Mossbauer spectra of orthopyroxenes at low temperature

Hafner, S. S.

Virgo, D.

English

NASA Technical Reports Server

Re-evaluation of t h e  Cation Dis t r ibu t ion  i n  Orthopyroxenes by t h e  
iY&sbauer Effec t  
David Virgo and Stefan S. Hafner 
Department of t he  Geophysical S c l  'ences . 
d h e  University of Ch.ica.go, Chicago, I l l i n o i s  60637 
~5 6 - 3 6 6  
Abstract 
D ( i n s e r t  a f t e r  t i t l e )  R653 Ju,y6B 
* e  
The reso lu t ion  of Plossbauer spee t rz  of orthopyroxenes i s  improved 
by measurement of t h e  absorbers a t  temperatures c lose  t o  l iquzd 
nitrogen. 
I' 
/ 
Some previously published data on t h e  fe r rous  :.on 
dis t r lbut j -on bykween M1 and M have been reevaluated. 2 
https://ntrs.nasa.gov/search.jsp?R=19680018510 2020-03-12T10:03:54+00:00Z
t 
- 1 -  
The hyperfine s p l i t t i n g  of FeS7 observed by Mossbauer resonance 
absorption has  been recent ly  used1-' t o  i nves t iga t e  t h e  d i s t r i b u t i o n  of 
Fe2' over t h e  two non-equivalent l a t t i c e  s i t e s  M and M 
pyroxenes (minerals c lose  t o  t h e  system FeSiO3-NgSiO ) .  3 
i n  ortho- 
Orbthopyroxene 
1 2 
spectra  cons is t  of two d i s t i n c t  doublets a r i s i n g  from t h e  nuclear  quad- 
2+ rupol-e in t e rac t ion  of Fe57 a t  M1 and M2. A s  a measure of t h e  Fe 
d i s t r ibu t ion ,  t h e  i n t e n s i t y  r a t i o  of t h e  doublets has  been used from 
spectra  taken a t  room temperature, and Fe2+ s i t e  occupancy f a c t o r s  f o r  
M 
data  published by difEerent groups on orthopyroxenes which a re  expected 
t o  exhib i t  s imi l a r  r e s u l t s ,  While e.g, Bancroft, Burns, and Howie 
found t h a t  orthopyroxenes from metamorphic rocks (specimens 68671, 355 
and K23) a r e  considerably disordered with respect  t o  Fe-Mg a t  El and M 
Evans, Ghose, and Hafner' quote s i t e  occupancy f a c t o r s  which ind ica te  
subs t an t i a l  ordering €or samples with s imi l a r  Fe/(Fe+Mg) r a t i o s .  The 
and M2 have been derived. However, t h e r e  i s  a discrepancy i n  t h e  1 
1 
1' 2 
inconsis tencies  appear t o  be pr imari ly  due t o  t h e  poor reso lu t ion  of t h e  
t w o  overlapping doublets a t  room temperature and t o  d i f fe rences  i n  t h e  
individual  techniques used t o  f i t  Lorentzian curves t o  t h e  absorption 
spectra .  
We have ovezcome t h i s  problem of poor reso lu t ion  by carrying out 
experiments with t h e  absorbers he ld  a t  temperatures c lose  t o  l iqu2d 
nitrogen. Resolution is  s t r i k i n g l y  improved due t o  a d i f f e r e n t i a l  
increase of t h e  quadrupole s p l i t t i n g  a t  M Measurements of t h e  
r e l a t i v e  i n t e n s i t i e s  of t h e  doublets using t h i s  technique have revealed 
and M 1 2'  
. 
