Cation distribution in clinopyroxenes, olivines, and feldspars separated from Apollo 11 and 12 rocks determined with Mossbauer spectroscopy of Fe-5

Hafner, S. S.

English

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64	 -t:
NATIONAL AERON1UTICS AND SPACh ADMINISTRATION
LUNAR SAMPLE ANALYSIS PROGRAM
CATION DISTRIBUTION STUDIES IN CLINOPYROXENES, OLIVINES AND FELDSPARS
USING MOSSBAUER SPECTROSCOPY OF 57Fe.I
Final Technical Progress Report
Period ending January 31, 1971
by
Stefan S. Hafner
Principal Investigator; Stefan S. Hafner
Final Report
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Prepared under Contract No. NAS-9-8080 	 v°0
by	 (	 W
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THE UNIVERSITY OF CHICAGO 	 z'
DEPARTMENT OF THE GEOPHYSICAL SCIENCES
CHICAGO, ILLINOIS 60637	 ► ^`+'z^	 o
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NATIONAL AERONAUTICS AND SPACE, ADMINISTRATION
Manned Spacecraft Center
Lunar Receiving Laboratory
Houston, Texas
1 Also submitted to the Second Lunar Science Conference, Houston,
January 1971.
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— 1 —
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ABSTRACT
Magnesium in clinopyroxenes with more than 20 percent wollastonite
occurs almost exclusively at the Ml position. In calcium-poor pigeonites
from rocks 12018, 12021, and 12053 Mg is significantly more disordered.
An olivine from 12018 exhibits 20 percent ordering of iron in one of
the M-positions. The Mg,Fe 2+ distribution in clinopyroxenes and olivines
is interpreted in terms of equilibrium distribution temperatures
estimated from heating experiments and subsolidus cooling history. Fe 2+
in feldspars occurs at calcium as well as aluminum positions. No Fe 3+
has been detected in our mineral separates.
Contents
Introduction
Experimental results
Conclusions
Publication list
References
Figure captions
Tables
i
a
f
- 2 -
INTRODUCTION
The Mossbauer absorption spectra of S7 Fein 15 clinopyroxenes,
olivines, and feldspars separated from Apollo 11 and 12 rocks, and a
pyroxene and a glass specimen from the Apollo 11 soil have been studied.
The aim was to obtain information on the oxidation state of iron and the
intracrystalline distribution of iron over the nonequivalent lattice sites.
The distribution of Mg t+ ,Fe2+ over the sites Ml and M2 in natural pyroxenes
is known to be dependent on the cooling history of the rock. Similar
relationships between Mgt+ , Fe2+, Ml, M2 and cooling history appear to
exist in olivines. We have made a number of heating experiments with
clinopyroxenes and oli vines to determine the temperatures of equilibrium
distribution in the natural crystals, and to investigate the ordering
and disordering kinetics of the Mg t+ , Fe 2+ exchange between the Ml and
M2 sites.
EXPERIMENTAL AND RESULTS
X-ray emission microanalysis, Mossbauer spectroscopy and heating
experiments were carried out as described previously l . The chemical
compositions of the clinopyroxene separates from Apollo 11 and 12 are
c
plotted on to the pyroxene quadrilateral En-Di-Hd-Fs in Figure 1.
Plotted along the En-Fs join are the compositions of the olivines from
12053-79 and 12018 9 35. Site occupancy data for the Apollo 12 clino-
pyroxeries are given in Table 1. Similar data on the Apollo 11 samples
6
•.
-'3 -
were reported in reference 1, Table 6.
The Mossbauer spectra of 57 Fein olivines were measured at absorber
temperatures between 300 and S00 0C since the quadrupole doublets at M1
and M2 are best resolved in that range 2. Spectra of an iron-rich olivine
(Rockport fayalite, Fa97) and an iron-poor sample (rock 12018,35 Fa 36 )
are compared in Figure 2. The resolution of both the high and low
velocity doublets shown in this figure is characteristic of the olivine
solid solutions. We generally take the area ratio of the high velocity
peaks as a measure for the distributioni of Fe 2+ over Ml and M2.
Unfortunately the assignment of the doublets to the M-sites is not
possible at this time 2 . The results which include distribution data
of five terrestrial samples are shown in Table 2.
CONCLUSIONS
Oxidation state of iron
Ferric iron was not detected in any of the ^_.:parates from the rocks
or the soil. In clinopyroxenes the Fe 3+/(Fe2++Fe3+) ratio was 0+0.002;
in the pyroxene dnd glass separates from soil 10084 this ratio was 0+0.01.
Approximately 7 percent of the total iron in this soil was metallic3.
The spectrum is indicative of the presence of some iron in a paramagnetic
state (less than 3 percent).
r-
	 Distribution of iron in feldspars
Iron in the plagioclase feldspars from 1004 11 and 12021 is in the
ferrous state and occurs at two nonequivalent positions with different
coordination. The nuclear quadrupole spli_ttings, isomer shifts, line-
'	 widths and area ratios are shown in Table 3. Pattern 1 probably results
•- 4 -
from Fe 2+ at the tetrahedrally coordinated Al 3+ positions whereas
pattern 2 may be assigned to the Ca-sites.
t
Distribution of iron in clinopyroxenes
The exchange energy of the reaction
t
Fe (M2) + Mg (1) 
"Z ' Mg (M2) + Fe (Ml)	 (1)
in pyroxenes within the enstatite-diopside-hedenbergite-ferrosilite
quadrilateral is primarily determined by the amount of calcium present
at the M2 sites, i.e. the amount of wollastonite component in the
solid solution. Space group symmetry and the presence of small amounts
of foreign cations at the M-sites appear to be of minor importance.
Our limited data suggest a simple, linear relationship between the
standard Gibbs-free energy difference, Q GE and the wollastonite con-
centration; but more data will be needed to establish this precisely.
Based on this assumption, plots of temperature versus the equilibrium
distribution constant k can be drawn where k is equal to the ratio
(Fe/Mg) Ml/(Fe/Mg)M2 . The equilibrium temperature which corresponds to
the observed Mg,Fe site occupancy (Table 1) can thus be deterr.ined from
this plot. This principle is shown in Figure 3.
F
	
