Investigation and simulation of metallic spherules from lunar soils

Gieochimica et Cosmochimica Acta,1975, Vol.S9,pp.305to324.Pergsmon Press.Printed inNorthern Ireland

Investigation and simulation of metallic spherules from lunar soils P. J. BLAU* and J. I. GOLDSTEIN Department of Metallurgy and Materials Science, Lehigh University, Bethlehem, Pa. 18015, U.S.A. (Received 19 March 1974;

accepted in revised form 28 June 1974)

Abstrac&Metallic spherules selected from the Apollo 11, 12, 14, 15 and 16 sites were studied by optical techniques as well as the electron probe and scanning electron microscope. In addition, metallic spherules of similar composition were produced experimentally. The structure of the metallic lunar spherules indicates an origin by solidification of molten globules of metal. The experimentally produced spherules have external morphologies, metallographic structures and solidification r&es (7 x lo2 to lo6 ‘C/set) similar to the luntlr spherules which have rapidly solidified. The majority of the lunar spherules are, however, either more slowly cooled or have been reheated in place with the lunar fragmental rocks, glass or soil. The heavy meteorite bombardment of the highlands is strongly reflected by the evidence of reheating and/or slow cooling of a majority of Apollo 14 and 16 spherules. The metallic spherules are probably produced from both lunar and meteoritic sources. Impact processes cause localized shock melting of metallic (and non-metallic) constituents et metal-sulfide phase interfaces in surface rocks and in the meteoritic projectile. The major source of metallic spherules is the metal phase present in the lunar rocks and soil. The large variation in spherule bulk compositions is attributed to the different meteoritic projectiles bombarding the Moon, metal phases of differing compositions in the lunar soils and rocks and to the experimental results which indicate that high S, high P alloys form two immiscible liquids when melted. INTRODUCTION

the lunar soil are a small number of metallic spherules which range in size from 50 pm to 5 mm in diameter (F~ONDEL et al., 1970, 1971; GOLDSTEIN et al., 1970; GOLDSTEIN and YAKOWLTZ,1971; GOLDSTEIN et al., 1972; GOLDSTEIN and WITHI-N

AXON, 1973;

MASON et al., 1970;

W~NKE

et al.,

1970,197l;

WLOTZKA et al.,

1972).

were formed directly from a liquid and their microstructure exhibits the original solidification features (Fig. 1). The presence of metallic spherules is, however, not unique to the lunar surface. Spherules with quite similar compositions and microstructures have been found on earth. One such occurrence is in the desert surrounding the Barringer Meteor Crater in Arizona. Several investigators have dealt with the origins and characteristics of these particles (NININGER, 1956; MEAD et al., 1965; RAMDOHR, 1966; BLAU et al., 1973). Similar particles have been observed in tektites (BRETT, 1967), and in ocean sediments (RAMDOHR, 1966 ; SCHIEIDT and KEIL, 1966). In general, there is good agreement that the metallic spherules produced by impact on the Earth are of meteoritic origin. Most of the lunar craters were produced by the impact of various sized bodies on the Moon’s surface. The material ejected from the lunar craters is largely composed of fragmented lunar rock (GAULT et aZ., 1963); however, small portions of the original projectile are likely to be mixed with it. The metallic spherules contain shock-melted and resolidified parts of the original projectile. Almost all of the metallic spherules recovered from the lunar soils have X-Co contents which are These

metallic

spherules

* Present address: 6

Wright Patterson

Air Force Base, Dayton, 30.i

Ohio 45433, U.S.A.

306

P. J. BLAU

and J. I. GOLDSTEIN

similar to those measured in ordinary chondrites and iron meteorites. It is therefore tempting to ascribe the origin of the lunar metallic spherules to meteoritic crater forming objects. On the other hand, it is well known that metal is a small but ever-present component of both the lunar soil (GOLDSTE~ et al., 1970 ; WXNKE et al., 1970 ; GOLDSTEIB and AXON, 1973), and the lunar rocks (DOWTY et al., 1973; BROWN et al., 1973; ALBEE et al., 1973). Much of this metal also has X-Co contents similar to

those measured in meteorites. In addition, magnetic studies have shown that large percentages of the metal content of lunar rocks is sub-microscopic (PEARCE et al., 1972). Subsequent reheating could form additional microscopically visible metal. It is therefore probable that impacts on the lunar surface produce shock melting of previously existing metal as well as metal from crater-forming objects.

The purpose of this study is to investigate the process by which the microstructures and chemistry of the metallic spherules were produced on the lunar surface and to determine the source(s) of the lunar spherules. Metallic spherules were produced experimentally from liquid metal of lunar spherule compositions in order to simulate conditions in the lunar environment. A comparison of the structures, chemistry and cooling rates of the experimental particles and those of lunar spherules from various landing sites are made. METHOD An apparatus was designed and built to produce metallic spherules experimentally

and to allow these spherules to solidify in an environment closely resembling lunar conditions. Various alloys of Fe-Ni-Co-S-P were melted, and extruded through a millimeter sized orifice into a vacuum. The liquid broke up into fine droplets which totally or partially solidified as they fell on to a bed of silica sand. A schematic diagram of the droplet-forming apparatus is shown in Fig. 2. The basic unit

is composed of glass with 2 chambers, a fused quartz melting tube (E), and a solidification chamber (H, I). A 3-4 g alloy charge was placed in a BN crucible which had a 06-l mm hole drilled in its bottom. This crucible was then held tightly in another BN holder (Fig. 2F) which

Fig. 2. Schematic diagram of the droplet-forming apparatus.

~'ig. 10. M e l t r e s i d u e s f r o m [~un VI ,sh.w ~ m a s s i v e l y c r a c k e d , d a r k g r a y FeS . ~ tho left, a n d a p h o s p h o r o u s e o m p o m , n t .m t h e r i g h t which e . n t a i n s Fe Ni metal grains, p h o s p h i d e - m e t a l e u t e e t i c , aw~tl s,~mc d a r k gray Vatchcs ~'f FeN. Field ~f view 320 • ;~.6~ /In~. Fig. l ] . A p o l l o 14 part, i~.lo M2'9 sh,~wi.g a .sulfide nn'tal I . i s k s u r w , u m l i . g a 9 0 / i r a di~l,, e . r e o f m e t q L a n d I ) h - s l ) h i d e - m e t a l e . t c ( ' t i ( ' Itl.el3" g r . n . b l r ~lre'asl. L I,er ~-4.~( ~dtaL ~qch.

