Antiferromagnetic inclusions in lunar glass

Earth and Planetary Science Letters, 21 (1973) 85-90 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands

ANTIFERROMAGNETIC

kLA

INCLUSIONS IN LUNAR GLASS

A.N. THORPE 1,2, F.E. SENFTLE l, C. BRIGGS z and C. ALEXANDER 1 I U.S. Geological Survey, Washington, D.C. (USA) 2Howard University, Washington, D.C. {USA) Received December 18, 1973 Revised version received February 15, 1974

The magnetic susceptibility of 11 glass spherules from the Apollo 15, 16, and 17 fines and two specimens of a relatively large glass spherical shell were studied as a function of temperature from room temperature to liquid helium temperatures. All but one specimen showed the presence of antiferromagnetic inclusions. Closely spaced temperature measurements of the magnetic susceptibility below 77 K on five of the specimens showed antiferromagnetic temperature transitions (N6el transitions). With the exception of ilmenite in one specimen, these transitions did not correspond to any transitions in known antiferromagnetic compounds.

1. I n t r o d u c t i o n

Magnetic susceptibility measurements by Thorpe, Senftle and co-workers [1, 2] between 4.2 and 300 K have shown indirectly the presence of antiferromagnetic inclusions in individual lunar glass spherules; however, no antiferromagnetic (N6el) transitions were observed. The experimental magnetic susceptibilities determined by Thorpe, Senftle and others may be expressed as the sum of an antiferromagnetic term and a paramagnetic term; the antiferromagnetic term is proportional t o the fraction of antiferromagnetic minerals in the glass. In our present investigation, we have selected individual glass spherules removed from lunar fine samples for which the antiferromagnetic term is large. Attempts were then made to observe the N4el transitions directly from careful susceptibility measurements at closely spaced temperature intervals between 4 and 80 K. Nagata et al. [3] have made similar measurements on the bulk fines of Apollo 11 which contain a higher concentration of antiferromagnetic minerals. They found two antiferromagnetic transitions at 41 K and 56 K which they suggest are due to the presence of flmenite and ferrosillite. In contrast, Nagata et al. [4] found no antiferromagnetic transitions in their studies of the fines and crystalline rock of the Apollo 12 mission. In a later investigation of the

Apollo 14 and 15 rocks, they [5] indicated that several antiferromagnetic mineral species may be present in lunar rocks even though antiferromagnetic transitions were not directly observed.

2. Antiferromagnetism in lunar rocks and glass Magnetic-susceptibility measurements of individual glass spherules from the lunar fines have shown that the glass is generally not a single paramagnetic phase but includes minor amounts of metallic iron or nickel iron spherules as well as several antiferromagnetic mineral phases (see [1, 2]). The paramagnetic susceptibility of the glass was shown to be due to the basic paramagnetism of the ferrous ions dissolved in the glass which is altered by the axial distortion of the octahedral crystal field caused by the surrounding oxygen ions and the low field temperature-independent contribution of the metallic spherules. Considering the lunar samples as a mixture of these three phases, then, the total observed susceptibility X may be written as: X = Xg + Xa + X~ _ 2N~2g2 C' 3 Z k Z [f(~) + 2h(~)] + T ~ O - +XI a

* Seeon next page.

(1)

86

A.N. THORPE ET AL.

The term × g is the contribution from isolated (no exchange interaction) ferrous ions in the glass phase and is of the general form C/T, where C is the Curie constant. In the two-term expanded form of Xg which is necessary to consider the effect of distortion of the ligand field about the ferrous ions [6, 7], N is the number of Fe 2+ ions per gram,/3 is the unit Bohr magneton, g is the spectroscopic splitting factor, k is Boltzmanns constant, T is the absolute temperature. Also : Z

= 2 + 2 exp ( - 3~) + exp ( - 4~)

f(~)

=4+exp(-3~)

h(~) = (2/3~) [1 - e x p ( -

3~)] + (3/~)

× [exp ( - 3~) - exp ( - 4~)] g

=2

= D/k T. D is the zero-field splitting parameter and is a measure of the degree of distortion of the electric field of the oxygen atoms about the Fe 2+ ions in the glass. The term "X a is the contribution of the antiferromagnetic inclusions. Above the N6el (antiferromagnetic transition) temperature, the antiferromagnetic susceptibility is given by: C'

