Shock-produced vapor-grown crystals in the Y anzhuang meteorite
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VOI. 40 NO. 2 SCIENCE IN CHINA (Series D) April 1997
Shock-produced vapor-grown crystals in the
Y anzhuang meteorite *
XIE Xiande ($%sf@)
and CHEN Ming (R @))
(Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China)
Received October 3, 1996
Abstract Vapor-grown crystals intimately related to shock metamorphism of meteorites were found in the
Yanzhuang (H6) chondrite which had been heavily impacted in the space. These crystals include: (i) subhedral low-
Ca pyroxene occurring on the wall of the pores within a silicate melt pocket that experienced a shock temperature high-
er than 1 500C. (ii)Fe-Ni needle-whiskers (taenite) occurring in the cracks in the partially melted chondritic facies
that experienced a shock temperature of 850-1 3 0 0 C , (iii) troilite with abundant microholes occurring in the cracks
in the brecciated facies and the lightly deformed chondritic facies that experienced a shock temperature lower than
850C . The occurrence and mineralogical features of vapor-grown crystals show that vaporization of minerals could be
produced in heavily impacted meteorites and that a small amount of crystals could be deposited in situ from vapor phas-
es.
Keywords: Yanzhuang meteorite, shock metamorphism, vapor-grown crystal.
Vugs with vapor-grown crystals are found to be common in ordinary chondrites[ll, and they
have also been identified in the lunar b r e c c i a ~ [ ~There
I. are some arguments about the origins of
these vapor-grown crystals. Ref. [2] concluded that these crystals could have formed through va-
por deposition during the shock-induced thermal metamorphism in these meteorites or in the lunar
breccias. Some authors considered that the formation of vapor-grown crystals was not due to im-
pact events. As indicated by ~ i e f f l e r ' ~ ] Stsffler et a1 . [41, the environment within the impact-
and
ed meteorites could not be suitable for the formation of vugs with vapor-grown crystals. Olsen's
investigation''] showed that there is no correlation between the presence o r absence of vugs and
the level of shock in these chondrites because the vugs occur in many lightly to heavily shocked or-
dinary chondrites. It remains open whether vapor-grown crystals could be produced in meteorites
by impact events.
Temperature is crucial for inducing vaporization and deposition of minerals. Since the equilib-
rium temperature of thermal metamorphism of chondrites induced through decay of short-lived ra-
dio nuclides or accretion of host materials reached 730-960°C in the H6 chondrites and 810-
1 100°C in the L6 c h o n d r i t e ~ ' ~ ]the
, high temperature could induce vaporization and deposition of
some minerals. Even if the thermal metamorphism in the chondritic parent bodies could trigger
the formation of some vapor-grown crystals, the crystals from this origin should not characterize
the relation to impact events. T o better understand the occurrence and mineralogical characteris-
tics of shock-related vapor-grown crystals, we conducted an investigation on the heavily impacted
Yanzhuang ( H6 ) chondrites.
* Project supported by the Natural Science Foundation of Guangdong Province.114 SCIENCE IN CHINA (Series D) Vol. 40
1 Experimental methods
Thin sections of Yanzhuang meteorite were prepared for petrologic study. Several small
pieces of stone containing pores, cracks and vugs were taken from parts of meteorite, and the oc-
currence and mineralogical features of vapor-grown crystals were studied by using optical micro-
scopes and scanning electron microscope (SEM) . X-ray powder diffraction analysis was conducted
to further identify some vapor-grown crystals. Mineral compositions were measured on the pol-
ished sections using an ARL-SEMQ electron probe microanalyzer (EPMA) at 15 kV acceleration
voltage, 25 nA sample current and ZAF data correction. Some vapor-grown crystals occurring in
very small pores and cracks prevent us from preparing polished sections for compositional measure-
ment using EPMA. In this case, a Tracor Northern 5500 energy dispersive spectrometer (EDS)
installed on an SEM was used to determine the surface compositions of crystals using standardless
semi-quantitative analysis program, 20 kV acceleration voltage and ZAF data correction.
