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57Fe MÖSSBAUER STUDY OF THE CRYSTAL CHEMISTRY OF STRUNZITE, MnFe2(PO4)2, (OH)2 .
6H2O
R. Vogel, B. Evans
To cite this version:
R. Vogel, B. Evans. 57Fe MÖSSBAUER STUDY OF THE CRYSTAL CHEMISTRY OF STRUN-
ZITE, MnFe2(PO4)2, (OH)2 . 6H2O. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-297-C1-
298. �10.1051/jphyscol:19801106�. �jpa-00219587�
JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au
n
O 1, Tome 41, janvier 1980, page C1-2975 7 ~ e M~SSBAUER STUDY
M
THE CRYSTAL CHEMISTRYOF
STRIJNZITE, M n ~ e ~ ( ~ 0 ~ ) 2 ( ~ ) 2,
6H20 R. Vogel and B.J. EvansDepartment of C7remistry, University of Michigan, Ann Arbor, Michigan 48109.
S t r u n z i t e i s a n a t u r a l l y o c c u r r i n g manganese i r o n phosphate w i t h t h e p o s t u l a t e d i d e a l composi- t i o n MnFe2(P04)2(0H)2.6H20. S t r u n z i t e occurs w i t h a n e e d l e l i k e h a b i t and v a r i e s from straw y e l l o w t o brownish y e l l o w i n c o l o r . Remarkable v a r i a t i o n s i n composition w i t h Mn/Fe r a t i o s ranging from 0 t o t h e i d e a l value o f 2 have a l s o been noted. The op- t i c a l and x-ray d i f f r a c t i o n p r o p e r t i e s a r e s i m i - l a r l y v a r i a b l e b u t no d e f i n i t i v e c o r r e l a t i o n s be- tween t h e composition, o p t i c a l and x-ray d i f f r a c - t i o n p r o p e r t i e s have emerged /1,2/. The absence o f a simple c o r r e l a t i o n between t h e Mn/Fe r a t i o and p h y s i c a l p r o p e r t i e s of s t r u n z i t e may be due t o t h e f a c t t h a t Fe s u b s t i t u t i o n f o r Mn may n o t occur according t o t h e simple mechanism i n (1).
Mn2+ + ( l - x ) ~ n ~ +
+
x Fe 2+ ( 1 ) Other p o s s i b l e mechanisms f o r t h e s u b s t i t u t i o n o f Mn by Fe a r e given below:t4n2+ + H20 + (1-x)bln2++ x ~ e ~ + + x OH'-+X/Z Hz ( 2 ) M ~ ~ + + O H ~ - + (1-x)Mn2++x ~ e ~ + + x / 2 0'-+x/2 HZ ( 3 ) Even though t h e p o s s i b i l i t y o f o x i d a t i o n i n Strun- z i t e according t o 92) and ( 3 ) has been mentioned 111, these two mechanisms have n o t been given s e r i - ous consideration.
The r e c e n t r e p o r t o f t h e s t r u c t u r e determin- t i o n o f s t r u n z i t e o f unknown composition /3/ has increased t h e p r o b a b i l i t y o f a successful i n v e s t i - g a t i o n o f s t r u c t u r e - p r o p e r t y r e l a t i o n s and o f t h e mechanism f o r t h e s u b s t i t u t i o n o f Mn by Fe. Assum- i n g t h e m a t e r i a l i n /3/ t o have had t h e i d e a l com- p o s i t i o n , t h e s t r u c t u r e has been described as con-
s i s t i n g o f F ~ ~ + ( O H ) ~ ( H ~ O ) O ~ octahedra which a r e j o i n e d t o g e t h e r v i a OH- groups a t opposite v e r t i c e s t o form i n f i n i t e chains o f v e r t e x - s h a r i n g octahedra.
There a r e two c r y s t a l l o g r a p h i c a l l y d i s t i n c t octa- hedra i n t h e chains. Each octahedral c h a i n i s bonded t o two o t h e r chains by means o f PO:' groups t o form a s l a b of mixed, vertex-sharing octahedra and t e t r a h e d r a . These slabs are bound together t o form a t h r e e dimensional s t r u c t u r e by means of t r a n ~ - * n ~ + ( ~ ~ 0 ) ~ 0 ~ octahedra which share v e r t i c e s w i t h t h e po3- groups. There i s no water of c r y s t a l -
4
l i z a t i o n i n s t r u n z i t e , and t h e charge compensation mechanisms i n ( 2 ) and (3) must r e s u l t i n changes i n t h e nearest-neighbor environments, a t l e a s t , o f the metal i o n s .
Therefore, we have i n v e s t i g a t e d by means o f 5 7 ~ e MGssbauer spectroscopy a s t r u n z i t e specimen from Hagendorf, Bavaria, West Germany, i n o r d e r t o c h a r a c t e r i z e t h e l o c a l c r y s t a l chemistry o f Fe i n s t r u n z i t e s w i t h Mn/Fe r a t i o s l e s s than 2. Previous chen~ i c a l a n a l y s i s o f s t r u n z i t e from Hagendorf had shownit t o have a Mn/Fe r a t i o o f 0.285. Trans- mission spectra were obtained by means o f an e l e c - tromechanical transducer and 51 2 channels of 1024 channel multichannel analyzer w i t h t h e source and absorber a t 298 K.
