MÖSSBAUER MEASUREMENTS OF THE SOVIET LUNA-16 AND LUNA-20 SAMPLES

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MÖSSBAUER MEASUREMENTS OF THE SOVIET LUNA-16 AND LUNA-20 SAMPLES

T. Zemčík, K. Raclavskí

To cite this version:

T. Zemčík, K. Raclavskí. MÖSSBAUER MEASUREMENTS OF THE SOVIET LUNA-16 AND LUNA-20 SAMPLES. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-549-C6-551.

�10.1051/jphyscol:19746117�. �jpa-00215729�

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 35, Dkcembre 1974, page C6-549

MOSSBAUER MEASUREMENTS OF THE SOVIET LUNA-16 AND LUNA-20 SAMPLES

T. ZEMC~K and K. RACLAVSK~

Institute of Physical Metallurgy, Czechoslovak Academy of Sciences, Brno, Czechoslovakia and Mining University of Ostrava, Ostrava, Czechoslovakia

Rbsum6. - On presente des resultats recents de spectroscopie Mossbauer de 57Fe a temperature ambiante dans la regolithe des Luna-16 et Luna-20 sovittiques. Dans la partie non magnttique du spectre, six composantes quadrupolaires sont r6solues, correspondant a deux types de fer dans des oxydes mixtes Fe-Ti (ilmenite, spinelle) et ^9, quatre dans des silicates (olivine, deux sites de pyroxene et une fraction vitreuse). Des positions des raies magnetiques externes, mesurtes en utilisant une fenstre de vitesse, on a deduit I'alliage du fer lunaire avec le nickel. On compare la distribution quan- titative estimee dans les matkriaux de Luna-16 et de Luna-20 ; ce dernier contient plus d'olivine, moins d'ilmenite et de metal et est plus riche en nickel. A cBtt des kchantillons macroscopiques, on a etudik des particules individuelles dans lesquelles on a identifie quelques-uns des minkraux cites (metal, ilmenite et pyroxhne avec olivine).

Abstract. - Up-to-date results of the room temperature Mossbauer measurements on Fe57 in the Soviet Luna-16 and Luna-20 regolith are presented. In the non-magnetic part of the spectra six quadrupole components were resolved, corresponding to two Fe types in mixed Fe-Ti oxides (ilme- nite, spinel) and four in silicates (olivine, two pyroxene sites and a glassy fraction). From the outer- most magnetic lines positions, measured in ^<( velocity-window >> mode, the alloying of lunar iron with nickel was deduced. Altogether, the quantitative estimate of iron distribution and content of the above phases is compared for the Luna-16 and Luna-20 materials, the latter containing more olivine, but less ilmenite and metal. Along with macroscopic samples, individual particles were investigated in which some of the discussed minerals were identified (metal, ilmenite and pyroxene with olivine).

1. Introduction. -Because of the abundance of iron - as one of the most significant cosmochemical elements - in most of the main mineral phases of lunar regolith, Fe57 Mossbauer spectroscopy belongs to the principal methods for studying these materials.

Due to the variability of iron bonding in these phases, the resolution was until now not satisfqctory for their unambiguous identification. In this respect, the situa- tion seems to be critical in the middle, non-magnetic part of the lunar regolith spectra.

In our previous works [I, 21 we have gradually increased the number of resolved quadrupole compo- nents up to five, which was still not satisfactory and led to the discrepancies in the interpretation of our as well as literature data. In this work we have distin- guished and interpreted six quadrupole components in the spectra of the Luna-16 and Luna-20 regolith.

Besides that, taking into account also the magnetic contribution of lunar metal, the iron distribution among individual phases and their relative content was estimated. From the hyperfine fields of the magnetic component we determined the alloying of the lunar iron with nickel for both of the above materials. Based on the achieved systematics of the quadrupole split- tings of phases present in macroscopic samples we succeeded to identify some of the corresponding minerals in selected small particles of lunar regolith.

2. Experiments and results. - Macroscopic samples of the Czechoslovak portion of the Luna-16 regolith were encapsulated into two flat acrylic containers, the first containing the average material (40 mg/cm2, 13.7 % Fe, sample L-16,) and the second the fine frac- tion below 0.05 mm (42 mg/cm2, 5.1 % Fe, L-16,).

Similarly, the sample of the Luna-20 average material was prepared (1 52 mg/cm2, 5.1 % Fe, L-20,).

All spectra were taken at the double parabolic time mode Mossbauer spectrometer NP-255 (KFKI, Buda- pest, Hungary) at room temperature with the ca 5 mCi Co57 in Pd source in transmission arrangement.

Punch tape output served for further data processing at the ZPA-600 computer with the aid of a versatile system of least-square programs allowing different constraints. Calibration of the velocity scale was performed by measuring the powdered sodium nitro- prusside, or, in the case of ⁽⁽ velocity-window ⁾⁾ measurements, the absorbers containing powdered Fe and Fe-Ni alloy in the mixture of terrestrial mine- rals [2]. Isomer shifts are related to the used palladium source.

