 +
F e I i o n i s formed and t h e r e f o r e , a h i g h AE v a l u e Q may i n d e e d b e e x p e c t e d f o r d o u b l e t 0 3 . The s m a l l
2 + 5E f o r 0 4 a n d t h e s h a r p l i n e w i d t h s f o r F e I I and
Q
~e;; f u r t h e r s u g g e s t t h a t t h e H20 l i g a n d s o f t h e e d g e - s h a r i n g o c t a h e d r a r e m a i n u n a l t e r e d o n o x i d a - t i o n . T h i s , however, would s l i g h t l y b r e a k t h e c h a r - g e b a l a n c e i n t h e c r y s t a l , u n l e s s some H20 molecu- l e s a r e randomly s u b s t i t u t e d by OH-. B e c a u s e of t h e
3 +
s m a l l amount of FeII i n v o l v e d , a n y i n f l u e n c e on t h e Mijssbauer s p e c t r a i s t h e n h a r d l y t o b e o b s e r v e d .
The s a m p l e s f r o m Cameroun a n d f r o m A u s t r a l i a a r e c l e a r l y l e s s o x i d i z e d t h a n t h e B e l ~ i a n s a m p l e s . T h i s i s p r o b a b l y d u e t o t h e d i f f e r e n t n a t u r a l e n - v i r o n m e n t s i n w h i c h t h e s e v i v i a n i t e s w e r e f o r m e d a n d k e p t b e f o r e b e i n g c o l l e c t e d . F o r s e v e r a l B e l g i a n s a m p l e s , a l l f o u n d i n t h e same r e g i o n , t h e F e 2+ /
3 +
F e r a t i o h a s b e e n d e t e r m i n e d by c h e m i c a l a n a l y s e s The r a t i o was f o u n d t o r a n g e b e t w e e n 2.18 and 3 . 7 5
( 4 1 which i s i n r e a s o n a b l e a g r e e m e n t w i t h t h e v a l u e of 2.2 - + 0 . 3 c a l c u l a t e d f r o m t h e r e l a t i v e d o u b l e t a r e a s S . T a b l e I1 f u r t h e r shows t h a t t h e o x i d a t i o n o f F e 2+ t o ~ e ~ ~ p r e f e r e n t i a l l y o c c u r s i n t h e s i n g l e
3 +
o c t a h e d r a . T h e amount o f F e f o u n d i n t h e e d g e - s h a r i n g s i t e s i s o n l y a b o u t 2 B 5 % o f t h e t o t a l i r o n c o n t e n t . The o b s e r v a t i o n o f t h i s a l t h o u g h small
3 +
F e I I c o n c e n t r a t i o n i s however i n a c c o r d a n c e w i t h t h e p r e v i o u s l y s u g g e s t e d e x p l a n a t i o n o f t h e c o l o u r a n d p l e o c h r o I s m o f p a r t i a l l y o x i d i z e d v i v i a n i t e [3l,
3 + i . e . t h e e l e c t r o n h o p p i n g b e t w e e n ~ e " and Fe on s i t e s 11. T h i s h o p p i n g i s s l o w e r t h a n t h e Mijss-
- 8
b a u e r m e a s u r i n g t i m e [ a b o u t 1 0 s 1 b e c a u s e i f n o t ,
2+ 3 +
t h e FeII and F e I I a b s o r p t i o n s would h a v e b e e n un- d i s t i n g u i s h a b l e i n t h e M 6 s s b a u e r s p e c t r a .
F i n a l l y , t h e I R s p e c t r a o b t a i n e d f o r a B e l g i a n s a m p l e and f o r t h e Cameroun s a m p l e c o n f i r m t h e p r e - s e n c e o f OH- g r o u p s and t h e d i f f e r e n c e i n o x i d a t i o n d e g r e e o f t h e two s p e c i m e n s . The l a t t e r i s c l e a r l y o b s e r v e d f r o m t h e d i f f e r e n c e s i n f r e q u e n c y a n d r e - l a t i v e i n t e n s i t y of t h e d i s t i n c t a b s o r p t i o n s a s s o - c i a t e d w i t h F e - 0 and H-0 v i b r a t i o n s . F u r t h e r , no s i g n i f i c a n t d i f f e r e n c e s a r e s e e n f o r t h e P-0 v i b r a -
3 -
t i o n s , w h i c h means t h a t t h e PO4 g r o u p i s n o t s e n s i t i v e t o t h e d e g r e e o f o x i d a t i o n .
The a u t h o r s w i s h t o t h a n k P r o f . 0 r . G . R o b b r e c h t and P r o f . 0 r . G . S t o o p s f o r h e l p f u l a n d e n c o u r a g i n g d i s c u s s i o n s . They a r e much i n d e b t e d t,o F.K.F.O.
f o r f i n a n c i a l s u p p o r t . The T e c h n i c a l a s s i s t a n c e o f Mrs. L.Bauwens and Miss A . F o n t a i n e i s g r e a t l y a c k n o w l e d g e d .
[ I ) G o n s e r , U., a n d G r a n t , R.W., P h y s . S t a t . S o 1 .
2
[ I 967 I 331
( 2 ) T i e n , P.. Waugh, T . C . , A m e r . M i n e r . E ( l 9 6 9 ) 1 3 5 5 ( 3 1 F a y e , G . H., Manning, P.G.. N i c k e l , E . H . , Amer.
M i n e r .
53
( 1 9 6 8 1 1 1 7 4141 V o c h t e n , R., Oe G r a v e , E . , S t o o p s , G., Neues J a h r b . M i n e r a l . N o n a t s h . ( i n p r e s s )