Amorphous magnesium silicate smoke particles were condensed from hydrogen and argon atmospheres containing Mg and Si0. A wide range of initial compositions were observed but all particles could be recrystallized into forsterite (Mg2Si04), by heating to 1000 C in vacuum. The amount of smoke formed decreased rapidly with temperatures between 300 and 800 K at reactant partial pressures of about 1 torr

Day, K. L.

Donn, B.

English

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https://ntrs.nasa.gov/search.jsp?R=19780009230 2020-03-22T06:29:23+00:00Z
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NASA
Technical Memorandum 78060
An Experimental Investigation of the
Condensation of Silicate Grains
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1	 1	 X[E 
INVESTICATI CN OF I	 :CN11	 Al 104 IF
SILICATE (;FAINS 	 17 F EC A')2/,IF A01C ]CI. C7C	 I) tic laS
;3/25 05048
K. L Day and B. D. Dom
JANUARY 1978
'National Aeronautics and
Space Administration
Goddard Spmm Flight Center
Greenbelt, Mary!and 20771
w.
{AN EXPERIMENTAL INVESTIGATION OF THE CONDENSATION OF SILICATE GRAINS
by
K. L. Day
Lunar and Planetary Laboratory
University of Arizona
Tucson, Arizona
and
B. Donn
Laboratory for Extraterrestrial Physics
NASA/Goddard Space Flight Center
Greenbelt, Maryland
ABSTRACT
•!	 Amorphous magnesium silicate smoke particles were condensed from
hydrogen and argon atmospheres containing Mg and SiO. A wide range
of initial compositions were observed but all particles could be
recrystallized into forsterite (Mg 2 SiO4 ) by heating to 1000 0C in
vacuum. The amount of smoke formed decreased rapidly with temperatures
between 300 and 800a K at reactant partial pressures of about 1 torr.
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1 .	 I N 1'RPPULPTI oN
We report hcic • Lhc ttrst :laboratory resulte; of an exper:[numtal.
and theoretleal study of the format [oil 	 solid parLic'los in astronomical
system . An Lnttial theoretleal analysts was provlously publLnhed
(Donn, 1976).
The prevailing belief is that silicates are the basic component
of micron size grains observed In the tnterstelI,lV medium, circ •um-
stellar clouds except around carbon stars, and commas (Woolf, 1474;
Field, 1974; Noy, 1477). In those objects the iufenr.ed spectrum bas
a broad, strueturclosm Blum feature and n generally weaker one at. 20mn.
These have boon attrLbuted to the 81-0 stretching and bending modes,
respectivel y (Nov, 1477).
Pay (1076) ham prepared amorphous hydrous silicates by proctpita-
tton from solution and tound Soho agreement with Lhe Mum Infrared
spectrum of the 1leekltn-Nougebauer object in Orion as well as w!Lh the
matrix material of tvpes I and 11 carbonneeous chnndritic meteorites.
Zaikowskl et al. (1475) :showed that the 1DUm spectra of some crystalline
hydrous silicates also had a close ronvuMlonce to the astronomical
spectra.
Additional support for the identification of si'.11eato material bag
been based on thermodynamic calculations of the stability of si.licata
minerals (Cilmr.m, 1964; Grossman and harimor, 1974; Field, 1974). This
procedure has berm criticized by Donn (1976). Condensation theory will
be reexamined in a forthcoming; paper.
Because of the Importance of silicate material in grain models, we
began with an Investigation of the condensation of magnesium silicates
from the gas phase.
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II. DESCRIPTION OF 111E EXPERIMENT
Magnesium silicate grains were condensed by simultaneously evaporating
Mg and SiO solids into an atmosphere of argon or hydrogen at a pressure
of a few torn. The two source materials were placed in crucibles or
molybdenum baskets about a centimeter apart. The temperatures of the
two were independently controlled through the use of two variacs. The
temperatures were monitored by Pt-Pt/13% Rh thermocouples. Average
temperatures were around 700 00 for Mg and 14000C for SiO. By varying
the temperatures, silicates with a large range of Mg/Si ratio were obtained.
The actual ratios were qualitatively determined in a few cases.