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Investigation and simulation of metallic spherules from lunar soils

307

was previously sealed within the quartz tube (E). Prior to melting, the 2 chambers are evacuated through valves (B) and (K). The alloy charge was melted by an external induction coil (G) and the melting operation was observed through a Pyrex window at the top of the apparatus (C). As the alloy is melted, surface tension holds the liquid charge from dropping through the hole at the bottom of the crucible and the liquid melt seals the melting chamber from the solidiflcation chamber. The upper chamber was then backfilled with gas @r/HZ) through (A) after closing the valve at (B). The solidification chamber continued to be evacuated during this process. Up to 1 atm pressure was produced in the upper chamber causing the melt to be expelled through the orifice. Under the action of surface tension and orifice perturbations, the stream broke up into droplets which fell through the vacuum within the solidification chamber into a dish of silica sand (L) at the bottom of the apparatus. The length of fall for the droplets was either 66 or 89 cm and the break-up length for the liquid was estimated at about 1 cm. The velocity of the spherules down the glass tube (H) after break-up varied from 0 to approximately 650 cm/set depending on the pressure (up to 1 atm) obtained in the upper chamber. It was calculated, BLAU (1973), that all particles with an initial velocity of 550 cm/set, which were smaller than 22 ,am, will be completely solidified by radiation for the 56 cm length of fall and allparticles, which were smaller than 35 Pm, will be completely solidified by radiation for the 89 cm length of fall. If one considers that the spherules need not be 100 per cent solid to retain some rigidity upon landing, all particles with an initial velocity of 650 cmlsec and smaller than 42 ,um will be 50 per cent solid for the 56 cm length of fall. For the 89 cm length of fall, particles smaller than 70 pm will be 50 per cent solid. If the spheruleshad initial velocities close to 0 cm/set, the solidification time available would increase and 60 ym particles could completely solidify for the 56 cm length of fall, while 80 pm particles would completely solidify for the 89 cm length of fall. For 60 per cent solidification the spherule sizes are 160 and 200 pm, respectively. Larger particles will be cooled primarily by conduction with the quartz sand contained in the Pyrex dish at the bottom of the solidification chamber. The basic metal alloy was supplied by International Nickel Company (INCO). Its composition was given as 7.35 wt.% Ni, O-50wt.% Co, 0.078 wt.% S, O-l-O.413 wt.% P, 0.011 wt.% C and the balance Fe. Using this as the starting material, elemental S and red P were added to provide additional alloys containing large amounts of S and P individually and in combination. Finally, alloys with approximately the same composition as the lunar spherule average were prepared. The average alloy compositions obtained by analyzing a number of spherules from each run are given in Table 1. Simulated spherules were sized after having been magnetically removed from the sand. The metallic particles were mounted in epoxy and polished for optical microscopy and electron probe analysis. Diameters of the particles were measured by focussing a calibrated microscope on their transmitted light silhouettes in the epoxy. The metallic spherules were identified for study by optical examination. Table 1. Average compositions and composition ranges in run products Run number

Number of spherules analyzed

Ni

co

S

P

14 17 19 21 23 27 28 29 30 31

8 12 11 8 13 6 14 27 19 27

8.2 7.1 7.3 7.0 6.0 5.5 7.4 7.1 7.0 7.0

0.46 0.46 0.54 0.52 0.40 0.36 0.51 0.51 0.51 0.50

3.6 0.07 0.08 0.11 0.08 17.1 0.18 4.7 1.9 3.4

0.42 0.32 O-48 0.42 10.2 0.31 6.3 6.7 7.6 1.5

Average spherule bulk compositions (wt. ye)

* N = no apparent spread; all within counting error.

Range in spherule compositions (wt.%) P Ni Co S 5.3 0.8 0.5 0.2 0.1 0.4 1.3 2.2 1.4 0.6

N* N N N N N N N N N

1.6 N N N N l-6 0.1 19.9 5.1 1.9

0.1 o-3 o-3 N 0.7 N 3.7 10.7 4.6 0.3

P. J. BLAU and J. I. GOLDSTEIN

308

Bulk analyses for Fe, Ni, Co, S and P were made by scanning representative areas 100 x 100 ,um or smaller. Standards of Fe-Ni alloys, pure Co, Fess and schreibersite, (FeNi)sP, from the Canyon Diablo meteorite were used. After microprobe analysis, the samples were repolished and etched or stained for optical study. A 4 set 1 per cent nital etch was used for most samples; however, where the metallic and non-metallic phases were tlnely intergrown, it was helpful to immerse the sample in a solution of sodium metabisulfite and water (2 g/ 100 cm3) for about 45 set to selectively stain the metal phases. Metallographic and electron microprobe studies were also made of over 65 spherules from five Apollo missions: 11, 12, 14, 15 and 16. The spherules were included within the general samples of metallic particles separated from the lunar soils. Some of the larger spherules were hand picked from the metallic samples and observed in the scanning electron microscope prior to mounting. They were prepared for examination by light optical, microprobe, and scanning electron microscope analysis in the same manner as discussed for the experimentally produced particles. RESULTS

A. Lunar q&e&es The bulk compositions and sizes of the 65 lunar metallic spherules from the various Apollo sites are listed in Table 2. The Apollo 11, 12 and 14 results (15 samples) Table 2. Lunar spherule data Apollo mission (site) (11ob85) 12 (14YO3,

(

Sample number

Diameter (elm)

17 M F3.2 F26.6 F27.10 F28.3 M6.1 M6.3 M1l.l F14.17 F14.21 F16.5 M2.6 M2.7 M2.9 MIO-1 81.14 81.36 S8.6 S8.14 54.5 54.2 1 54.22 S4.25 815.25 515.4 F10.1 F10.5

3500 500 150 100 x 70 130 160 135 150 90 100 100 x 50 100 110 80 160 79 81 140 x 103 160 x 100 83 103 97 115 96 105 125 157

14Yi3)

(150::,081)

d.c. (pm) 40 15

10 6.4 4.0

2.5

2.0 5.0

1.0 3.0

Ni

co

S

P

17.0 -9-O 8.1 13.0 6.1 7.0 8.9 6.6 6.7 6.2 12.1 6.1 7.1 14.5 15.6 9.4 11.1 11.2 14.1 6.4 11.5 8.2 96 22.5 17.6 14.3 6.4

-0.5 0.4 0.45 0.25 O-6 0.3 0.3 0.3 0.3 0.45 0.4 0.35 0.75 0.45 0.4 0.5 0.55 0.6 0.3 0.4 0.4 0.45 1.1 0.85 0.5 0.3

6.0 -3.0 0.3 1.0 2.3 0.15 4.2 2.7 2.0 o-3 2.4 1.05 1.0 2.9 2.0 0.73 2.66 0.31 0.22 0.64 0.74 1.53 3.29 1.59 2.08 1.6 7.4