- 2 -  
l a rge  discregancies with already published r e s u l t s ,  and we f e e l  t h a t  revis ion 
of some of t h e  room-temperature data w i l l  be necessary. Sprenkel-Segel and 
per lo^^'^ have taken spec t ra  a t  77'K of chondr i t ic  meteori tes  which include 
pyroxenes as s i l i c a t e  phases. From a spectrum of t h e  Johnstown meteorite 
which contains orthopyroxene (Fe/(Fe+Mg) = 0.24) but no detectable  o l iv ine ,  
7 they found t h a t  6 percent of Fe2+ i s  located a t  M1. This value is  i n  surpr iz ing-  
l y  good agreement with t h e  assumption of a simple, i d e a l  so lu t ion  model a t  each 
s i t e  f o r  metamorphic samples of t h e  orthopyroxene system . 5 
The samples discussed i n  t h i s  paper include two of t h e  metamorphic 
orthopyroxenes inves t iga ted  by Bancroft, Burns and FIowie1'8 (Fe/(Fe+Mg) = 
0,531 and 0 .775) ,  a na tu ra l  and heated metamorphic sample discussed by Evans 
Ghose and Hafner' (Fe/(Fe+Mg) = 0 , 5 3 2 ) ,  a na tu ra l  and heated igneous mthopyrci- 
xene' (Fe/(Fe+Mg) = 0,269) , and a synthe t ic  orthopyroxene" (Fe/(Fe-i-Mg) = 0.80) . 
A CoS7 source diffused i n t o  Pd was used. The absorbers were prepared by mixing 
0.5 gr. l u c i t e  with 60-80 mgr. of powdered mineral and pressing t h e  mixture t o  a 
p e l l e t .  The mass of natupal  f e r rous  ion per  u n i t  area var ied from 3.1-4.6 mgr. 
Fe/cm . No cor rec t ion  f o r  s a tu ra t ion  (thickness) was made. The spectra  were 2 
f i t t e d  by Lorentzians y i thou t  using any cons t ra in ts .  The halfwidths of t h e  peaks 
7 ( f u l l  widths a t  ha l f  i n t ens i ty )  were found t o  be-  0.30 d s e c  a t  77'K. For 
f u r t h e r  d e t a i l s  abouk experiment and s p e c t r a l  ana lys i s ,  t h e  reader  i s  r e fe r r ed  
t o  r e f .  4 -  
It i s  known t h a t  t h e  i n t e n s i t i e s  of t h e  l e f t  and r i g h t  peaks of t h e  ortho- 
pyroxene doublets a r e  general ly  not symmetric . Employing t h e  same notat ion a s  4 
5 2' previously,  , t h e  r e l a t i v e  i n t e n s i t i e s  of t h e  two doublets a r e  (A1+B1] /(Al+B1SA2+B2) 
an.d (A 3-B )/(A1+Bl+A2+RZ) , respect ively,  A s  measure of t h e  i n t e n s i t i e s ,  t h e  a reas  
of t h e  f i t t e d  Lorentzians (product of halrwidth and maximum in t ens i ty )  were taken e 
2 2  
5 .  . e '  
I P 
Plo-ts  (Fig. . l -3)  of some of t hese  samples a t  77OK compared with t h e  
room-temperature spec t ra  i l l u s t r a t e  t h e  improved reso lu t ion  of Fe2+ a t  t h e  
two s i t e s .  Also ,  Figs. 2b and d and Figs. 3a and b demonstrate t h e  exchange 
of Fe2+ and Mg between s i t e s  by heatirrg a na?ural orthopyroxene a t  1000 C 
f o r  7 days. With i-ncreasing temperature, t he  f e r rous  ion concentration in-  
c reases  a t  t h e  M s i t e s  (outer  peaks i n  Figs.  2b,d and 3a,b) a t  t h e  expense 
of t h e  magnesium concentration. 
t a ined  f o r  a highly ordered Mg r i c h  orthopyroxene (Fe/(Fe+Mg) = 0.269) . 
0 
1 
Fig. 3a shows a l i m i t  t o ' t h e  reso lu t ion  ob- 
However, t h e  spectrum of t h i s  specimen heat  t r e a t e d  a t  1000°C f o r  7 days, 
c l e a r l y  exh ib i t s  t h e  presence of  divalent: i r o n  a t  both s i t e s  (Fig. 3b). 
Tables 1 and 2 exhib i t  s i t e  occupancy f a c t o r s  of t h e  fe r rous  ion 
ca lcu la ted  from t h e  Fe dis t r i .but ion between t h e  s i t e s  according to t h e  method 
used by Bancroft, Eurns, and Howie (Tb. l), and a l s o  t h a t  used by Evans, Ghose 1 
4, 5 and Hafner (Tb. 2 ) .  Using e i t h e r  of these  methods, it i s  apparent t h a t  
deviations e x i s t  between t h e  r e s u l t s  a t  77'K and the  values derived from room- - 
temperature spec t ra  (c f .  Tb. 1). We bel ieve t h a t  these  e r r o r s  must be 
a t t r i b u t e d  to t h e  individual  computer f i t t i n g  technique when applied t o  
unresolved o r  poor1.y resolved absorption peaks. 
