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CAN ELECTRON EXCHANGE PROCESSES IN VESUVIANITES BE DETECTED BY MÖSSBAUER
SPECTROSCOPY ?
M. Tricker, P. Manning
To cite this version:
M. Tricker, P. Manning. CAN ELECTRON EXCHANGE PROCESSES IN VESUVIANITES BE
DETECTED BY MÖSSBAUER SPECTROSCOPY ?. Journal de Physique Colloques, 1979, 40 (C2),
pp.C2-477-C2-479. �10.1051/jphyscol:19792165�. �jpa-00218540�
JOURNAL DE PHYSIQUE Colloque C2, supplkment au n o 3, Tome 40, mars 1979, page C2-477
CAN ELECTRON EXCHANGE ?ROCESSES I N V E S U V I A N I T E S B E DETECTED B'/ M ~ s s B A U E R S2ECT2OSCOQY
?
M. J. Tricker and P. G. ~ a n n i n ~ *
Department of Chemistry, Heriot-Natt University, Sdinburgh, U.K.
*.Environment Canada, BurZingtun, Ontario, Canada.
RQsum6.- On presente des observations par spectroscopie Ygssbauer du 5 7 ~ e dans des vesuvianites montrant des effets de transferts glectroniques.
Abstract.- 5 7 ~ e Mzssbauar evidence for the occurrence of electron exchange processes in vesuvianites is presented.
Vesuvianites are complex alumino-silicate minerals for rapid electron exchange within vesuvianites.
of formula [ ~ a l S ~ l ~ F e ( ~ l , ~ g , ~ e ) 8 Si18070 (OH)
which display a variety of colours ranging from Table I : Summary of M~ssbauer parameters and as- signments of quadrupole doublets obser- greens to browns. Recent crystallographic studies ved in vesuvianites at room temperature.
/l/ demonstrate that they contain three tetrahe- dral sites occupied by si4+, four eight coordinate sites, (C is a square antiprism) Ca(l), Ca(2), Ca(3) and C, occupied mainly by ca2+, two six coordinate "octahedral" sites (A1 and AllFe) and a five coordinate square pyramidal B-site. Along the four-fold axis eight coordinate C-sites share faces leading to short cation-cation separations of 2.60A. Edge sharing occurs between and amongst the remaining six and eight coordinate polyhedra lea- ding to cation-cation separations ranging from 2.8 A to 3.3 ft. Strongly polarised intervalence charge transfer bonds have been observed in the optical spectra of these minerals and assigned to interactions between eitter lJe2 +/Fe3 + or Fe2 +/Ti4 +
ion pairs located on adjacent sites. 12-41. In a previous Mgssbauer study of vesuvianites we obser- ved complex spectra consisting of superpositions of doublets arising from iron ions distributed over the possible sites. In additon to parameters cor- responding to "normal" Fe 2+ and Fe 3+ ion satisfac- tory fits to the data for two samples could only be obtained by inclusion of doublets with parame- ters intermediate between those expected for nor- mal "frozen" Fe2+and Fe 3 + iron (see table I for summary). In view of recent work on electron ex- change prbcesses in minerals 151 and spinels 161 it now seems reasonable to associate these para- meters with "averaged" doublets arising from rapid electron exchange between adjacent Fe +/Fe 3 + or Fe2'/Ti 4+ ion pairs. We have now extended our ori- ginal study to further vesuvianite samples and to lower temperature and we report further evidence
I.Results.- We report here results of some of the features of representative spectra of the new sam- ples. In general the gross features of the spectra can be accounted for in terms of the parameters as- sociated with various sites determined previously (see figures). However there are new features which we stress here.
I. 1 .S_+np:e_ -!:- At room temperature sample one dis- plays a spectrum (Fig. la) whose gross features arise from major amounts of ~ e iron in six coor- ~ + dinate sites, minor amounts of Fe3+ iron on eight coordinate sites and Fe2+ on six coordinate sites.
An intermediate absorption (feature 1A Fig. la) is also seen corresponding to doublet W-4. At 80 K the (Fig. la) intensity of this doublet is diminished and a new doublet with parameters Az2.5 mm-' and
&=l .0 ms-' appers. This new doublet (designated T-l) reasonably corresponds to frozen Fe2+ iron in six coordination.