0.1 -1.0 4.5 6.4 0.9 0.2 6.5 2.9 2.3 2.2 6.5 1.65 3.5 5.7 7.7 1.8 4.85 4.3 3.55 2.35 10.2 6.7 2.4 6.5 4.62 1.48 0.2

*

1 2

1 1 1 1 1 1 I,3 1 1

1

309

Investigation and simulation of metallic spherules from lunar soils Table 2 (contiraued) Apollo mission (site)

Sample number

Diameter (tim)

56.3 S6.9 57.22 SlO*1 510.2 S10.6 s11.2 511.14 Sll.17 Fl-1 Fl.2 F2.5 Sl8.14 Sl8,18 519.2 519.3 519-34 SZO-36 s20.47 Fl4-8 Fl4.10 F15.1 F15.11 F2-16 53.10 54.15 M2.3 F1.6 Fl-15 F4-12 F4.13 F4.17 F3.16 F3.19 F3.24 F3.30 F6-22 F6.24

110 120 118 96 110 103 85 116 90 180 150 x 182 310 x 126 110 88 120 x 188 170 125 103 103 147 300 x 600 300 270 188 x 94 120 157 x 125 1100 157 125 x 205 209 209 125 219 282 157 282 138 251

(1512561) (ltz61)

(lj12571)

(63YOl)

(6201) (68Zl)

d.c. (pm) 1.0

3-O 6.3

7.7

4*3

7.5

18.

15.

6-O

Xi 9.2 8-O 6-O 6.7 7-o 13.0 3.3 18.7 1.5 o-2 5-3 8.7 8-O 9.2 5-l 15.7 11-5 6-7 6.0 6-4 10.2 5.2 4.8 15.7 8.5 20.8 16 & 3 6.45 9.5 6.6 6.4 14.9 6-l 7.5 7.1 8.5 9.8 7.2

co 0.35 o-45 o-3 o-35 0.32 0.52 o-2 0.6 0.25 2.35 0.25 0.6 0.4 0.45 0.25 o-55 0.6 0.35 0.4 0.3 0.45 -0-l -0.1 0‘71 0.47 0.7 0.45 0.35 O-64 0.35 o-4 0.65 0.30 0.35 0.4 0.30 0.34 0.35

* Footnotes 1. 2. 3. 4. 5. 6. 7. 8.

Reheated or very slow cooled. Partly surrounded by a sulfide husk. Completely surrounded by a sulfide husk. Partly surrounded by phosp~de~metal eutectic. Transition between spherules and fragments of spherules. Transition between spherule and metal plus sulfide. Reheated slightly, recrystallized. Several reheating cycles.

S

P

1.5 0.08 5.95 1.25 6.6 1.31 3.36 o-44 13.8 O-52 o-04 5.0 O-36 0.71 0.37 1.74 0.71 l-2 5-95 0.61 0.75 25.5 26.0 0.7 o-05 2.85 5*5 2.5 0.8 o-1 O-6 o-1 0.65 4.5 1.1 0.35 O-8 o-41

7.05 2.36 o-29 5-3 l-18 3.32 0.42 4.65 o-19 0.77 4.16 0-21 1.7 10.4 0.24 o-21 3.02 7-3 l-01 13.2 4.66 0.03 o-02 2-6 3.2 4-4 6 f 2.5 0.35 O-85 2.6 O-6 6-O 2.6 o-5 2.8 4.9 5.9 5.65

* I,4

1

1

1

5 f,S 2 2 1,6 7

1 1 1 1 8

P. J. BLAU and J. I. GOLDSTEIN

310

have been reported previously (GOLDSTEIN et al., 1970; GOLDSTEIN and YAKOWITZ, 1971; GOLDSTEINet al., 1972). Spherule sizes from 80 ,um up to 3.5 mm were examined. Some of the particles are non-spherical (Fig. 1 and Table 2) and most of them were coated with small amounts of silicate when observed in the scanning electron microscope. A considerable range of compositions, from O-2 to -22.5 wt. % Ni, O-1 to 2.35 wt. % Co, 0.04 to 26.0 wt. % S, and O-02to 13.25 wt. % P were observed (Figs. 3;4). As a point of reference, the average composition of the 65 spherules is 9.45 wt. % Ni, 0.46 wt. % Co, 2.74 wt. % S and 3.45 wt. % P with the balance Fe. Over half of the spherules (45) are highly enriched in S or P, that is with a ratio of S to total S + P or a ratio of P to total S + P greater than O-75. Of these spherules 31 are highly enriched in P, and 14 are highly enriched in S. Only 6 spherules have Ni-Co bulk compositions outside of the meteoritic composition range as outlined by GOLDSTE~ and YAKO~ITZ (1971) and are noted on Figs. 3 and 4. Particles Fl.l, S11.2 and 511.17 from soil 15261 have Ni contents less than 4 wt. % (Fig. 3, Table 2). Particle F1.l has the lowest Ni content, O-2 wt. %, and the highest Co content, 2.36 wt. %. Particles F15.1 and F15.11 from soil 15271 contain metal globules in a sulfide matrix. These two particles have low Co contents, O-1wt. %, but have the highest S contents, 25.5 and 26-O wt. % S (Fig. 4). In addition, particle S15.25 from soil 15071/81 has a non-meteoritic Co content of 1.1 wt. % as well as the highest Ni content observed, 22-5 wt. %.

AVERAGE

12.-

.

02466

IO

:

WEIGHT

9.45

12

14

16

16

20

22

24

26

34 NICKEL

::

5

AVERAGE

0.46

WEIGHT

K

C06ALT

Fig. 3. Bulk Ni and Co distributionsin spherulesfrom Apollo 11, 12, 14, 15 and 16 soils. Spheruleswith Ni-Co bulk compositionsoutsidethe meteoriticrange are outlinedin black.

Investigation and simulation of metallic spherules from lunar soils

311

16-W Ifi.l4-

L

AVERAGE 2.74 , l

12-

I

0 z

2

4

6

14 16 6 IO 12 WEIGHT % SULFUR

IJ IO WEIQHT

,n, 12

14

I 16

16

20

22

24

26

, 16

I 20

I 22

I 24

! 26

% PHOSPHORUS

Fig. 4. Bulk S and P distributions in spherules from Apollo 11, 12, 14, 15 and 16 soils. Spherules with Ni-Co compositions outside the meteoritic range are outlined in black.