When the temperatures became high enough a mixed Mg-SiO cloud
formed within a few centimeters of the sources and smoke would condense.
Convection currents carried these upward and deposited them on a stain-
less steel collecting plate about 15 cm above the crucibles.
In a few runs the particles were subsequently dispersed on formvar
films and examined by electron microscopy and electron diffraction.
For some other runs an electron microprobe was used to determine com-
position. In other cases, X-ray flourescence was used. Following
every run some of the particles were dispersed in KDr pellets for examina-
tion of their infrared spectrum.
For most runs, samples were annealed at 500 0C and 10000C for one
hour in vacuum. In some cases, intermediate temperatures were used to
better define transition points. With the later experiments in the
series the condensation temperature was controlled by having the
sources approximately centrally located within a resistively healed
aluminum oxide tube. This auxiliary furnace was 10cm diameter and
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and 20 cm high. Van temperatures within the furnace were measured with
a thermocouple. This was adopted an the nominal condensation tempera-
ture. The result we found,	 that the quantity of smoke produced had
a high dependence on the nominal tcmperature,Indicates that the source
heating produced only a small modification of the temperature. A
diagram of the experimental arrangement is given in Figure 1.
Slightly under 0.5g each of pig and Sio were vaporized in approxi-
mately 5 minutes during an experiment. An estimate of the steady state
reactant pressures from convection or from diffusion indicate about
1 torr.
111. RESULTS
The condensed grains were quite unlike common terrestrial silicates.
They were tan to ioL black and exhibited various degrees of instability.
Most of the black material exploded violently when ignited after removal
from the chamber. Some samples burned leaving a white or gray ash.
The explosive quality would greatly diminish if the grain were exposed
to air overnight when it could slowly oxidize. Vaporization of Sio
by itself always yielded mainly Si 203 smoke (Day and Donn, 1977) rather
than Sio which occurs with vacuum deposited films.
Electron microscopy showed that the individual grains were on the
order of few hundred angstroms. Diffraction studies were carried out
on samples from several. experiments. The initial condensates were
amorphous grains. Heating to 1000 00 converted them to crystalline
forsterite (Mg`004 ). This was true no matter, what the starting
composition .vas. We were never able to produce enstatite (MSSiO 3) or any
other magnesium silicate, in dercctahle amounts.
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Our microprobe measurements were preliminary and very incomplete
but certain trends were evident. The grains condensed without regard
to mineral stochlometry and ranged from some with very high magnesium
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to almost pure silicon oxide. The Mg/Si ratio also varied within a given I
experiment for grains condensed from the same cloud. Vie Mg/Si vapor I
ratio within the cloud would be expected to vary considerably also.
The infrared spectrum was obtained for essentially al]. samples.
T9ie infrared spectra of the amorphous material (Figures 1 and 2) of
imost runs bear a strong resemblance to the often observed interstellar
and circumstellar 9.711 feature. It is interesting to note that varia-
tiono in the Mg/Si ratio produce some shift in the position of the
I
principle peak near 10p. High-Mg varieties peak near 10.011, whereas
high-Si ones peak around 9.5 pm, thus bracketing the range in which
	 I
the astronomical feature occurs. Substantial differences in the ratio	 {
of 10 to 20 peak heights were found. In every case, the spectra of 	 j
the 1000°C annealed sample was thatof Mg2 S104 . In a few instances
I	 ''
evidence for amorphous SiO 2 or of Si 203 was found. These were
associated with excess Sio in the condensate. In a number of runs
	 I
the initial condensate had a very weak 101m peak and a much stronger
20pm peak. Heating to 500°C always increased the 1011 band relative
to the 20. Even for these samples, the 1000°C anneal produced a
,
fosterite spectrum as in Figures 1 and 2.	 1,,
The auxiliary furnace was provided to study condensation at con-	 l#
trolled temperature, This had two objectives: (1) to determine the
temperature dependence of condensation and (2) to compare the nature
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of the condensate and annealed grains as function of temperature.