The spectrum a t 298 K i s shown i n F i g . 1.
The s o l i d l i n e i n F i g . l a represents a f i t o f two quadrupole doublets s u b j e c t t o t h e c o n s t r a i n t t h a t AiZ = hAi where Ail and Ahi a r e t h e i n t e g r a t e d
i n t e n s i t i e s of t h e low and h i g h v e l o c i t y components
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801106
c1-298 JOURNAL DE
o f t h e doublet i
,
r e s p e c t i v e l y . This f i t i s inadequate g i v i n g a X2 and MISFIT g o o d n e s s - o f - f i t parameters approximately two and f o u r times l a r g e r than expected, r e s p e c t i v e l y . The s o l i d l i n e i n Fig. l b represents a t h r e e quadrupole doublet f i t which r e s u l t e d i n acceptable g o o d n e s s - o f - f i t para- meters; t h e h y p e r f i n e parameters a r e given i n Table 1 The isomer s h i f t s a r e r e p o r t e d r e l a t i v e t o Fe metal.I -215 -210 -115 -1!0 -015 0!0 015 1!0 1 5 210 215
VELOCITY MM/SEC
Figure I . 5 7 ~ e ~ & s b a u e r Spectrum a t 298X of S t r u n z i t e from Hagendorf: ( l a ) two quadrupole doublet f i t ; note strong s t r u c t u r e i n residual;
(Ibl t h r e e quadrupole doublet f i t .
Subpattern
AEQ
&IS i- Area(mm s - l ) (nun s-') (mm s - l ) (per c e n t )
X2 = 569., MISFIT = (0.016 f 0.005") %
. . .
Three aspects of t h e spectrum a r e remarkable:
(1) The i r o n i s present o n l y as ~ e ~ ' and t h e f o r - mula MnFe2(P04)2(0H)2'6H20 cannot apply t o t h e Fe-
PHYSIQUE
r i c h compositions. ( 2 ) The two i r o n s i t e s w i t h i n t h e i n f i n i t e chains e x h i b i t such s i m i l a r ~ 6 s s b a u e r parameters t h a t t h e i r 5 7 ~ e p a t t e r n s appear as an
apparent
s i n g l e doublet (subpatterr: 11) w i t h s l i g h t l y broadened l i n e s . (3) The i r o n species g i v i n g r i s e t o sub-pattern I does n o t occupy a l a t t i c e s i t e whose l i g a n d environment i s s i m i l a r t o those enumerated i n /3/.The r e l a t i v e i n t e g r a t e d i n t e n s i t i e s o f sub- p a t t e r n s I1 and I 1 1 a r e i n good accord w i t h t h e s i t e m u l t i p l i c i t y and Mn/Fe r a t i o o f t h e Hagendorf specimen. Assuming t h e c h a i n s i t e s t o be occupied o n l y by ~ e ~ + , t h e r e a r e 0.67 Mn atoms and 0.33 atoms o f ~ e on t h e i n t e r l a y e r s i t e s . ~ + Consequently, t h e r e l a t i v e i n t e n s i t i e s o f t h e p a t t e r n s a r i s i n g from t h e i n f i n i t e c h a i n and i n t e r l a y e r Fe ions a r e c a l c u l a t e d t o be 86% and 14%, r e s p e c t i v e l y , i n q u a l i t a t i v e agreement w i t h t h e r e s u l t s i n Table 1 i f subpatterns I 1 and I 1 1 a r e assigned t o t h e i n - f i n i t e c h a i n and i n t e r l a y e r ~e~~ ions, r e s p e c t i v e l y . B e t t e r agreement i s obtained i f subpattern I i s assigned t o an i n f i n i t e c h a i n s i t e . The Fln/Fe r a t i o and t h e r e l a t i v e i n t e n s i t i e s o f t h e t h r e e p a t t e r n s a r e i n e x c e l l e n t agreement w i t h a !.In-Fe s u b s t i t u t i o n / c h a r g e compensation mechanism i n which 1/2 mole o f H20 coordinated t o an i n f i n i t e
c h a i n s i t e i s reduced t o 0'- f o r each mole o f ~ e ~ ' s u b s t i t u t e d f o r Mn 2+ a t an i n t e r l a y e r s i t e .
References
/1/ Frondel, C., N. Jb. Miner, Mn. Jg.
1957,
222.
/2/ Correia Neves, J. M., Pub. d. Mus. e Lab.
Min. e Geol. Univ. Coimbra
50
(1960) 1./3/ Fanfani, L., Tomassini, M., Zanazzi, P.F., and Zanzari, A. R., Tschermaks Min. P e t r . M i t t .