C,

T + O a - E n T+On where n = 1, 2 . . . . . C n is the Curie constant of any given antiferromagnetic species and 0 n is the associated Curie-Weiss temperature. (C' and 0 a are the effective values summed over all the antiferromagnetic inclusions present). Rather abrupt changes in ×a occur near the N6el temperature; these anomalies may be used to identify the antiferromagnetic mineral inclusions in the lunar samples. The term X1 is the contribution from the temperature-independent phase, such as metallic iron spherules in the glass matrix. When plotting the magnetic susceptibility as a func* In two of our previous papers [ 1, 2] the temperature-independent term was inadvertantly given the symbol ×t in the text and ×I in the tables. The latter symbol, XI, will be used henceforth.

tton of the reciprocal temperature for the lunar glasses, we have previously used eq. 1 to fit the experimental data above 55 K, but not at lower temperatures. It is reasonable to assume that the antiferromagnetic contribution to the total susceptibility is probably small in comparison with the paramagnetism of the dissolved ions (primarily iron) at temperatures below the mean N6el temperature. Therefore, to a first approximation, we considered the antiferromagnetic contribution relatively small and essentially constant below the mean Ngel temperature and thus were able to make a fit to the experimental data at all temperatures from 4 K to 300 K. Using this procedure, it was found that for the lunar glass spherules, a mean Ngel temperature of 55 K and a mean Curie-Weiss temperature of 20 degrees gave the best fit to the experimental data. As 55 K is very close to the Ngel temperature of ilmenite (12), the data suggest that ilmenite is the antiferromagnetic inclusion in lunar glass. However, 55 K is only the mean N~el temperature, which gave the best overall fit, and other antiferromagnetic species in the proper ratio could give similar results. Although it is clear that other antiferromagnetic mineral inclusions are probably present in the glass, it is desirable to prove their presence and to determine, if possible, what species are present. Results similar to ours were obtained by Nagata et al. [5] who also found that they had to add a term to the basic magnetization equation to account for the antiferromagnetic inclusions in their magnetic measurements on lunar rocks and fines. They were able to more reasonably describe their results by an expression of the form**:

M(H, T)=

H + H Ei ~

+Ms(H)

(2)

l

where M is the magnetization, H is the magnetic field, and M s is the magnetization of the temperature-independent phase. The second term is a summation term which includes contributions from the several antiferromagnetic species present. By plotting ~X/3(1/T) as a function of temperature, they were ** In the paper by Nagata et al. [5], the additional term was expressed as H(Ei(Ci/T-Oi)). In eq. 2 we have used the positive form of the Curie-Weiss temperature to conform with the symbols in our previous papers.

ANTIFERROMAGNETICINCLUSIONSIN LUNAR GLASS able to show the presence of the ilmenite transition at 56 K and possibly several unidentified transitions below 50 K. Below the lowest transition ( ~ 30 K) their value of aX/a(1/T) continued to decrease and approached zero. For a true paramagnetic specimen containing antiferromagnetic inclusions, O×/a(1/T) should remain almost constant below the Ngel temperature. The decrease in a×/a(1/T) that Nagata et al. [5] observed at low temperatures is probably due to ligand field interactions which were not accounted for in their theoretical treatment. However, as shown previously, we have been able to fit X versus I/T data from lunar specimens containing antiferromagnetic inclusions by taking into account the ligand field interactions.

87 1412~I0

6 4

2t+ o

3. Experimental procedure and results Eleven glass spherules from the Apollo 15, 16, and 17 fines were first selected for the magnetic studies, primarily on the basis of size. These spheres were of good solid glass with a minimum number of vesicles as observed with an optical microscope. In addition, two pieces of glass from a relatively large ( 3 - 4 cm diameter) glass spherical shell (15017) were also measured. Magnetic-susceptibility measurements were made on a quartz helical spring balance, using the Faraday method as described more fully elsewhere [8, 9]. After a general study of the magnetic properties from room temperature to 77 K, further measurements were made on all the specimens down to liquid helium temperatures to determine the value of the antiferromagnetic Curie constant, C'. The experimental and mathematically derived results are presented in Table 1. X0 and o are the room-temperature values of the magnetic susceptibility and soft component of the magnetization, respectively. C is the Curie constant determined from the data taken from room temperature down to 77 K, i.e., in the region where the Curie law holds. C', 0 a and D are as previously defined. TN is the measured N6el temperature-transition. According to the model describing the magnetic susceptibility of the glass spherules as proposed by Thorpe et al. [1] and Senftle et al. [2], the value of the total measured Curie constant, C, for spherules