The. d i s t i n c t i o n  between t h e  doublets at Ml and M2 depends on t h e  Fe/Mg 
4 r a t i o  of t h e  orthopyroxene . Only f o r  compositions very r i c h  i n  i r o n  
(Fe/(Fe+Mg)) 0.75 :an t h e  room-temperature spectrum be r e l i a b l y  f i t t e d  without 
cons t ra in ts .  
fc r  ha l f  widths and pos i t ions  of  t h e  doublets i n  t h e i r  l e a s t  squares ana1ysj.s. 
We bel ieve t h a t  using cons t r a in t s  f o r  strongly overlapping peaks i s  a r a t h e r  
involved problem, and possible  e r r o r  sources were discussed i n  d e t a i l  3 y  
For r a t i o s  I'e/(Fe+Mg)( 0.75, Evans e t  al.4 introduced cons t r z in t s  
'. * '  
m . - l ! -  
these  authors. 
probably accidental .  Al ternat ively , Bancroft e t  a1  f i t t e d  orthopyroxene 
The agreement between t h e  two s e t s  of data i n  Table 2 i s  
spec t ra  w i t h  Fe/(Fe+Mg) r a t i o s  of about 0.5 without any cons t ra in ts .  
of t h e i r  samples reported t o  be considerably disordered t u r n  out t o  be 
Some 
nearly ordered (Tb, 1). 
There a r e  addi t iona l  inconsis tencies  between t h e  data of Tables 1 and 2, 
F i r s t l y ,  Bancroft e t  al.' used a constant C = 0.9 t o  account f o r  t h e  non- 
equivalent r e c o i l l e s s  f r a c t i o n  of FeS7 a t  M and M while Evans e t  a l .  1 2 .  
assume 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  i s  t h e  same a t  both s i t e s .  We ca re fu l ly  
s tudied t h e  spectrum of orthoferrosilite'O (FeSi03) and found 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  i s  i n  f a c t  very near ly  t h e  same (C = 1.0 5 0.1). I n  
addi t ion,  i f  a d i f f e r e n t i a l  r e c o i l l e s s  f r a c t i o n  ex is ted  a t  M1 and I?$, C i s  
expected t o  be dependenr on t h e  temperature. 
t h i s  reason, C = 1 was used i n  Table 2 as prevlously. 
column, C was s e t  C = 0.9 t o  provide t h e  proper b a s i s  f o r  comparison with 
data of columns 4 and 5. 
This  was not  obscrved. For 
I n  Table 1, 1.a.st 
I 
1 
1 _-  
Secondly, t he re  i s  an ambiguity i n  deriving t h e  Fe2+ s i t e  occupancy 
fac-tors of Ml and M2 from t h e  Fe2+ d i s t r ibu t ion .  We have ca lcu la ted  these  
f a c t o r s  by mul t ip l ica t ion  of t h e  i n t e n s i t y  r a t i o  of t h e  M1 and M2 doublets 
, 
by t h e  t o t a l  r a t i o  of 2 Fe/(Fe+Mg) of t h e  mineral according t o  two formula 
u n i t s  per u n i t  eel?.. Other groups132 prefer red  t o  derive t h e  f a c t o r s  
from t h e  product of Yhe i n t e n s i t y  r a t i o  of t h e  M 
ca t ion  percent Fe2', ca lcu la ted  on the  b a s i s  of 6 oxygen atoms pe r  formula 
un i t .  
by a few percent. 
and M 1 2 doublets and. t h e  
These two techniques r e s u l t  i n  s i t e  occupancy f a c t o r s  which d i f f e r  
While t h e  77'K spectra  demonstrate t h e  p o t e n t i a l  of t h e  Mossbauer 
method t o  determine the i ron  d i s t r i b u t i o n  very accurately,  possibly w i t h  
a prec is ion  of a f r a c t i o n  of one percent,  t h e  data  reported i n  t h i s  note 
are st i l l  regarded a s  preliminary. We h e s i t a t e  t o  ass ign appropriate  e r r o r  
estimates a t  t h i s  time. 
the hyperfine parameters as funct ion of t he  degree of €e/Mg order will be 
submitted elsewhere. 