2 Yanzhuang meteorite and its shock metamorphism
The Yanzhuang meteorite is a fall that had been heavily impacted in the A total of
3 . 5 kg meteoritic materials were recovered, and the largest piece of fragments weighs 823 g. The
meteorite consists of unmelted chondritic parts and shock-produced melt facies including the net-
works of veins ranging in width from 0 . 1 to 15 mm, and the pockets are up to 24 cm3. The
chemical composition of melt facies is the same as that of the unmelted chondritic parts[71. The
rock-forming minerals in chondrite are olivine, low-Ca pyroxene, plagioclase, kamacite, taenite
and troilite. The Yanzhuang meteorite is a highly equilibrated ordinary chondrite characteristic of
poorly-defined chondrules and high degree of crystallinity of matrixes among chondrules. Olivine
and low-Ca pyroxene in the chondritic part are compositionally homogeneous with -
Fal9 and
-FsI6, respectively. The Yanzhuang meteorite was classified as an H6 chondriteL6'.
The texture and structure of Yanzhuang meteorite are heterogeneous. Four shock metamor-
phic facies have been revealed from the meteorite (fig. 1( a ) ) : ( i ) Slightly deformed chondritic
facies. The facies makes up about 40 % by volume of stone. Major metamorphic characteristics of
this facies results from mechanical deformation of minerals. Olivine displays intense undulatory
extinction, four to five sets of planar fractures and mosaicism with domains of 10 to 15 pm in di-
ameter. Since a majority of minerals including troilite were not melted during the impact event,
the shock temperature should be below 850°C . (ii) Brecciated facies. This facies makes up about
20 % by volume of stone. Silicates were broken into fragments of micrometer size. Plagioclase was
transformed into maskelynite. (iii) Partially melted facies. This facies occurs on neighboring melt
veins and melt pockets, and it makes up about 10 % by volume of stone. All plagioclase was melt-
ed, while troilite, metal, olivine and pyroxene were partially melted. Solid state recrystallization
of oilvine and pyroxene has occurred extensively. Melted metal and troilite penetrated into the
fractures of silicates, darkening these areas. The shock temperature is estimated to be from 850 to
1 300°C because the rock-forming minerals were partially melted. (iv) Melt facies including melt
veins and melt pockets. The veins and pockets which connect with each other and penetrate the
whole meteorite make up about 30 % by volume of stone. The facies consists of recrystallized mi-
crocrystalline olivine and pyroxene ( l to 5 pm in size), silicate melt glass and metal-troiliteNo. 2 SHOCK-PROIIUCEI~VAPOR-GROWN (:KYSI'.AI-S 1N YANZHUANG MEfE0KI'I.E 115
eutectic nodules. 'Ihe total melting of this facies during impact event is indicative of shock tem-
peratures higher than 1 500°C . T h e cooling history ol melt facies in the interval from 1 400 to
950°C has been revealed as 100-400°C /s in the melt veins and 6-30°C / s in the melt p c k e t s L 8 ] .
Fig. I ( a ) Heav~lyshocked Yanzhuang meteorite consisting of four shock met;irnorphisrn fac~eh: ( 1 ) slightly de-
formed chor~driticfacics and brccciated facies; ( 2 ) partially mclt f;icies; ( 3 ) nlelt facies-veins; and ( 4 ) melt facies-
pockets. (1)) Subhedral vapor-grown low-Ca pyroxene dep~)site(l(111 a porr wall within a melt pocket. Material of
pore w;ill is thv nrtdt facies consisting of microcrystalline olivirlt, ;irrd pyroxene crystallized from melt, as well as silicate
glass. (1.) \.'iip(,r-grown F v - U I metal needles occur in a crack lrl the partially meltt.d facies. ( d ) The ring growth
steps with e q ~ ~ ;distance
il on the surface of metal needle. ( e ) V;ipor-grown troilite deposited in a crack in the slightly
deformed chcjndritic facics. Note the abundant microholes distril~utingin some crystals ( a r r o w s ) . ( f ) Networks of
microholv~111 v;ipor-grown troilitc.