Designation
W- l W-2 W- 3 W- 4 W-5 LE-4 LO-2
Isomer shift
(ZO.OZ)II~.S-'
0.39 0.82 1.32 0.87
1. 13 0.85 0.62
Quadrupole Splitting (fo.oZ)m-s-'
0.48 0.39 3.44
' 1.55 2.74 1.04 0.89
Assignment
6CN-Fe 3C
5CN-Fe 2+
8CN-Fe 2+
Averaged
~ c N - F ~ ~ + Averaged
~CN-Fe 3+
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792165
JOURNAL DE PHYSIQUE
5 7
Fig. I : Fe Mijssbauer spectra of sample l recorded at 300 K (a) and 80 K (b). See table and text for key.
1.2.Samvle
2.-
The room temperature spectrum of a representative-
Wakefield type-
sample is shown in figure 2a. In addition to previously observed absorptions corresponding to frozen Fe2+ on five, six and eight coordinate sites together with Fe3+iron on six-coordinate sites a 1A like intermediate absorption is also observed. At 80 K the high ener- gy arm of the T-l doublet appears in an analogous manner to sample 1 but in this case residual inter mediate absorption is still observed. (Fig. 2b)
1.3._S_apJ_e-3,- In figure 3 we show spectra of a third vesuvianite recorded at various temperatures.
At 300 K the spectrum is broad and ill defined with virtually continuous absorption at intermediate velocities although a peak (3A) may be distingui- shed occurring at slightly higher energies than feature 1A. As the temperature decreases the spectra sharpen, the high energy component of the W-5 dou- blet moves to higher energy and the high energy component of the T-l doublet Fe2+ peak grows in intensity. At 80 K intermediate absorption occurs and feature 3A is still present. The growth of T-l in figure 3 is reminiscent of sample l and there is possibly a concomitant decrease in the absorption in the 1A region although this is now obscured by the feature 3A. It should be further noted that the
Fig. 2 : 5 7 Fe ~zssbauer spectra of sample 2 at 300K (a) and 80K (b). See table and text for kev.
2. Discussion.- Most of the spectral features of the new samples are explicable in terms of the previous- ly observed parameters for vesuvianites. However in"
view of the relative quadrupole splittings of the two six coordinate Fe2+ doublets it would now seem reasonable to assign the doublet T-l to Fe2+ ions in the distorted Al/Fe-sites and W-5 to Fe2+ ions in the tight more synrmetrical Al-sites. It appears that Fe2+ ions on Al/Fe-sites can participate in fast electron exchange with either Fe3+ or ~i'+ on neighbouring six or eight coordinate sites and thus give rise to the average doublet W-4. This behaviour is most clearly observed in the spectra of sample I.
Other processes involving Fe2+/Fe3+ or Fe2+/~i'+ on adjacent eight coordinate C-sites are also possible and these averaged spectra, together perhaps with continuous absorption of the Van Diepen /6/ type, in all probability contribute towards the complexity of the spectra of sample 3.
It should be finally pointed out that the occurrence of intervalence charge transfer in mine- rals is not uncommon and that these processes con- siderably complicate the determination of Fe2+/Fe3+
ratios and site populations by Mijssbauer spectros- COPY
-
position of the high energy arm of the T-l doublet is virtually temperature independent.
References
/l / Rucklidge, J .C., Kocman, V., Whitlow, S .H., Gabe, E.J., Can. Miner.
13
(1975) 15./ 2 / Manning, P.G., Can. Miner.
13
(1975) 110, /3/ Manning, P.G., Can. Miner.3
(1976) 216./ 4 / Manning, P .G., Tricker, M.J., Can. Miner.
2
(1975) 259.
1 5 1 Dambly, M., Pollack, H., Quartier, R. and Bruyneek
J. Physique Colloq.
3
C6-806.161 Lotgering, F.K. and Van Diepen, A.M., 3. Phys.
Chem. Solids,
2
(1977) 565.-2 0" 2 mm<'
Fig. 3 : ''~e Mgssbauer spectra of sample 3 at 300 K (a), 140 K (b) and 80 K (c). See table and text for key.