Texturally, the lunar spherules can be classiCed into one group which has largely retained the original high temperature solidification microstructures, and another group which shows evidence of slow cooling or possible reheating cycles of greater than several hours. This differentiation was based both upon the appearance of the metallic phases and upon the absence or presence of solid-state phosphide exsolution. Rapidly solidiCed metals frequently take on a branching, dendritic primary crystal morphology. With slower solidification rates, coarser structures develop as the primary crystals thicken and agglomerate. The photomicrographs in Fig. 1 provide evidence for a large range of solidification rates for the lunar spherules. The top two particles contain fine, branching metal dendrites and probably solidified most quickly. The individual dendrites are single crystals that nucleate and grow into treelike structures during the solitication of liquid-metal systems. The last liquid to freeze outlines the individual dendrites. Figure la shows a considerable amount of interdendritic schreibersite [(FeNi),P], and sulfide, troilite [FeS], (gray areas surrounding the lighter metal). The metal in the spherule in Fig. lb appears dark. Figures lc and d show slower cooled and coarser metal phases in a cellular structure and also show considerable non-metallic phases surrounding the metal grains. Particle lc is partially surrounded by sulfide-metal eutectic. The cellular structure is denoted by rows of what appear to be unattached dendrites or cells.

312

P. J. BUU and J. I. GOLDSTEIN

The center-to-center distance between dendrite side branches (secondary arms) was measured in a number of rapidly solidified spherules (e.g. Fig. la and b). Average secondary arm spacings are given in Table 2 in the column labeled d, and ranged from 1 to 6 ,am in size. In less rapidly solidified particles, elongated cells developed. The average width of these cells (e.g. Fig. lc and d) was also measured where possible. Cell size data is given in Table 2, in the column labeled c. For the lunar metallic spherules, cell sizes ranged from 3 to 40 pm. In total about 30 per cent of the lunar spherules solidified with a dendritic or cellular structure. More irregularly shaped, globular metal phases appear in Fig. le and f. Spherules with this structure cooled more slowly than the dendritic or cellular structure metal. The globules do not have an elongated or branching appearance and they appear mostly as rounded convex grains. The additional cooling time allows original solidified metal areas to coarsen, producing large grained metal structures. In these coarse samples, each metal grain is a substantial fraction of the overall particle diameter in size. In low P spherules the phosphide tended to form thin filamentary networks, whereas the sulfide tended to form globular blebs in the larger interstices (Fig. le). The majority (60 per cent) of the spherules have the slow cooled microstructure containing globular metal. Because of the relatively slow solidification rates of spherules with globular metal, P trapped in the rapidly quenched Fe-Ni phase may exsolve in the solid state from the metal in the form of schreibersite, (FeNi),P. Figure 5 shows a microstructure of a spherule with globular metal in which schreibersite has begun to precipitate inside the metal phase (solid arrow). As schreibersite-metal eutectic which formed upon original solidification is also visible (open arrow). Alternatively, this solid-state precipitation could occur if the spherule was reheated after solidification and cooled slowly. The presence of solid-state precipitation therefore, indicates slow cooling or a reheating cycle. Time periods of from 1 h to several months will allow for the solid-state precipitation by either process. Specific lunar spherules which show evidence of solid-state precipitation are noted in Table 2. Based upon such microstructural evidence, about 60 per cent of the Apollo 14 spherules, 20 per cent of the Apollo 15 spherules, and 50 per cent of the Apollo 16 spherules were subjected to slow cooling or a reheating cycle. In addition the vast majority of samples with phosphide precipitates had the globular metal characteristic of rather slow solidification rates. Areas of eutectic containing about 11 wt. % P were observed in a number of lunar metallic spherules with bulk P contents over about l-2 wt. %. A eutectic of a-Fe/(FeNi),P at 11 wt. ‘A P is predicted by the Fe-Ni-P equilibrium diagram (DOAN and GOLDSTEIN,1970). In particles containing over 2 wt. % S a eutectic of y-Fe/FeS was sometimes observed (Fig. le). Such a eutectic is expected from a study of the Fe-Ni-S phase diagram (KULLERUD,1962). In both phosphide and sulfide eutectics, the metal appears discontinuous in the microstructure. B. Experimental

metallic spherules

Measurements of individual spherule compositions produced during each experimental run were made. The number of spherules analyzed and the range of

Investigation

and simulation of metallic spherules from lunar soils

313

spherule compositions within each experimental run are listed in Table 1. In most cases, except for Co, this range of compositions far exceeds the compositional accuracy and counting rate limits (Ni rt 0.09 wt. %, Co rfr 0.OGwt. %, S f 0.07 wt. %, and P f 0.07 wt.%). A very small compositional spread was measured for the spherules produced from the INCO alloys (Run Number 17,19,21). Only relatively small compositional spreads were measured for Runs 14 and 27 where large amounts of S were added and for Runs 23 and 28 where large amounts of P were added. On the other hand, large compositional spreads appear to occur in Runs 29 and 30 where large amounts of both S and P were added to the basic INCO alloy. Apparently melts which are rich in both P and S allow spherules to form with quite different P and S contents. Local inhomogeneities in the original melt, on the scale of (100 pm, were not measured. Examination of the external features of the simulated metal particles revealed a great diversity in appearance. Figure 6 shows numerous configurations of simulated metallic spherules and several other configurations of metal with adhering silica grains. Geometries ranged from nearly perfect freestanding metal spheres to ellipsoidal metal particles to flattened irregular ‘splats’ of metal with sand grains attached to their undersides. There is a much higher proportion of spherules in the smaller size fractions and the nearly perfect freestanding metal spheres are almost always 1100 pm in size. It is apparent that the smaller spherules solidified by radiation cooling before landing on the silica sand. Larger particles landed while partially solidified. It should be noted that several large metal particles have smaller spherules attached to their surface (Fig. 6). Because of the large variations in spherule velocities after extrusion through the crucible, it is probable that various sized particles impact on each other causing the spherules to become attached. The external morphologies of the experimentally produced metallic spherules are similar in many ways to those of the lunar samples. Metallographic preparation of the experimentally produced spherules revealed internal structures similar in size and morphology to the rapidly solidified lunar particles. Over 75 per cent of the spherules in any experimental run solidified rapidly having either dendritic or cellular structure. By contrast, only 30 per cent of the lunar spherules showed such texture. Several examples of simulated spherule microstructures are shown in Fig. 7. They compare favorably to the lunar sample photomicrographs in Fig. 1. The same four phases were observed: a-Fe, y-Fe, FeS and (Fe, Ni),P. Where sufficient S and P were present, as in lunar samples, phosphide-metal eutectic and/or sulfide-metal eutectic was observed. Measurements of dendrite secondary arm spacings and cell sizes were made on the experimentally produced spherules. Arm spacings and cell sizes from particles in high S run 14, high P run 28 and high S-high P runs 29, 30 and 31 are plotted versus particle diameter in Fig. 8. The open symbols indicate cell sizes while the solid symbols indicate dendrite sizes. For a given spherule diameter, as much as an order of magnitude variation in solidification element spacing was observed. In general, the cell sizes lay above those for. the arm spacings. It is clear that different rates of cooling are obtained for particles of the same diameter. The amount of variation of cooling rate is discussed in the following section. The measured dendrite arm spacings and cell sizes overlap those determined for the lunar spherules.