D i ~ e ~ u i l i b r i u shock
ni effect is a common phenomenon in many heavily impacted chondritic
meteorites. 'The formation of this effect is believed to result from local deviations of shock pressure
and temperature from the equilibrium pressure and temperature experienced by the whole mete-
orite. Four met;imorphism facies occur in the Yanzhuang meteorite, thus indicating that the116 SCIENCE IN CHINA (Series D) Vol. 40
meteorite experienced disequilibrium pressures and temperatures induced by shock wave within its
stone.
The Yanzhuang meteorite was intersected by abundant cracks. It is probable that the cracks
were produced through the rarefaction wave relaxation after the shock wave had passed. There are
some small pores with diameters from 0 . 5 to 1 . 5 mm in melt pockets. These pores should have
been produced through vapor expansion of components vaporized from melt. Such pores are absent
in the melt veins. Cooling in the melt veins could be too fast to vaporize sufficient volatile matter
from melt to produce pores.
3 Vapor-grown crystals in the pores and cracks of meteorite
The primary pores and vugs with vapor-grown crystals were not found in the chondritic part.
However, some vapor-grown crystals found in some pores and cracks of the meteorite are consid-
ered to be of shock-related origin.
3.1 Low-Ca pyroxene
Vapor-grown subhedral low-Ca pyroxene ranging in size from 10 to 30 pm occurs on the pore
walls within melt pockets (fig. l ( a ) - 2 ) ) . Even though the melt pocket itself consists of abundant
microcrystalline low-Ca pyroxene crystallized from melt, we can distinguish the pyroxene crystal-
lized from melt (less than 5 pm) from vapor-grown pyroxene by grain size. The vapor-grown py-
roxene shows prismatic crystals in the combinated form of { 100 / , 1010 / , { 210 1 and 1 101 1 . Most
of these crystals have parallel C-axis, and are in well-developed 1 100 1 , { 010 / and { 210 1 forms.
The compositional measurement of crystal surfaces shows that the low-Ca pyroxene has a composi-
tion similar to those in the melt facies and the chondritic part (table 1 ) . The major difference is
that the vapor-grown low-Ca pyroxene contains very low A1203-content ( n o aluminum has been
detected by EDS) in contrast to high aluminum content of the low-Ca pyroxene (*0.7 % A1203)
in the melt facies and that (-0.2 % A1203) in the chondrite.
Table 1 Compositions of low-Ca pyroxene (weight percentage)
Vapor-grown Low-Ca pyroxene Low-Ca pyroxene
low-Ca pyroxenes' in chondriteb) in melt faciesb)
(14) ( s . d. ) (6) (s. d. ) (14) (s. d. )
NazO b. d. b. d. 0.02 0.01 0.07 0.06
MgO 32.0 1.9 30.79 0.56 30.28 1.65
CaO 0.8 0.3 0.68 0.04 1.39 0.97
FeO 10.2 3.7 10.96 0.32 10.19 0.54
MnO 0.4 0.1 0.47 0.02 0.41 0.04
4 2 0 3 b. d . b.d. 0.23 0.07 0.73 0.66
Cr203 0.9 0.3 0.15 0.05 0.76 0.40
vzO3 b.d. b. d. b. d. b. d. 0.04 0.06
TiOz b. d. b. d. 0.17 0.06 0.11 0.06
Si02 56.2 1.5 56.24 0.51 54.84 1.28
Totality 100.5 99.71 98.82
( ), Number of analyses; s. d . , standard deviation ( l o ) . a) by EDS; b) by EPMA.