The main attention was paid to the middle part of the spectra ( 5 ca 4 mm/s range) measured into 256 channels with a rather high counting statistics (up to lo7 counts/channel). Except for the L-16, sample (which was only shortly available for investi-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746117

C6-559 T. Z E M ~ ~ K AND K. RACLAVSKf

gation) at least two runs of measurements were per-

formed. Examples of the measured spectra are shown ^{2 3 4 5 6}

in figure 1. L - T + R U N I y s ^~6

R U N Z A r o 4 - 1 6

TZ9 p78 *23

4 7 2 4% 4s 4 2 5

W A j m &? -. 12977 0 2 4

L-16, O ' ) " * m "?*a "?s &15 4 2 2

L - 3 RUN 1 + 4 ++ ^{+s +a +s} p

RUNZ 4 ^{$ 6 +22 +24 +76 +a}

W A 4 4 +ti +e +2? +s

L

ILMENITE bm

' I

,

>

;

ULVOSPINEL j ~ 5 7 ; =&

8 t

>

OLIVINE i

^+lo2

I I

VOLC GLASS j ' A m

AUGITE \ ^{M2 : M I} ;

+42 ,

GABERO 4s _{tj 77} &? ,,L

L-16 PARTICLE ", +w . ^{C;"3 ,'}

L-20 PARTICLE ril2 b 5 3

0 02 04 06 08 70 I2 14 16

- ^A / 2 rmm/sl FIG. 2. - Quadrupole splittings of components in the spectra of lunar and some terrestrial samples. Top figures indicate -2 -1 0 - 1 relative velooty [mm/g 2 3 4 components, small figures their relative areas.

FIG. 1. - Mossbauer spectra of average samples of Luna-16 window >> spectra taken with extremely high counting

and Luna-20 regolith. Smooth lines represent least-square fit.

Numbering of components corresponds to figure 2 and Table I. statistics (ca lo7 counts/channe13 12'

By linear interpolation of the line positions between Fe Six asymmetric quadrupole doublets of Lorenzian

lines with equal line widths were fitted to the spectra.

In spite of this substantial constraint, the reproduci- bility seem to justify the procedure necessary with respect to the computational limitations. Thus achieved quadrupole splittings are graphically displayed in figure 2 along with those of some comparative terres- tial minerals (for details see [I]).

To gain the complete distribution of iron among individual iron-bearing phases present, the whole spectra were measured (f ca 7 mm/s, 512 channels).

The separate fitting of the outer part of spectra was performed, from which the intensity of the magne- tically split component resulting from the metallic iron

and Fe-4.8 % Ni standards, the content of nickel in the Luna-20 metal of (2.01 f 0.84) % was deter- mined, compared with the previously reported (1.50 + ^0.96) % Ni in the Luna-16 metal [2]. Details will be published elsewhere.

At the last Mossbauer conference in Bratislava we have shown the possibility of measuring small, isolated particles of lunar regolith in a special transmission arrangement. Similarly as in the Luna-16 particle [3], three different (quadrupole split) components were distinguished in the measured spectrum of ca 0.5 mg gabbroidic particle from Luna-20 regolith. Values of the quadrupole splittings for both these particles are arranged at the bottom of figure 2.

has been evaluated. Relative areas of all the compo- 3. Discussion. ^- The number of the resolved compo- nents are lilted in Table I. nents made it possible to relate them to all major types Instead of less accurate hyperfine field measurement of iron known in lunar regolith (Fig. 2). The compo- from the whole spectra, the positions of the outermost nent 1 refers unambiguously to ilmenite [4], the magnetic lines of the ferromagnetic phase, presumably component 6 belongs to olivine [5]. Component 2 has Fe-Ni alloy, were determined from the ((velocity- been proposed to be interpreted as a spinel, near to

Sample -+

Phase (Component No.) Ilmenite (1)

Spinel (2) Pyroxene (3 + 5) Glass (4)

Olivine (6) Metal

TABLE I

Phase distribution of iron in lunar regolith

L- 16, L- 16,

Rel. area Content Rel. area Content

r ^{%I [ %I [%I [%I} Rel. area Content

r %I %I

MOSSBAUER MEASUREMENTS OF THE SOVIET LUNA-16 AND LUNA-20 SAMPLES C6-551

ulvospinel [6], although the reported quadrupole splitting is usually higher (ca 0.85 mm/s [7]). In our comparative measurement of synthetic ulvospinel we distinguished, besides the main component corres- ponding to Fe2+ in tetrahedral sites, a weaker contri- bution of Fez+ in octahedral sites. In case they were unresolved, the obtained mean splitting would corres- pond to that quoted above. It could therefore be concluded, that our component 2 originates from the tetrahedral iron site in spinel structure.