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Cop It, r.: tlIt kit tit It' s o  ranokl . were ob Lit Inoil wItIt the uuxIIIary Iurnact'
Col d. The gas temperatur e In the condellnitt toll region was heated to
ti(1(1 t C to congtaro the smoke with t;nloke annealed In 500
00, 'to oltr
nuvpr Ino, lit , rolldellnat iell Occurred, A goo d usppl y Was nhtalut'd at
h1tl0C and a ;;,,tall :amount at 500 0C, A trace of smoke wan collocted
0at '+Utl C when the ,,Duerr trmper,tture yl were raised, Increasing the
reactant concontraLlon In the cloud.
IV. [IISCttSSTni
The ntlicatt • material We condensed Was in a highly reduced condl-
Lion and e:et •med to be the same whether tho ambient atmosphere Wa y; argon
or hydrogen. At tvmperaturen up to BOOK (50;1 0 ;'.) amorphous, non
stoehiom,vic (:rain y: Were obtained, During the 1270K (100000) annrai
a mlagrionIsm mirror deposited in the cooler region outside the furnace,
al' .I ;obit I:.6 yellow Ilit• aceo.. deposit Would occur. The amorphous
grainy Were very .:table, rvstating reerastallratIon to above 11000K.
The vapor proosuce of :dlicatem is W remm >ly low below 800 K. For
lunar rocks, the partial pressure ol. $10 at 1700 K Is 111 1 atm. " ;1,71,
Parr (de Maria vt al,, 1071). A gross oxt'apotation of their linear
in P vs Ill curve yields	 1'(800 K) = 5x1;1 f 'Cot's'. For Bill, (cristobalite)
Schictc (100)(1) in his veview reports P(Slt l)	 ax10-7 '1•ore at 1 6170 K.
Despite tilt• extre zlmoly low etluilibrtum vapor pressure at 800 K for
silicateK alld consequent super;;atlr:"Ons in our experiments greater
than 10 5, neither ma4uo8illmt silicates nor silicon oxidt •s conden80d. A
first order comparison of the taboratory and astronomical systems in
i;ivell ill 'fable 1,	 thin compares: the important parameters, pressure,
distance scales and time ,:tale;:. The last row lists the quantity
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which is physically most relevent, the total number of collisions. We
note that this quantity is generally comparable among laboratory-cloud
pairs. In the forthcoming theoretical analysis it will be shown that
dependence of processes ,involved in condensation substantially inhibits
nucleation at low pressures characteristic of clouds.
The fact that it is very difficult to condense refractory grains
at ambient temperatures of a few hundred degrees celsius, even with
very high supersaturations present indicates that the barriers to
nucleation of silicates from a homogeneous vapor are quite formidable.
Even simple systems, such as the condensation of metallic iron from
the vapor, as discussed by Blander and Katz (1 0,67) call 	 high
supersaturation without nucleating.
Experimentally, rrurip and Bauer (1977) obtained critical super-
saturations in the range of a hundred to several thousand for iron,
lead and bismuth. Since none of the common terrestrial magnesium
silicates (Mg2SiO4' N9SiO3 , etc.) has been recorded in the vapor state,
the problem of nucleation from a mixture of component vapors is even
more difficult (Donn, 1976). This problem has received inadequate
attention and the theory is presently inadequate to deal with it.
However, the tmp'lications for the early stages of condensation in
astronomical clouds are quite profound. The experimental approach
appears to be quite fruitful and is being extended by the authors to
cover more complex situations and to make more quantitive experiments.
V. CONCLUSIONS
This paper presents the first results from a series of experiments
designed to explore the nucleation of small silicate grains from a vapor
of astrophysically significant elements and compounds. We conclude:
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1. At low temperatures (to a few hundred degrees Celsius) the
rondrns:atrs are amorphous and have widely varying stoichiometrics.
The compounds: most stable thermodynamically (Mg 2 Sio4'
 QUO and Si3O4)
do not readily form.
2. At higher temperatures (above 700 0K), it becomes very difficult
to nucleate any silicates at all, and those that did form were amorphous.
This shows that surface energies and kinetic effects, which determine
the stabil-tty and growth of small clusters, are very important in
determining under what conditions condensation will occur.
1. Wo infrared spectra of the experimentally produced magnesium
silicates have a strong resemblance: to those observed in many astro-
nomical clouds.