3 . 2 Fe-Ni metal
Vapor-grown Fe-Ni metal needle-whiskers with a curvilinear shape were found in severalNo. 2 SHOCK-PRODUCED VAPOR-GROWN CRYSTALS IN YANZHUANG METEORITE 117
cracks of 0.05 to 1 mm in width within the partially melted chondritic facies (fig. l ( c ) ) . Euhe-
dral metal crystals are absent at the same sites. These needle-whiskers have diameters from 1 . 5 to
7 pm and lengths from 20 to 100 pm. Two sets of ring growth steps with equal distance are sym-
metrically distributed on the surface of each needle-whisker (fig. l ( d ) ) . The morphology of nee-
dle-whiskers is a result of skeletal growth crystallized rapidly from vapor phases. Fe-Ni metal nee-
dle-whiskers have an average Ni concentration of 42 % (weight percent) on their surfaces, which
is much higher than those of kamacite (*6.2 % Ni) and taenite ( = 2 8 . 8 % Ni) in the chondrite
(table 2 ) . Based on the high Ni content of needle-whiskers, they should be taenite. It has been
analyzed that the Ni-concentrations of metal dendrite in the melt facies are about 20 % (weight
percent) Ni in the dendritic rims and about 7 . 9 % (weight percent) Ni in the i n t e r i o u ~ ' ~ ]The
.
morphological and compositional characteristics of vapor-grown Fe-Ni needle-whiskers are appar-
ently different from other metal phases, such as the kamacite and taenite in the chondrite as well
as those metal dendrite in the melt facies.
Table 2 Compositions of Fe-Ni metal and troilite (weight percentage)
Vapor-grown Kamacite Taenite Vapor-grown
Fe-Ni needlea) in chondriteb) in chondriteb) troiliteb'
(15) (5) (5) (12)
Ni 42.6f 15.3 6.20k0.78 28.83k 1.40 n.a.
Co n.a. 0.65k0.13 0.23k0.04 n. a.
Fe 57.4"" 92.03f1.11 69.84k1.83 63.27k0.62
S n.a. n. a. n. a. 35.31f0.54
Totalitv 98.88 98.90 98.58
( ),Number of analyses; n . a . , not analyzed; * , average Fe content = 100Ni content; a) by EDS; b) by EPMA.
3 . 3 Troilite
In the brecciated facies and the slightly deformed chondritic facies, some cracks 0.01-0.5
mm in width were partially filled with granular and subhedral troilite having the grain sizes from
100 to 300 pm (fig. 1( e ) ) . Most of the crystals have well-developed 1 10711 form. Some troilites
contain abundant microholes with diameters from 2 to 5 pm. The microholes usually connect each
other and constitute networks ( fig. 1( f ) ) . Analyses by SEM and X-ray power diffraction show
that there are no other solid-state ~ h a s e sfilling in or attaching to the microholes. Compositions of
the troilite are 63.27 % (weight percent) Fe and 35.31 % (weight percent) S (table 2 ) .
We conclude that shock-compressed gaseous phases were trapped within vapor-grown
troilite. As soon as pressure was released and temperature decreased, the gases trapped in the mi-
croholes in troilite escaped, thus leaving microholes empty. The gases filling the microholes could
include &.
4 Discussion
Experimental vaporization of a chondrite by Gooding and ~ u e n o w ' indicated
~] that the sulfur
gas (s) appeared at above 800°C and reached the maximum abundance at about 1 250"C, and
that Fe- and Ni-vapor phases evidently increased above 1 100°C . As the Yanzhuang meteorite had
been heavily impacted, the severely shock-metamorphosed lithologies including the melt facies and
the partially melt facies make up about 40% volume of stone. These lithologies experienced shock118 SCIENCE IN CHINA (Series D) Vol. 40
temperatures from > 1 500°C to > 8 5 0 C . The shock temperatures were high enough t o induce
thermal decomposition and vaporization of some minerals, thus resulting in crystals including py-
roxene, Fe-Ni metal needle-whiskers and troilite deposited from shock-produced gaseous phases in
the pores and cracks of meteorite.