	 A GE in clinopyroxenes with WO25-36 is approximately 8-10 kcal
per formula unit M2Si 206 . Therefore, only very small changes in the
equilibrium site occupancy of Mg and Fe at Ml and M2 are expected in the
temperature range from 0 to 1000°C. This has been confirmed by
experiments (Table 1). Precise occupancy determinations are hampered
-'S -
by the large chemical zoning and the pigeonite exsolution, character-
istic of these crystals. Our data indicate, however, complete ordering,
at least within the experimental error, with Mg exclusively at the Ml
sites (cf. also ref. 1). We do not believe that there is any indication
of Ca disordering. The apparent excess of cations Ca and Fe at M2 in
some samples of Table 1 can be largely accounted for by corrections
which result from the elements Ti, Mn, Cr, Al.
AGE in apparently homogeneous pigeonites with compositions WO5-10
is approximately 4-S kcal per formula unit. Disordering as well as
ordering experiments of the yellow-green colored pigeonite, 12021,150
suggest a close similarity to that of the orthopyroxene system 4 . Some
data for ortho- and clinopyroxenes with compositions within the pyroxene
quadrilateral are plotted in Figure 3. The natural Mg,Fe distribution
observed in the yellow-green pigeonite 12021,150-P1 corresponds to an
	
`•	 equilibrium temperature of 570°C. This temperature is substantially
	
!	 lower than the quench-in 'temperature for the short range ordering
	
'	 (rate constant K1 29 cf. ref. 1) which was determined from an ordering
experiment (Table 1) to be 810°C. The natural Mg,Fe distributions in
orthopyroxenes from Hawaiian lavas l correspond to equilibrium tempera-
;
tures generally higher than the quench-in temperature for orthopyroxenes.
The Mg,Fe distribution observed in the pigeonite 12053,79-P1 (Table 1)
represents an equilibrium temperature of 710°C. Ordering experiments
have not been carried out but the equilibrium temperature does suggest
a more rapid cooling of rock 12053 through the critical range of 600-500°C,
compared with rock 12021.
- 6 -
In clinopyroxenes with compositions Wo10-20 the observed degree
of Mg,Fe disorder over Ml and M2 is high. It seems likely that the
ordering processes in these chemically inhomogeneous pyroxenes are
hindered by the complexity of the augite-pigeonite exsolution.
Distribution of iron in olivines
The natural Mg,Fe distribution in an olivine separate (Fa 36)from
rock 12018 exhibits approximately 20 percent more iron in one of the two
nonequivalent M-positions (Table 2). After heat treatment at 1155°C for
i
7 days the distribution was found to be almost completely disordered. A
somewhat ordered terrestrial olivine (Bl) did not show disorder after
heating at 915 0C for 1 day or 10500C for 5 days. It appears that the
Mg,Fe exchange in olivines is quenched-in at much higher temperatures
i
than in pyroxenes. But more experiments will be needed on olivines to
establish the nature of this exchange reaction between the M-sites and
its relationship to the cooling history of the mineral.
6
•- 7 -
Publications
H. Fernandez-Moran, S.S. Hafner, M. Ohtsuki and D. Virgo: Mossbauer
effect and high-voltage electron microscopy of pyroxenes in
Type B samples. Science, 167, 686-688 (1970).