P. J. BLAU and J. I. GOLDSTEIN

314

0 a 0 0 x

RUN RUN RUN RUN RUN

0.1

10

I.000

loo OIAYETER.

29 30 31 14 28

I

10.000

miuonr

Fig. 8. Summary of dendrite secondary arm spacings and cell sizes for esperimental spherules of various diameters. Solid symbols are used for dendrite secondary arm spacings, and open symbols indicate cell sizes. The estimated cooling razes (right ordinate) and heat transfer calculations (dashed lines) are represented.

A.

~~0~~~~ rates

of~~~~r~~e~a~ ~~~~~~

~~h~r~l~~

FLEMINM et aE. (1967) have used dendrite secondary arm spacings, and LTTAMIS and ELEMINGS (1966) have used cell spacing (‘cell size’) measurements to

arrive at cooling rate estimates for Fe-Ni alloys. As in most materials, cooling rate decreases with increasing solidification element spacing. The cooling rates of the e~erimentally produced spherules can be obtained through this relationship for Fe-Ni alloys and is represented in Fig. 8 by the right and left vertical axes. Cooling rates for the experimental spherules vary from 700’C/sec up to 106”C/sec depending on the diameter of the particle. For cooling rates of lo6 “C/set and above, spiat cooled particles are usually observed. Up to three orders of magnitude differences in cooling rate are measured for particles of the same diameter (Fig. 8). It should also be noted that spherules with dendritic str~ctu~s cooled faster than spherules with cellular structures. In order to understand the range of cooling rates obtained, heat transfer calculations for radiation to space and conduction to silicate substrates nvere made. The heat loss Q (calories) required to solidify a molten Fe-Ni sphere and cool it through the solid plus liquid temperature range is given by

Q = Qx +&a &I = VP&

(1) (-"I

Q2 = V&AT,

(3)

Investigation

and simulation of metallic spherules from lunar soils

315

where Q1 is the heat loss from the heat of fusion, L, Q2 is the heat loss during cooling through the solidification temperature range AT, V is the volume of the droplet, p is its density, and C is the specific heat. The heat transfer relations used (HOLMAN, 1972),

were qT = FeAa(

T,4

-

Ta4)

(4

for radiation, and qcond =

kA(T,

-

Ta)

iI, q-‘”

(5)

for conductive heat transfer through a droplet surface of area A into a semi in&rite solid, where 9; is the radiative rate of heat transfer, qcondis the conduction heat transfer rate, F is the geometrical view factor, E is the emissivity, T, is the initial temperature of the droplet, T, is the temperature of the environment, k is the thermal conductivity of the substrate, C, is the heat capacity of the substrate, ps is the density of the substrate and T is the time after the hot sphere contacts the environment, (Table 3). The final cooling rates of a given spherule, R(“C/sec) were found from R = AT (%ond +

a,)/&.

(6)

The calculated relations of cooling rates as a function of spherule diameter are plotted on Fig. 8 for radiation cooling and conduction cooling in solid silica. As Table 3. Values used in cooling rate calculations Term

Value

P

8.0 g/cm3

C

PS

2.24 g/cm3 (silica) 0.117 c&l/g Oc!

C* L

0.191 c&l/g OC 67 c&l/g

AT

5ooY!

F E

T,

1.0 0.28 (metallic) 0.90 (silica) 1.336 x lo-l2 cal/sec cm3 ‘C2 4.38 x lo9 cal/l/sec cm2 (‘C/cm) 1260%

TlI 7

20% No value assigned

k”

Source Fe-Ni alloys, AMER. SOC. FOR MIETD (1962), p. 62 FOUST et ol. (1960), p. 660 Fe-Ni ahoys, An5ZR. SOC. FOR &&T&S (1962), pp. 22-23 FOTJSTet OZ. (1960), p. 666 Fe, ARRR. SW. FOR &TAI.s (1962) p. 46 Based on Fe-Ni-P and Fe-Ni-S phase diagrams, DOAR and GOLDSTEIN (1970), Kuzn~~ UD (1962) Radiation in all directions FOUST et uZ. (1960), p. 667 FOUST et al. (1960), p. 667 Stefan-Boltzman conscant FOUST et al. (1960), p. 660 Midpoint of a solidifkation range between 1600’ and 1000% Becomes equal to time to solidify, and drops out in the solution of equation (6)

316

P. J. BLAU and J. I. GOLDSTEIN

indicated, the slowest cooled samples correspond more closely with radiation cooling conditions, E = O-28 and O-90, and the rapidly solidified samples cooled by conduction into surrounding solid silica. The maximum solidification time for a spherule to cool by radiation during travel down the length (89 cm) of the solidification chamber is approximately 0.4 set providing the spherule has a small initial velocity, of ~0 cm/set as it leaves the melting crucible. If cooling proceeds through a solidification temperature range of 5OO”C,the slowest possible cooling rate is 1180”C/sec. If one considers that the spherule cools through a solidification range from liquid to solid of 250°C, the slowest cooling rate that can be obtained by radiation in the experimental apparatus is 590’C/sec. Our slowest cooling rates obtained for the experimental alloys lie within this cooling range (590-118O”C), Fig. 8. Particles which are truly spherical in shape were found only in a size range below 20pm These spherules were cooled by radiation in the vacuum of the solidification chamber Also no textural differences were seen across the slow, radiation cooled spherules, indicating that no temperature differences were present across the spherule during cooling. The minimum solidification time or maximum cooling rate for the experimental spherules should be given by the curve calculated for conduction heat transfer to solid silica (Fig. 8). Our experimental measurements indicate a faster cooling rate than the calculated curve by a factor of from 4 to 7. Considering the difficulty in modeling a process by which molten spheres of metal solidify when impacting at velocities up to 550 cm/set on solid-irregular silica grains, an error of up to one order of magnitude might be expected. Intermediate cooling rates are obtained (Fig. 8) when cooling occurs by a combination of radiation and conduction cooling and the spherules impinge upon the silica when already partially solidified. B. Cooling rates of lunar metallic spherules The data for lunar spherules, arm spacing (d) and cell size (c), are plotted vs particle diameter in Fig. 9. It would appear that these lunar spherules experienced cooling rates approximately the same as those of the experimentally produced spherules and may therefore have cooled under similar conditions. It should be noted that several lunar spherules, >200 pm diameter, partially or completely solidified by radiation cooling in the lunar environment. As discussed previously, it was not possible for us to simulate radiation cooling for spherules > 200 pm diameter in our apparatus. Furthermore, in accordance with simulation data, the cellular lunar samples (open symbols) had coarser spacings than did the dendritic samples (solid symbols). Cooling rates ranged from 10 to 5 x 105’C/sec among all the speicmens. For the 100 pm diameter samples, cooling rates spanned over three orders of magnitude from the limits of conduction to radiation cooling. The Apollo 11 spherule from Table 2, the ‘minimoon’ (MASON et al. 1970; GOLDSTEIN et al. 1970), has the slowest cooling rate, lO”C/sec. The large variation in lunar metallic spherule cooling rates indicates that solidification has proceeded by a number of different processes. Some spherules partially or completely solidified by a radiation process in the Moon’s vacuum when liquid metal was ejected from the projectile impact site (6 or about 10 per cent of the lunar spherules investigated). A smaller number of spherules partially or completely