ACKNt1WGEDG;IENTS
One of us (KLD) wishes; to acknowledge the support NASA Goddard
Space Flight Center, which provided funding through rho Visiting
Scientist Program. This work was also partially supported by NSF.
S
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REFERENCES
Blander, M., and Katz, J.L., 1.967, Geochim. Cosmochim. Acta, 31, 1025.
Day, K.L., 1976, Icarus 27, 561; Ap. J. 210, 614.
Day, K.L. and Donn, B., 1977, to be published.
DeMaria, G., Balducci, G., Guido, 4%., Piacente, V,, 1971, Proc. 2nd
Lunar Sci. Conf. V. 2 1 1367, M.L.T. Press, Cambridge, Mass.
r;
Donn, B.D., 1976, Mem. Soc. Roy Sci. Liege, 6 Ser. 9, 499.
Field, G.B., 1974, in The Dusty Universe, ed. by Field, G. B. and
Cameron, A.C.W., p. 88, Neale Acad. Pub. N.Y.
°	 Frurip, D. J. and Bauer, S.H., 1977, J. Phys. Chem., 81, 1001.
Gilman, R., 1969, Ap. J. 155, L185.
Grossman, L. and Latimer, J.W., 1974, Rev. Geophys. Sp. Phys. 12, 71, 	 1{
Ney, E.P., 1977, Science, 195, 541.
Schick, H.L., 1960, Chem. Rev. August, 331.
Woolf, N., 1974, in The Dusty Universe, ed. Field, G.B. and Cameron,
t
A.G.W., p. 60, Neale Acad. Pub., N.Y.	 f
I
	
	 1,
Zaikowski, A., Knacke, R.P., and Porco, C.C., 1975, Astrophys. Sp.
Sci., 35, 97.
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TABLE I
Condensation Conditions in Laboratory and Clouds
Clouds
Lab. A B
Total Pressure (torr) 5 10-1 10
Condensible Pressure (torr) 1 10 7 10 10
Distance Scale (cm) 10 1013 1013
Time Scale (s) 1-10 109 107
No. of Collisions a PT 1-10 102 10 3
A = primordial nebula
B = circumstellar shell
C = dark cloud
C
< 10-6
< 10 12
1016
1012
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FIGURE CAPTIONS
Figure 1. Dingram of apparatus.
Figure 2. Infrared spectra of nominally high Mg smoke condensed at
300aK, ---- initial condensate; .--,heated to 800°K in
vacuum;	 heated to 13000K in vacua. The last is the
spectrum of crystalline forsterite, TI92S30lf.
Figure 3. Infrared spectra of nominally high Si smoke condensed at
300`. K. ---- initial. condensate; . 	 heated to 8000K in
vacua;	 heated to 13000K in vacuo. The last is
forsterite, with a small. percentage of amorphous quartz
admixed.
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1BIBLIOGRAPHIC DATA SHEET
1
For sale by the National Tachniml Information Service, Springfield, Virginia 22151,
14	 e
i
[.
I . Report No. 2. Government Accession No. 3, Recipient's Catalog No.
TM 78060
4. Title and Subtitle 5, Report Data
AN EXPERIMENTAL INVESTIGATION OF THE January 1978
6, Performing Organization CodeCONDENSATION OF SILICATE GRAINS
7. Author(s) 8. Performing Organization Report No.
K. L. Day and B. Donn
9. Performing Organization Name and Address 10. Work Unit No.
NASA/GSPC
Laboratory for Extraterrestrial Physics 11. Contract or Grant No.
Greenbelt, Maryland 20771
13. Type of Report and Period Covered
12. Sponsoring Agency Name and Address
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
16, Abstract
Amorphous magnesium silicate smoke particles were condensed from
hydrogen and argon atmospheres containing Mg and SiO. 	 A wide range of
initial compositions were observed but all particles could be re-
crystallized into forsterite (Mg2SiO4) by heating to 1000 0C in vacuum.
The amount of smoke formed decreased rapidly with temperatures between
300 and 8000K at reactant partial pressures of about 1 Corr.
17. Key Words (Selected by Author(s)) 18, Distribution Statement
Smoke particles, forsterite
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No, of Pages 22. Price
U U 14
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GSFC 25 .44 (10171)
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An experimental investigation of the condensation of silicate grains

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