H. Fernandez-Moran, M. Ohtsuki, S.S. Hafner and D. Virgo: High voltage
electron microscopy and electron diffraction of lunar pyroxenes.
Proc. Apollo 11 Lunar Sci. Conf. Vol. 1, 409- LI17 (1970).
S.S. Hafner and D. Virgo: Temperature-dependent cation distributions
in lunar and terrestrial pyroxenes. Proc. Apollo 11 Lunar Sci.
Conf. Vol. 3, 2183-2198.
S.S. Hafner, B. Janik and D. Virgo: State and location of iron in
Apollo 11 samples. Mossbauer Effect Methodology Vol. 6
(E. Gruverman ed.) , 193-207 (1971) .
Submitted, being submitted: Three additional papers on Apollo 11 and
12 samples.
- 8 -
References
1. S.S. Hafner and D. Virgo: '.emperature dependent cation distributions
in lunar and terrestrial pyroxenes. Proc. Apollo 11 Lunar Sci. Conf.
Vol. 3 9 2183-2198 (1970), and references therein.
2. W.R. Bush, S.S. Hafner, and D. Virgc: Some ordering of iron and
magnesium at the octahedrally coordinated sites in a magnesium-rich
olivine. Nature 227, 1339-1341 (1970).
3. S.S. Hafner, B. Janik, and D. Virgo: State and location of iron in
Apollo 11 samples. Mossbauer Effect Methodology Vol. 6, 193-207(1971.).
4. D. Virgo and S.S. Hafner: Fe 2+,Mg order-disorder in heated ortho-
pyroxenes. Mineral. Soc. Amer. Spec. Pap. 2, 67-81 (1969), and
references therein.
Figure captions
Fig. 1. Average compositions (molecular percents) of clinopyroxene
separates from rocks 12018 ( D), 12021 ('-^ and 12053 (p) .
The compositions of two olivine separates from 12018 (;,j)
i
and 12053 (.J) are plotted on the En-Fs join. Also plotted
are the previously published data from Apollo 11 rocksl.
Fig., 2. S7.resonant absorption spectra of an olivine separate from
rock 12018 Fa 36 (top) , and a fayalite Fa 97(bottom.
Fig. 3. Plot of temperature against the equilibrium distribution
constant k (Fe/Mg) X11/ (Fe/hIg) M2 for orthopyroxenes ((Mg, Fe) SiO3)
pigeonite 12021,150-P1 ((Mg,Fe)0.93Ca0.07SiO3); pigeonite
.!
__•^	 12053179-P1 ((Mg,Fe)0.90Ca0.10SiO3), and augite 10044,26-P2
((Mg,Fe)0.64Ca0.36SiO3). The slopes of the lines are con-
str42cted assuming a constant standard Gibbs free energy, AG E,
determined from a linear plot of 0 GE and molecular percent
6
- 9
Fig. 3.	 wol.lastonite in the sample (numbers in kcal per mole). The
(conttd.) crosses M correspond to the respective measured k values
for the unheated samples. For the orthopyroxene line, the
crosses correspond to volcanic samples from Hawaiil . The
circles (p) correspond to the final distributions in
orde2ing runs of initially disordered samples. For the
(1g,Fe)0.93Ca0.07SiO3 and (h'g,Fe)SiO 3 samples, these were
respectively 600 0C for 7 days and SSO 0C for 7 days.
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- 13 -
Table 3, Hyperfine interaction of 57 Fein lunar feldspars at 2980K.
afrom J. V. Smith et al., Proc. Apollo 11 Lunar Sci. Conf. Vol. 1, 897-925 (1970)
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Cation distribution studies in clinopyroxenes, olivines and feldspars using Moessbauer spectroscopy of Fe 57  Final technical progress report, period ending 31 Jan. 1971

Cation distribution studies in clinopyroxenes, olivines and feldspars using Moessbauer spectroscopy of Fe 57 Final technical progress report, period ending 31 Jan. 1971

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