Investigation and simulation of metallic spherules from lunar soils

0 0 0 0 0

DIAMETER.

APOLLO APOLLO APOLLO APOLLO APOLLO

317

II I2 I4 IS IS

nicrom

Fig. 9. Summary of dendrite secondary arm spacings and cell sizes for lunar spherules. Solid symbols are used for dendrite secondary arm spacings, and open symbols indicate cell sizes. The estimated cooling rates (right ordinate) and heat transfer calculations (dashed lines) are represented.

solidified by a conduction process on a silicate substrate almost immediately after liquid metal was ejected from the projectile impact site (3 or about 5 per cent of the lunar spherules investigated). Fragments of target or projectile material in the impact cloud may furnish this substrate material. About 9 (15 per cent) of the lunar spherules cooled by both radiation and conduction. FRONDEL et al. (1970) have studied metallic spherules which are not perfectly round. Two spherules showed unequal radii and it was postulated that these droplets were formed as mounds on glass surfaces which were subsequently detached. During an impact event such processes are quite possible due to the varieties of materials in the impact cloud and their various initial velocities and trajectories. MCKAY et al. (1971) have postulated that the ‘minimoon’, the large Apollo 11 spherule, and another particle with unequal radii were solidified as mounds attached to glass particles. It was removed from the glass at a later date. These processes are quite compatible with our cooling rate calculations and intermediate cooling rates either above or below the radiation cooling curves can be obtained depending on the temperature of the glass when solidification of the metal occurred. The majority of lunar spherules, approximately 70 per cent of our sample, are not represented in Fig. 9. The vast majority of these spherules have globular metal areas (Fig. le, f) which indicate slower cooling rates than the particles cooled by radiation in the lunar environment. Some of these particles have precipitates formed by solid-state precipitation either on slow cooling or reheating. The spherules with globular metal areas were probably solidified in place within a glassy matrix, the

318

P. J. BLAU and J. I. GOLDSTEIN

lunar soil, or in a fragmental rock after liquid metal was produced by the impact process. These various environments will lead to cooling rates slower than those predicted for radiation cooling. In many cases slow cooling of the surrounding silicate material allowed the metal phases to coarsen producing globular metal areas. In some cases solid state precipitation (Fig. 5) occurred at lower temperatures. Subsequent impact events presumably caused the metal to be fragmented out of the surrounding silicate. The structure of the metal in many chondrites, in which impact events have caused localized melting of metal-troilite areas, is similar to those of the globular metallic spherules, for example, Ramsdorf (BEGEMANNand WLOTZKA,1969). In these chondrites the molten metal has solidified in place, within the silicate. C. Sources

of the lunar metallic spherules

The sources of metallic spherules on the Earth have been discussed by various authors including NININGER(1956), MEAD et al. (1965), RAMDOHR(1966), BLAU et al. (1973) and BRETT (1967). There is almost universal agreement that the spherules were produced by meteorites impacting the Earth and the ultimate source of the spherules was the meteorite projectile itself. BLAU et al. (1973) proposed, for the Barringer Crater site, that the spherules were produced by the localized shock-melting of metallic and non-metallic constituents at metal-sulfide phase interfaces in the meteorite at the instant of impact. The molten liquid produced breaks into droplets which subsequently solidify. This same mechanism can be used to describe spherule formation from meteorite projectiles impacting on the Moon. If the impacting body was an iron meteorite it would be expected, as in the case of the Canyon Diablo iron meteorite (BLAU et al. 1973), that the metallic spherule would be sulfur rich. If the impacting body was chondritic, it would be expected, as in the case of the shocked melted chondrites, (BEQE~N and WOLTZIEA, 1969; TAYLORand HEYMAYNX, 1971), that the metallic spherules would also show large sulfur enrichments. This enrichment of sulfur in chondrites is due to the fact that FeS is a major component of the chondrites, often associated with the metal. Even though (FeNi),P, schreibersite, is usually not present in chondrites, TAYLOR and HEYMANN(1971), reported that phosphorus enrichment was observed in chondritic metal spherules. It has also been proposed that phosphorus enters the metal by reduction of chondritic phosphates during shock (TAYLORand HEYMANN, 1971). The phosphorus enrichment is usually much smaller than that of the sulfur, however. Therefore, it would be expected that metallic spherules produced on the Moon from the impacting meteorite projectile would be sulfur rich. It should be noted that the 6 spherules (Fig. 9) which cooled by radiation are all enriched in sulfur. These spherules were probably ejected away from the crater site after impact and solidified during passage back to the Moon’s surface. All the other spherules plotted in Fig. 9 are either phosphorus rich or have similar amounts of sulfur and phosphorus. It appears by analogy with meteoritic spherules that some of the lunar spherules have their immediate origin in meteorite projectiles which bombard the Moon. They have not been reworked before they were sampled. However, much of our data are not consistent with a meteoritic projectile source for the spherules, for example: (1) two thirds of the spherules are enriched in phosphorus with respect