Based on the melting point temperatures of FeS ( =Z 1 195°C ), Fe-Ni metal ( 1 536-
1 453°C ) and MgSi03 ( - 1 657°C ) [ l o ' , the pertinent minerals will vaporize when heated from
solid state to gaseous phases in the order: FeS > Fe-Ni > MgSi03. This order shows relative fu-
gacity of these gaseous components produced from a shocked chondrite. T h e order may explain
the occurrence of vapor-grown crystals in the Yanzhuang meteorite : ( i ) At given temperature,
the abundance of pertinent vapor components for the formation of low-Ca pyroxene might be the
lowest among all produced components. Vapor-grown low-Ca pyroxene could occur only in the
pores within melted pockets in which the peak shock temperature and the fugacity of each gaseous
component vaporized from pyroxene were the highest. ( i i ) In the partially melted facies with
shock temperatures about 850-1 300"C, gaseous phases Fe and Ni might be vaporized in situ or
partially came from shock-produced melt. Fe-Ni needle-whiskers deposited in cracks as tempera-
ture decreased. (iii) Shock-induced volatile phases FeS and Sz could be the most abundant among
the vaporized gaseous components. These gaseous phases might be transported a long distance
from hotter areas to cooler areas, or from high pressure areas to low pressure areas. Vapor-grown
troilite finally deposited in the cracks in the slightly deformed chondritic facies and the brecciated
facies. Compared to other vapor-grown crystals, the troilite is one of the most abundant crystals
deposited due to the lower thermal decomposition temperature of FeS.
Olivine crystallized from gaseous components was not found in the Yanzhuang meteorite.
Experiment results indicated that the equilibrium coefficients ( K ) between olivine (Mg2Si04) or
pyroxene (MgSi03) and their vaporized gaseous components Mg, Si02 and O2 are K = and
K= - respectively at 1 100°C ['I, showing that under the same temperature conditions,
vapor-grown pyroxene is more easily formed than vapor-grown olivine, which is consistent with
the properties of vapor-grown crystals in the Yanzhuang meteorite. For example, there is coarse-
grained vapor-grown low-Ca pyroxene in the pores of melt pockets, but no vapor-grown olivine
has been found.
The duration of high pressure induced by the impact between the chondritic parent bodies is
usually as short as a few microseconds'"', even though the duration would last for several seconds
in the impact between big asteroids. The cooling of shock-heated meteorites was usually rapid
(1-300°C /s[12]). Instantaneous high temperature induced by shock wave is not enough for va-
porization and deposition of vapor-grown crystals in a great quantity in meteorite. Although the
Yanzhuang meteorite was severely shock-metamorphosed, only a small amount of vapor-grown
crystals have been found, hence indicating that the meteorite experienced a very short duration of
high pressure and high temperature. Among the tens of cracks investigated in the Yanzhuang me-
teorite, we found vapor-grown metal needle-whiskers and troilite only in several cracks. The rapid
cooling history of the Yanzhuang meteorite after impact event has been confirmend by the study of
metallic dendritesL8'and by the study of radiogenic gases"31 . Vapor-grown low-Ca pyroxene only
occurred in the melt pockets that experienced relatively low cooling rates (6-30°C /s['] ) and notNo. 2 SHOCK-PRODUCED VAPOR-GROWN CRYSTALS IN YANZHUANG METEORITE 119
in the melt veins that experienced rapid cooling rates ( 100-400°C /sr8]) , although the peak
shock temperature in the melt facies including veins and pockets was higher than 1 500'C.
The occurrence of vapor-grown crystals in the Yanzhuang meteorite is indicative of shock-re-
lated origin. The severely shock-metamorphosed chondrites could result in the vaporization of
chondritic minerals and the in situ deposition of vapor-grown crystals. As is well known, the in-
stantaneous high pressure and high temperature experienced by meteorite was not enough for the
formation of vapor-grown crystals in a great quantity. Hence species and quantity of mineral de-
posited from gaseous components should be very limited. The shock-induced vapor-grown crystals
in the Yanzhuang meteorite have shown the mineralogical characteristicis of rapid crystallization,
such as Fe-Ni metal needle-whiskers with ring growth steps, troilite with networks of micro-
holes.
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