o Apollo 15 +Apollo 16 -,Apollo 17

+ I 2

I

I

I

I

J

4

6

8

I0

12

CI

Fig. 1. The total Curie constant, C, as a function of that part of the Curie constant, C', associated with the antiferromagnetic inclusions in the glass spherules. from a single sample of bulk fines is linearly related to the mean antiferromagnetic Curie constant, C'. A similar plot for glass spherules from Apollo 15, 16, and 17 reported in this paper could not be made for each sample of fines as the number of spherules from a given sample of fines was not sufficient. It is surprising and probably fortuitous that in spite of the fact that the spherules represent five different samples of fines and one spherical glass shell, most of the points fall along a given line, as shown in Fig. 1. The single Apollo 17 specimen which falls well off the curve is an orange glass spherules and may not be representative of the group. The linear relation of Fig. 1 shows that the ratio of the antiferromagnetic Curie constant to the total Curie constant in this group of spherules is essentially constant although the total concentration of paramagnetic plus antiferromagnetic phases varies from spherule to spherule. As the most common antiferromagnetic lunar minerals so far reported all contain iron (see Table 2), this ratio implies that the ratio of the antiferromagnetic iron to paramagnetic iron is also very nearly constant. Unlike the bulk fines as measured by Nagata et al. [3] most of the individual glass spherules, besides being in the microgram size range, contain a much lower overall concentration of paramagnetic plus antiferromagnetic phases, and

88

A.N. THORPE ET AL.

TABLE 1 Experimental and calculated results for the lunar glass specimens* Sample No.

Mass (mg)

15211,3 15211,4 15221,1 15221,2 66041,1 66041,2 66041,4 66041,5 66041,6 15017,10,1 15017,10,2 60095,8,1 74220,64,3

0.158 0.094 0.095 0.081 0.146 0.098 0.052 0.015 1.24 0.16 26.5 2.65 0.124

xo (X 10 - 6 emu/g) 18.7 41.4 23.1 34.3 19.7 18.9 87.4 17.3 62.1 205.0 165.0 55.1 31.1

o (X 10 - 4 emu/g) 152.6 312.0 276.8 398.5 333.5 163.0 350.4 2703.0 200.1 177.0 65.0 113.0 263.7

C (X 10 -3)

C' (X 10 -3)

4.92 7.01 6.57 2.46 5.05 1.99 1.92 1.07 2.13 7.34 4.38 2.27 12.82

1.08 2.28 2.28 0.53 1.45 0.11 0.85 0.00 0.40 2.33 2.06 2.62 9.76

0 (1~)

3.48 4.56 4.70 3.23 4.46 2.21 4.99 3.26 2,91 6.53 2.77 17.92

D (cm -1)

TN (K)

6.90 6.56 6.27 6.42 7.31 7.17 3.69 7.03 2.87 5.89 7.43 6.29

39.5; 56.5 29; 41.5 18; 31.5 15;37.5 25 ;42

x0 = magnetic susceptibility; o = magnetization; C = total Curie constant; C' = mean antiferromagnetic Curie constant; 0 = Curie-Weiss temperature; D = zero-field splitting factor; TN = N6el temperature. TABLE 2 N~el Temperatures of less than 77 K Formula

N~el Temperatures

Reference

Antiferromagnetic compounds reported in lunar samples FeTiO3 Fe2SiO 4 FeSiO 3 7-Fe FeP FeAI2 04

56 65, 20, ~ 30 10, 43 8, 55-67 125 8

[ 12] [10] [ 11 ] [10] [10] [ 10]

Possible but unreported an tiferromagnetic compounds MnTiO3 CoTiO3 NiTiO 3

Ni2SiO4 MnSiO 3 Co2 SiO4 CoSiO 3 Co2Ti CoTiO a

41-64, 66 38, 42 25, 23, 22 34 7 49 50 43 36

[10, 12] [10] [10, 12] [ 10] [ 11 ] [ 10] [10, 11] [ 10] [ 12 ]

that a t e m p e r a t u r e transition will be observed, as a large C ' could be the result o f very small concentrations o f m a n y different species o f antiferromagnetic mineral inclusions. The N~el temperatures of the antiferromagnetic lunar minerals are not very well k n o w n . The few N6el temperatures o f minerals which m a y occur on the lunar surface are shown in Table 2. Several o f the minerals are m e m b e r s of a mineralogical series, one end m e m b e r o f w h i c h is a k n o w n antiferromagnetic mineral. F o r instance, the c l i n o p y r o x e n e s and ortho30C