Investigation

and simulation of metallic spherules from lunar soils

319

to sulfur; (2) two of the metallic spherules have non-meteoritic Ni-Co contents similar to those of metal particles from Apollo 15 igneous basalts (DOWTY et al., 1973). In addition, cooling rates for most of the lunar spherules are slower than those due to a radiation cooling mechanism. These slow cooling rates indicate that most of the solidification has taken place within a glass, or silicate matrix, or within the lunar soil. However, cooling within a glass is not inconsistent with a meteoritie source. In summary, we propose that the major source of the lunar spherules is the metal present in the target material, the lunar rocks and soil. Although the vast majority of the metallic spherules have a Ni-Co content similar to that of ordinary chondrites and iron meteorites, the spherules could be produced from lunar target meterial if the metal in the lunar fragmental rocks and soil was meteoritic in composition. It is clear that many of the metal particles in breccia rocks (ALBEE et al., 1973; EL GORESYet al., 1973) and soils (GOLDSTEIN et a?., 1973 ; WLOTZKAet al., 1972) from the various Apollo sites do contain metal of meteoritic Ni-Co content. The source of some of the metal in the target material may be meteoritic but the metal has been reworked, mixed with lunar silicate and its original structure obliterated. If these rocks and soils are shock melted during an impact event, the metallic spherules produced from the metal in the target material would have similar Fe-Ni contents to those spherules produced from shock melted metal from the meteoritic projectile. An example of the production of spherules of meteoritic composition in a lunar rock by the heating effect of a glass coating, which was sufficient to produce partial melting within the host rock, was discussed by GRIEVEand PLANT (1973). Phosphorus enrichment of the lunar spherules can be produced upon impact by phosphate reduction during impact melting as proposed by GOLDSTIX.I~’ et al. (1972) and RIDLEY et aZ. (1972) for Fra Mauro basalt 14310. MCK.AYet al. (1973) suggest that P-rich spherules can be formed by reduction of Fe and P from a silicate melt similar in composition to that of KREEP. Reduction of Fea+ in the liquid with subsequent scavenging of phosphorus has been demonstrated in 1 h heattreatments. It is clear, however, that the production of isolated regions of iron in the size range 2100 pm cannot easily occur in the short time usually ascribed to an impact event, although smaller sizes of iron spherules (5 pm have been observed (SCLAII.et al., 1973). We suggest that the metal phase must be present initially and that during the impact event sufficient phosphorus can be reduced from the shocked and partially melted rock. GOOLEYet al. (1973) showed that slower cooled metal should contain more phosphorus, all else being equal. Sulfur enrichment can be produced by the presence of sulfides in lunar rocks often in close association with the metallic phases. Some of the impact produced metallic spherules from a lunar source may be ejected at high velocities away from the crater site. In many cases, however, the metal may be held by the surrounding soil, silicate and/or glass and may cool rather more slowly than allowed by radiation cooling. Subsequent bombardment of the host rock or soil can free the spherule to the lunar soil. This study has described metallic spherules extracted from various lunar soils. In addition, it should be noted that metal spherules are present within glass particles and glass coatings on rocks and soil particles (AGRELLet al., 1970; ALBEE and CHODOS,1970; FREDRIKSSON et al., 1970; SIMPSONand BOWIE, 1970; WOSINSEI

320

P. J. BLAU and J. I.GOLDSTEIX

et al., 1972 ; GRIEVEand PLANT, 1973)as mounds on the surface of glass particles (CBRTERand MACGREGOR,1979; MCKAY et al., 1970; VOX ENQELHARDTet al’., 1970; CARTER,~~~~; &&Kay etaZ.,1971;VON EKGEIXARDT etaE.,l971;CARTER and MCKAY, 1972 ; CARTER, 1972) and in lunar breocias and metamorphic rocks (HELZ, 1972; EL GORESEYet al., 1973; MCKAY et al., 1973). The structures of the metal spherules associated with glass and the lunar rocks range from X-poor iron to Ni-rich spheruIes, and incIudes troilite spheres and iron metal spheres which are enriched with Ni and Co. The latter type of spherule contains varying amounts of schreibersite and is often rimmed with troilite. Some of the particles show dendritic structures and some have meteoritic Ni-Co contents. Commonly, the larger spherules contain Ni-Co rich metal and sulfide while the smaller spherules often tend to be metallic iron. The mounds on glassy particles form in part from dropIets of those fiquids which are present in the vapor cloud generated by impact and solidify on the outside of the glass. Vapor deposition of some metallic mounds are also proposed (CARTER and MC&Y, 1972). Several authors, for example, AGRELLet al. (1970) and ALBEEand CHODOS(1970), have pointed out that the metal spherules associated with glass have both a lunar and meteoritic origin. They proposed that the metallic spherules are derived from both the melting of metal phases in surface rocks and the shock melting of meteoritic metallic phases in the impacting projectile. Our results for isolated metallic spherules are in agreement with their conclusions. On the Moon, the production of spherules is a continuous one. The spherules are produced at one period of time, in an isolated event, and then may be buried, incorporated in the soil or fragmental rocks, reheated several times and perhaps even remelted in a shock event at a later date. The fact that 60 per cent of the Apollo 14 spherules and 50 per cent of the Apollo 16 spherules showed structural evidence of at least one thermal cycle is consistent with the fact that these highland regions were formed during the early period of lunar history when the Moon’s surface was heavily bombarded by meteorites and asteroids. The Apollo 15 sitewhich shows less evidence of this bombardment, has only 20 per cent of its spherules showing effects of thermal cycling. D. Variation in the composition

of the Zunar spherules

The probIem of seIective enrichment or depletion of certain elements in meteoritic spherules found on Earth has been discussed in several papers (RAMDOHR, 1966, BLAU et al. 1973; BRETT, 1967).On the Earth, solidification of the molten droplets proceeds mainly by convection in the atmosphere, and Ni-Co enrichment is caused by partial oxidation of the molten spherules. By this mechanism, the source of sulfur and phosphorus is from the impacting body and the enrichment of Ni and Co is caused by oxidation. In the lunar case, sulfur and phosphorus e~ichment occurs by shock melting metal-sulfide phase interfaces, phosphorus reduction from surrounding phosphorus-rich phases and/or from phosphides in the shocked material. At very high shock pressures, shock melting can also occur at any point in the metal phase and no enrichment of phosphorus and sulfur need occur. It is of interest to ask what happens chemically to the phosphorus, sulfur-rich melt which is ejected from the crater site.