A de ta i l ed  e r r o r  ana lys i s  and an inves t iga t ion  of 
F ina l ly ,  it is  t o  be noted t h a t  measurements of orthopyroxene spec t ra  
between 77OK and 1.7OK do not  l ead  t o  a f u r t h e r  improvement of the reso lu t ion  
of t h e  overlapping doublets. 
complicated by re laxa t ion  e f f e c t s  . 
A t  temperatures below 77OK, the  spec t ra  a re  
12 
P 
Acknowledgements: 
Foundation (Grant GA-113q) . One of us (D.V.) g r a t e fu l ly  acknowledges 
This work was supported by t h e  National Science 
a postdoctoral  fellowship supported by N.A.S .A. (Grant NSG-366 of 
D r .  Edward Anders) . 
. .  
. . .  
REFERENCES 
1. G.M. Bancroft, R.G. Burns., and R.A. Howie., Nature 2 - 3, 1 2 2 1  1 67).  
2. G.M. Bancroft, and R.G. Burns., Earth and Planetary Science Le t t e r s  
- 3, 1 2 5  (1967). 
3 .  R.W. Dundon, and L.S. Walter, Earth and Planetary Science Le t t e r s  
- 2, 372 (1967).  
- 4.- -B.J.  Evans, S ,  Ghose and S.S. Hafner, Jour. of Geology 75, 306 (1967).  
5. . S. Ghose, and S.S. Hafner, 2 .  Kris ta l logr .  125, 1 5 7  (1967).  
6. E.L. Sprenkel-Segel and G . J .  Perlow, Meteori t ics  3, 126 (1967). 
7 .  E.L. Sprenkel-Segel and G . J .  Perlow, Icarus  8, 66 (1968) . 
8. W e  a r e  indebted t o  Professor R.A. Howie from t h e  University of  
Cambridge f o r  supplying t h e  samples. 
9 .  We a r e  indebted t o  M r .  I .S ,  McCalltim f o r  supplying t h e  sample which 
was col lec ted  from t h e  lower gabbro zone i n  t h e  St i l l -Water  
complex. 
1 0 ,  We a r e  indebted t o  Professor S;I .  Akimoto from t h e  University of 
Tokyo f o r  t h e  synthesis  of samples. 
11. R.A, IIowie, Min. Soc. Am. Spec. Paper 1, 213 (1963)-  
1 2 .  G.M. Kalvius and S.S. Hafner, Bull. Amer. Phys. SOC. - 1 3 ,  29 (1968).  
Figure captions 
Figure 1 .  a) Spectrum of orthopyroxenes (Fe/(Fe+Mg) = 0.80) synthesized 
10 at 12OO0C, h 5  K bars for 40 minutes and measured at 
room t ernpera t ure . 
0 
b) Same sample measured ato 77 K. The sol id 1 ines are the 
Lorentzi an f i ts. 
Figure 2. a) Spectrum of natural orthopyroxenes 372.18 (Fe/(Fe+Mg) = 0.532) 
measured at room temperature. (Fig. 5 of ref, 4). 
0 b )  Same sample measured at 77 K. The solid lines are the 
Lorentzian fits. 
c )  Spectrum of orthopyroxene 37a18 (Fe/(Fe+Mg) = 0.532 heated 
at 1000°C for 7 days and measured at room temperature. 
(Fig. 7 of ref. 4) .  __  
d) Same sample measured at 77OK.  The sol id 1 ines are the 
Lorentzian fits. 
Figure 3. a) Spectrum of nahural orthopyroxene N-5 (Fe/(Fe+Mg) = 0.269 9 
measured at 77 K. The solid Tines are the Lorentzian fits. 
, .  
b)  Same sample heated at 1000°C for 7 days and measured at 77OK.  
The solid lines are the Lorentzian fits. 
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Re-evaluation of the cation distribution in orthopyroxenes by the Moessbauer effect

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