The prevailing part of absorption is caused by components 3, 4 and 5. The doublets 3 and 5 corres- pond to two inequivalent Fe2+ sites (M 2, M 1) in pyroxenes as it follows from the comparison with the augite (from the basalt) or pyroxenes in gabbro (Skaergaard massif) in agreement with the literature data (see e. g. [8]). In spite of the strong overlap of these three doublets, quadrupole splitting of compo- nent 4 matches that of terrestrial volcanic glass.

Based on the above interpretation, in the Luna-16 particle, besides the previously reported metallic iron, probably pyroxene with an admixture of olivine is present, while the Luna-20 particle contains ilmenite and a mixture of pyroxene and olivine with the higher content of olivine (compare the quadrupole splittings of their unresolved contribution in Fig. 2).

In principle, the hyperfine splitting parameters yield the quantitative information of composition of indivi- dual phases. Unfortunately, in our case these possi- bilities are rather limited. Ilmenite is known to be stoichiometric [4] in opposite to all others. Compara- tively large errors of the quadrupole splittings due to the overlap of the components in comparison with the variations in figure 2 prevent to judge e. g. the relative positions in isomorphous series of olivines and the site occupancy in pyroxenes. Good exception of well resolved lines represents the magnetic splitting of the metallic phase, in which the nickel content has been determined, slightly higher in the Luna-20 sample.

Supposing that our samples were sufficiently thin (maximum absorption 3 %) and that f-factors of the components do not differ substantially, the relative areas indicated in figure 2 and summarized in Table I, correspond to the iron distribution among individual phases or iron positions. From these a quantitative phase analysis could be made supposing that the content of iron in the phases is known. In addition to ilmenite and metal whose composition is exactly known, we assumed the following : stoichiometric

[I] ZEMMK, T., RACLAVSK~, K., Geokhimia (1974), 1084.

[2] Z E M ~ ~ K , T., RACLAVSKP, K., Proc. 5th Int. Conf. Mossbauer Spectroscopy, Bratislava 1973 (in press).

[3] Z E M ~ ~ K , T., RACLAVSK*, K., LAUERMANNOV~, J., P ~ o c . 5th Int. Conf. Mossbauer Spectroscopy, Bratislava 1973 (in press).

[4] MUIR, A. H., HOUSLEY, R. M., GRANT, R. W., ABDEL- GAWAD, M., BLANDER, M., Science 167 (1970) 688.

ulvospinel, olivine with a reasonable substitution ((Feo,,Mgo.,)SiO,), and a representative content of 12 % iron in pyroxenes and glass. On these assump- tions, the relative content of iron-bearing phases in the samples was calculated (Table I).

Disregarding the strong overlapping contributions of pyroxenes and glass and leaving aside the not yet definitive interpretation of spinel, valuable information of the differences between the samples can still be gained. Although some differences between the fine average Luna-16 material do exist (e. g. enrichment of the former with metal, probably due to its very fine nature), the samples of different origin are remarkably distinct. This can be even qualitatively seen in figure 1, where pronounced innermost ilmenite doublet (compo- nent 1) in Luna-16 and clearly sharper olivine lines (component 6) in Luna-20 are to be noticed. To sum- marize, it may be stated that the Luna-20 samples contain appreciably less ilmenite and metal, but more olivine than those of Luna-16.

4. Conclusion. - In this work we succeeded to distinguish as many components in the Mossbauer spectra as the number of major iron-bearing phases or iron positions, which could reasonably be supposed.

A reliable quantitative estimate of the content and composition of some these phases was achieved. This illustrates diverse characteristics of lunar regolith from Mare region (Luna-16) and from the lunar mountains (Luna-20).

We are aware of some weak points in our proceed- ings. The limitation in the line width is a substantial one, namely in the case of the glassy fraction in the sample, where an increased line width is to be expected due to the non-uniformity of iron environments in the amorphous substances. Avoiding this would probably ask for more powerful computing technique and for yet higher counting statistics.

We believe that our technique of Mossbauer measu- rements of small particles provides a new, unique tool for studying isolated, non-average parts of lunar regolith. Further measurements of this kind are in progress and will be published separately.

Acknowledgment. - We wish to express our thanks to the co-workers in the Institute of Physical Metal- lurgy, Ing. S. HavliEek and Mr. S. JurneEka, for their experimental help, and to the staff of the computing centre for performing the computations.

rences

[5] MALYSHEVA, T. V., KURASH, V. V., ERMAKOV, A. N., Geokhimia (1969) 1405.

[6] MALYSHEVA, T. V., Geokhimia (1973) 1079.

[7] AVRAHAMI, M., GOLDING, R. M., N. 2. J. Sci. 12 (1969) 594.

[8] HAFNER, S. S., VIRGO, D., PYOC. Apollo 11 Lunar Sci. Conf.,

(1970), Vol. 3,2183.