Investigation

and simulation of metallic spherules from lunar soils

321

Bulk chemical analyses of simulated spherules provided information about the range of com~sitions which could be produced from a single melt (Table 1). As mentioned earlier, the runs which produced the largest composition changes from one spherule to another were those with large amounts of S and P in combination (Runs 29 and 30). To investigate further the problem of bulk composition distributions, polished mounts were made of the residues removed from the crucibles used in some of the runs after the melts had been partially extruded. Figure 10 is a photomicrograph of the residue from the high P, high S, Run 29. Run 29 residues show evidence that a system of two immiscible liquids may have existed in melts with large S and P contents. The sulfide component at the left is porous and contains troilite (FeS) with some small inclusions of nickel-iron. The phosphide component at the right contained most of the P, Ni and Co, and took the form of nickel-iron grains surrounded by sc~ibersi~, (FeNi),P, and a few tiny blebs of troilite. Thus, a shock melted portion of a meteorib or a lunar metal particle might contain two liquids, each quite different from the other with respect to chemical composition. Since spherules formed from different parts of a segregated melt crould exhibit quite different compositions, the bulk chemistry of the starting liquid for any given lunar spherule can not be accurately estimated. Analogous two-liquid spherules have been observed in the lunar samples. Figure 11 shows an Apollo 14 spherule containing a metallic core with a sulfide husk around it, and Fig. lc shows a more commonly observed situation in which most of the sulfide husk seems to have been broken away from the metal core. A large amount of fine-grained scbreibersite with a-FeNi eutectic is present in these metal cores. Samples with sulfide husks are noted in Table 2. Since the sulfide husk is brittle, it is quite probable that many of the lunar spherules were encased in sulfide which has been removed by subsequent shock processes. An example of an experimental spherule with a sulfide husk is shown in Fig. 70 and comes from the high (S + P) combination Run 29. Spherules 811.17 from soil 15261 and F15.1, Fl5.11 from soil 15271 (Table 1) have very high S contents, >13 per cent. They may represent fragments of sulfide husks and their low Co content may be due to formation by shook produced high-P, high-S liquids in which Co and Ni preferentially segregates into the high-P liquid. CoNoLUsIoNs

The experimentally produced metallic spherules have structures similar in size, morphology and cooling rates (7 x 10~106D~~sec) to the most rapidly solidified lunar spherules. The majority of the lunar spherules however, have either cooled more slowly than the experimentally produced spherules or have been reheated in place within lunar fragmental rocks, glass or soil. 2. The structural evidence of reheating and/or slow cooling of a majority of Apollo 14 and 16 spherules reflects the heavy meteoritic bombardment which is evident in the highlands. 3. There are two sources for the lunar metallic spherules. One source is the meteoritic projectiles which bombard the lunar surface. These spherules are generally sulfur rich and solidify by radiation cooling in the lunar atmosphere. The major source of metallic spherules is the metal phases present in the lunar rocks and soil. These metal phases are shock melted during impact. The primary origin of this metal may be meteoritic but the metal has been reworked and its original structure obliterated. It has also been mixed with the lunar rocks and soil. These metallic 1.

322

P. J. BLATJand J. I. GOLDSTEIN

spherules produced from the target material are often greatly enriched in phosphorus and have slow cooling rates controlled by solidification within the lunar rocks and soil. Further bombardment releases these spherules from their host rocks and soil. 4. The mechanisms responsible for S and P enrichments in lunar spherules produced from either source are shock melting at metal-sulfide interfaces and/or metal-phosphide interfaces as well as reduction from surrounding phosphorus rich phases during the shock melting process. 5. The large variation in bulk spherule compositions (0.2-22.5 wt.% Ni, O*l2.35 wt.% Co, O-04-26 wt.% S, 0.02-13-25 wt.% P) is attributed to the different meteoritic projectiles bombarding the Moon, metal phases of differing compositions in the lunar soils and rock and to the experimental results which indicate that high-S, high-P alloys form 2 immiscible liquids when melted. The latter process is responsible for the sulfide husks which often form around lunar spherules and for the segregation of Co and Ni into the high-P liquid. Acknoutledgements-The authors wish to thank Dr. H. J. AXON, University of Manchester, Great Britain, for his valuable advice and encouragement. They also wish to thank Dr. H. D. BRODY, University of Pittsburgh, and Dr. R. M. POND, Sr., John Hopkins University for their helpful discussions and Dr. H. YAKOWI~Zand Mr. D. B&LARD, Natiomxl Bureau of Star.&rds, for the Scanning Electron Microscopy. The teohnical assistrtnceof Messrs. D. BUSH,M. SHESKA, and W. WALER of Lehigh is acknowledged. This study was supported through grant no. NCR 39-007-056 from the National Aeronautics and Space Administration. REFERENCES AQIZELLS. O., SCOONJ. H., Mum I. D., LONG J. V. P., MCCONXELJ. D. C. and PECKETTA. (1970) Observations on the chemistry, mineralogy and petrology of some Apollo 11 lunar samples. Proc. Apollo 11 Lunar Sci. Conf., Geochim. Coanzochim. Acta Sup@ 1, Vol. 1, pp. 93-128. Perg8rnon Press. ALBEE A. L. and CHODOSA. A. (1970) Microprobe investigations on Apollo 11 samples. Proc. .4pollo 11 Lunar&i. Conf., Geochim. Cosmochim. ActaSuppl. 1, Vol. 1, pp. 135-157. Pergamon . Press. ALBEEA. L., GANCARZA. J. and CHODOSA. A. (1973) Metamorphism of Apollo 16 and 17 and Luna 20 metaclastic rocks at about 395 AE: samples 61156, 64423, 14-2, 65015, 67483, 15-2, 76055, 22006 and 22007. Proc. Fourth Lunar Sci. Conf., Geochim. Cosmochim Acta Suppl. 4, Vol. 1, pp. 569-595. Pergamon Press. A~~ERICAN sot. FORME+~s (1962) Metals Handbook, Vol. 1. Metals Park, Ohio. BEQEXANNF. and WLOT~KAF. (1969) Shock induced thermal metamorphism and mechanical deformations in the Ramsdorf Chondrite. Geochim. Cosmochim. Acta 33, 1351-1370. BLAU P. J. (1973) Investigation and simulation of metallic spherules from lunar soils. M. S. Thesis, Lehigh University. BLAU P. J., AXON H. J. and GOLDSTEIN J. I. (1973) Investigation of the Canyon Diablo metallic spheroids and their relationship to the breakup of the Canyon Diablo meteorite. J. Geophys. Res. 78, 363-374. BIDETS R. (1967) Metallic spherulesin impactite and tektite glasses. Amer. Mineral. s&721-733. BROU~NG. M., PECKETTA., PHILLIPSR. and E~~ELEUS C. H. (1973) Mineral-chemical variations in the Apollo 16 magnesia-feldspathic highland rocks. Proc. Fourth Lunar Sci. Conf., Geochina. Cosmochim. Acta SuppZ. 4, Vol. 1, pp. 505-518. Pergamon Press. CARTERJ. L. (1971) Chemistry and surface morphology of fragments from Apollo 12 soil. Proc. Second Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 2, Vol. 1, pp. 873-892. M.I.T. Press. CASTERJ. L. (1972) Morphology and chemistry of glass surface of breccia 15015,36. In The Apollo 15 Lunar Samples, (editors J. W. Chamberlain and C. Watkins), pp. 51-53. Lunar Science Institute, Houston. CARTERJ. L. and MACGBECORI. D. (1970) Mineralogy, petrology and surface features of some

Investigation and simulation of metallic apherulesfrom lunar eoils

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