,

74220 64-3

0I

,

20,

TEMPERATURE (K) 40 ' 6,0,

" \~ 'x ,, ~

580

2O0 460 ~6o

\

,522,,,

~

",,

,5o,~, o

120

thus the m e a s u r e m e n t is m u c h m o r e difficult. As the c o n c e n t r a t i o n m u s t be high e n o u g h to yield a temperature transition observable above the paramagnetic background, only those specimens having a relatively large value of C ' were selected for the detailed temperature measurements. Even so, this does n o t insure

8(620

80

,20 S 380

,~~

",,,,,,~

60095.8

340

300 0

20

40

60

80

I00

TEMPERATURE (K)

Fig. 2. Magnetic susceptibility of the glass spherules as a function of temperature showing the N~el transitions.

ANTIFERROMAGNETIC INCLUSIONS IN LUNAR GLASS

89

pyroxene have antiferromagnetic ferrosillite (FeSiOa) as an end member and in the olivine series, antiferromagnetic fayalite (Fe2 SiO4) is an end member. In general, but not always, the N6el temperatures tend to decrease as one moves away from the antiferromagnetic end member, and at some point the solid solu. tion ceases to be,antiferromagnetic. Thus, it is to be expected that positive identification of an antiferromagnetic inclusion based on the N6el temperature will be difficult. The change in the magnetic susceptibility as a function of temperature for five of the glasses is shown in Fig. 2, in which a number of transitions were observed. Specimen 15221,1 shows a wellformed transition at 56.5 K and a small one at 39.5 K. From the known information on N6el temperatures shown in Table 2, the former transition is undoubtedly due to ilmenite. The lowtemperature transition is slightly less than that of ferrosillite and may be due to clinopyroxene. Specimen 16041,1 also shows a transition at 41.5 K and a small inflection at 27 K. The low-temperature transition cannot be correlated with any known antiferromagnetic mineral, but the higher one is close to that of ferrosillite. Again, the glass may contain one or more clinopyroxene inclusions. Specimen 60095,8,1 has transition points at 15 K and about 37.5 K, and specimen 15017,10,1 shows transitions at 18 K and 31.5 K. None of these transitions correspond to known N6el temperatures of antiferromagnetic minerals. The orange glass from Apollo 17, specimen 74220,64,3, shows only a single transition at 42 K which closely corresponds to ferrosillite. Other minor transitions can be seen in the figure, but their significance is questionable.

measured. A number of the antiferromagnetic transitions were also observed in the same specimens. Thus, the assumption of the presence of antiferromagnetic inclusions in the lunar glass spherules which was made previously is shown to be valid. Each of the five specimens measured had different antiferromagnetic transitions which are attributed to differences in type of antiferromagnetic species. For instance, the antiferromagnetic transition will depend on the chemical composition of a member of a solid-solution series. If the lunar glass spherules contain mineral inclusions that are members of a solid-solution series, one can easily account for the relatively large number and variability of the observed transitions.

Conclusions Magnetic susceptibility measurements as a function of temperature on selected individual lunar glass spherules from 4 K to 77 K indicate that although the total concentration of paramagnetic plus antiferromagnetic phases varies from spherule to spherule, the ratio of the iron in the antiferromagnetic state to the iron in the paramagnetic state is essentially constant for the Apollo 15 and 16 spherules

Acknowledgements The authors thank the National Aeronautics and Space Administration for partial support of this work under contract NGL-09-11-006. We are also grateful to Edward Dwornik for help in examining the bulk samples for individual specimen selection and to Drs. T. Tsang and P. Wasilewski for their helpful suggestions.

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A.N. THORPE ET AL 9 F.E. Senftle and A.N.iThorpe, Technique and interpretation of magnetic susceptibility measurements of water in normal and tumor time, Instr. Soc. Am. Trans. 2 (1963) 117-120. 10 T.F. Conolly and E.D. Copenhaver, Bibliography of magnetic materials and tabulation of magnetic transition temperatures, Solid State Phys. Literature Guides, 5 (Plenum Press, 1972). 11 A. Sawaoka, S. Miyabara and S. Akimoto, Magnetic properties of several metasilicates and metagermanates with pyroxene structure, J. Phys. Soc. Japan, 25 (1968) 1253-1257. 12 J.J. Stickler, S. Kern, A. Wold and G.S. Heller, Magnetic resonance and susceptibility of several ilmenite powders, Phys. Rev., 164 (1967) 765-767.