MÖSSBAUER SPECTROSCOPY OF ELECTRON IRRADIATION EFFECTS IN ORTHOPYROXENES

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MÖSSBAUER SPECTROSCOPY OF ELECTRON IRRADIATION EFFECTS IN ORTHOPYROXENES

R. Scorzelli, E. Saitovitch, J. Danon

To cite this version:

R. Scorzelli, E. Saitovitch, J. Danon. MÖSSBAUER SPECTROSCOPY OF ELECTRON IRRADI-

ATION EFFECTS IN ORTHOPYROXENES. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-

459-C1-460. �10.1051/jphyscol:19801180�. �jpa-00219669�

JOURNAL DE PHYSIQUE Colloque C1, supplkment au n ^O 1 , Tome 41, janvier 1980, page C 1-459

~@sSBAUER SPECTROSCOPY OF ELECTRON IRRADIATION EFFECTS

R.B. Scorzelli, E.B. Saitovitch and J. Danon

Centro BrasiZeiro de Pesquisas Fisicas, Av. Wenceh

Orthopyroxenes (Fe,Mg)2Si206 occur commonly as a major constituent of mafic, igneous and metamorphic rocks, being a m&

neral of great mineralogic and geophysi- cal interest. The orthopyroxene crystal structure provides two non-equivalent oc tahedrally-coordinated positions, M1 and M2, and the only cations present in subs- tantial quantities at these positions are magnesium and iron (1). The site popula- tion observed in natural and synthetic orthopyroxenes can be used as a thermal indicator on the basis of thermodynamic and kinetic arguments (2). Several factors modify the distribution equilibrium in these minerals, such as high pressures arising from shock (meteoritic impact).

Radiation effects from particle bombarde-

I

ment (cosmic rays and natural radioactivL ty) can possible disturb the equilibrium site population.

In this paper an attempt has been ma- de to determine the influence of a 2 MeV electron irradiation on the orthopyroxene structure. We present the results of a Mhsbauer study of the distribution of

~ e in the irradiated orthopyroxene as ~ + a function of the radiation doses and the annealing behaviour of the system.

The studied samples are highly puri- fied natural OPX orthopyroxene with the composition Fe0.55Mg0.47Si03. The experi- ments have been performed with powder sam ples both source and absorber being kept at llOK. A 2 .MeV electron beam had been used for the irradiation experiments according to previous description ( 3 ) .

The hyperfine spectrum of the orthopy roxene Fe0.88Mg0.12Si03~OPX) at llOK con- sists of: a inner doublet assigned to pe2+ ions in the M2 site (IS=O. 90 mm/s AE -2.08 rnm/s) and the outer one assigned toQ;e2+ in the M1 site (TS=0.92 mm/s

:Zau Brag, ^, 71, Rio de, Janeiro, Brasi 2.

AE =2.85 mm/s). The intensitiep of the ,Q

inner two .peaks are not equal, due,to tex ture effects, the line at lower velocity being the more intense.

. < - . . . . . " , -

'The M6ssbauer spectrum of the irradis ted ~ P X ortho.pyxoxene-~ shows an. alteration comp,ared, 'to the .,spectrbm o£ khe unirradia ted ~q+ple . , .d=th the, app&ar,$nc~:+of a .third, d o u b ' k I I&=O i9i mm/s! :,, AE '=2{$9 mm/s)

,

< , , . , Q

which ean ;be gttributed to

ai

1 ,. , '

med at ex~ens&s of irdi in

he

1

]

1

-

I

By keeping the irradiated sample in liquid nitrogen the third doublet.has been~founa .to disappear e-xponentially with the time that has elapsed after the end .of.-the, ,i~radi,ation ,(see Table, I),

.

Similar results have been,fou$ in a XYZ ,orthopyroxe,ne [,FeO 88Mg0i 12Si03) ,which has .better ,resolved :doub+ets for iron. in the ML..and .M2 >sites.. -In ,this case ,the po- sition of the new doublet can be seen in the Mdssbquer sge,ctru,m :as is illustrated in fig. 1,.

T+e e£fect,of,,electron irradiation on orthopyroxenes differ completely from that of shocks (4,5). When shock is applied to orthopyroxene crystals, a permanent alte- ration of the cation distribution takes place, meaning that the initial crystal

31 irrldiated

'

' ~ $ ! ~ ~ k '

24 haurs after irradiation 48 hqurs after

irradiation lg2

irradiat~on

fotal errors less than + ^{0 05 mmlr Table I}

..

2 68 2.01 2.19,

..

2-70 2.03 zf1, 2 85 2 0 2 2.21 2.91 2.00 2 14 . . . . . . , , . ,

-2.92 2 05 2 09 26 29 53.48 20 20'

21.i0 11 00 21 64 2092 57 72 22.95 19.49 58.69 18.79 21 52 68.62 ' 9.48

0 33 q 34; 0.32 . - 0.32: S;0.36 0 33 0 31 %\o 35 0 3 5 0 2 8 ~ 0 39 0.28

. - . . -

b

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801180

c

structure has been destroyed and that a new phase has been reached.

Our experiments with 2 MeV electron.

irradiation reveal a different behaviour, namely that there is a complete recovery even at low temperature. This means that irradiation with a doses up to 5400 Mrad produces only a transient effect, at least as seen by M6ssbauer spectroscopy, connec ted to a local lattice disturbance.

T R A N S M I T A N C E ( % I

I

F i g . 1

References

1. Hafner, S.S. and Virgo, D., Am. Miner.

56 (1971) 850.

-

2. Bancroft, G.M., Mbssbauer Spectroscopy (Mc Graw Hill) 1973.

3. ScorZelli,R.B., Saitovitch, E.B. and Danon, J., J.Phys. C6, 307 (1976) 801.

4. Dundon, R.W. and Hafner, S.S., Science 147 (1971) 4009.

5. Hafner

,

.

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