Interplanetary dust particles: reproducing GEMS-like structure inthe laboratory (Forwarded)

Journal Astronomy & Astrophysics
Paris, France

Contact persons:

Science:

Dr. Louis d'Hendecourt
"Astrochimie Expérimentale" - Institut d'Astrophysique Spatiale
Campus d'Orsay, Bat 121
91405 Orsay, cedex - France
Phone: +33 1 69 85 86 40
Fax: +33 1 69 85 86 75

Dr. Hugues Leroux
Laboratoire de Structure et Propriétés de l'Etat Solide, Bat C6
Université des Sciences et Technologies de Lille
59655 Villeneuve d'Ascq - France
Phone: +33 3 20 33 64 16

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Dr. Jennifer Martin
Journal Astronomy & Astrophysics
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Phone: +33 1 43 29 05 41

Released: February 14th, 2006

Interplanetary dust particles: reproducing GEMS-like structure in the
laboratory

"The origin of GEMS in IDPs as deduced from microstructural evolution of
amorphous silicates with annealing", by C. Davoisne et al.

To be published in Astronomy & Astrophysics.

For the first time, a team of French scientists [1] were able to
reproduce the structure of the exotic GEMS in the laboratory. The
results of their experiments will soon be published in Astronomy &
Astrophysics. GEMS (glass with embedded metal and sulphides) is a major
component of primitive interplanetary dust. To understand its origin is
one of the primary objectives of planetary science, and especially of
the recently successful Stardust mission.

In a coming issue, Astronomy & Astrophysics presents new laboratory
results that provide some important clues to the possible origins of
exotic mineral grains in interplanetary dust. Studying interplanetary
grains is currently a hot topic within the framework of the NASA
Stardust mission, which recently brought back some samples of these
grains. They are among the most primitive material ever collected. Their
study will lead to a better understanding of the formation and evolution
of our Solar System.

Through dedicated laboratory experiments aimed at simulating the
possible evolution of cosmic materials in space, C. Davoisne and her
colleagues explored the origin of the so-called GEMS (glass with
embedded metal and sulphides). GEMS is a major component of the
primitive interplanetary dust particles (IDPs). They are a few 100 nm in
size and are composed of a silicate glass that includes small, rounded
grains of iron/nickel and metal sulphide (Figure 1). A small fraction of
the GEMS (less than 5%) have presolar composition and could therefore
have an interstellar origin. The remainder have solar composition and
may have been formed or processed in the early Solar System. The varied
compositions of the GEMS make it difficult to arrive at a consensus
regarding their origin and formation process.

The team first postulates that the GEMS precursors originated in the
interstellar medium and were progressively heated in the protosolar
nebula. To test the validity of this hypothesis a joint experimental
project involving two French laboratories, the Laboratoire de Structure
et Propriétés de l'Etat Solide (LSPES) in Lille and the Institut
d'Astrophysique Spatiale (IAS) in Orsay, was set up. Z. Djouadi, at the
IAS, heated various amorphous samples of olivine ((Mg,Fe)2SiO4) under
high vacuum and at temperatures ranging from 500 to 750 C. After
heating, the samples show microstructures that closely resemble those of
the GEMS, with rounded iron nanograins that are seen to be embedded in a
silicate glass (Figure 2).

This is the first time that a GEMS-like structure has been reproduced by
laboratory experiments. There, they show that the iron oxide (FeO)
component of the amorphous silicates has undergone a chemical reaction
known as reduction, in which the iron gains electrons and releases its
oxygen, to precipitate in a metallic form. Since the GEMS component in
IDPs is usually closely associated with carbonaceous matter, the
reaction FeO + C -- Fe + CO will be at the source of the metallic iron
nanograins in these IDP's. Such conditions may have been encountered in
the primitive solar nebula. This reaction has been known of for
centuries by metallurgists, but the originality of the LSPES/IAS
approach is the application of material science concepts to extreme
astrophysical environments.

In addition, the scientists found that, in the heated sample,
practically no iron remains in the silicate glass, since all the iron
has migrated into the metal grains. The team is thus able to explain why
the dust observed around evolved stars and in comets is mainly composed
of magnesium-rich silicates where iron is apparently lacking. Indeed,
iron in metallic spherules becomes totally undetectable by the usual
remote spectroscopic techniques. This work could therefore provide an
important and new insight into the composition of interstellar grains as
well.

The team shows that GEMS could form through a specific heating process
that would affect grains of various origins. The process may be very
common and could occur both in the Solar System and around other stars.
The GEMS could thus have diverse origins. Scientists now eagerly await
the analysis of grains collected by Stardust to find out for certain
that some GEMS truly come from the interstellar medium.

[1] The team includes C. Davoisne, H. Leroux (from LSPES, Lille,
France), Z. Djouadi, L. d'Hendecourt, A. P. Jones and D. Deboffle (from
IAS, Orsay, France).

The origin of GEMS in IDPs as deduced from microstructural evolution of
amorphous silicates with annealing
By C. Davoisne, Z. Djouadi, H. Leroux, L. d'Hendecourt, A. P. Jones, and
D. Deboffle
To be published in Astronomy & Astrophysics volume 448, issue 1, pp.
L1-L4 (DOI number: 10.1051/0004-6361:200600002)

Full article available in PDF format,
http://www.edpsciences.org/articles/...PRAA200603.pdf

IMAGE CAPTIONS:

[Fig. 1:
http://www.edpsciences.org/papers/aa...hl144_fig1.jpg
(51KB)]
Image of a GEMS in an interplanetary dust particle. Copyright: NASA

[Fig. 2:
http://www.edpsciences.org/papers/aa...hl144_fig2.jpg
(47KB)]
Iron grain embedded in silicate glass.

Andrew Yee wrote:

The varied
compositions of the GEMS make it difficult to arrive at a consensus
regarding their origin and formation process.

The glassy substance in GEMS is forsterite, Mg2SiO4. The SiO4
portion, as well as the entire salt, requires water for its formation.
Si* + 4 H2O --- H4SiO4 (silicic acid) + 2 H2
Water is also the preferred medium for the formation of sulfides
in GEMS. Free metals in GEMS require a reducing environment,
such as hydrogen.
Aqueous and reducing conditions exist around evolved stars,
whose temperatures are below sun spot temperature (3500 ? K),
where water first appears. Subsequent reactions around evolved
stars are water-based, such as the formation of carbon monoxide:
C* + H2O --- CO + H2
A closer picture of these aqueous reactions can be gained from
an inspection of the deep sea volcanos, where conditions are
also reducing; note the presence of silicic acid, sulfides,
hydrogen and forsterite in basalt:
http://www.pmel.noaa.gov/vents/chemi...ges/vents2.gif
John Curtis

In message . com, John
Curtis writes
Andrew Yee wrote:

The varied
compositions of the GEMS make it difficult to arrive at a consensus
regarding their origin and formation process.

The glassy substance in GEMS is forsterite, Mg2SiO4. The SiO4
portion, as well as the entire salt, requires water for its formation.
Si* + 4 H2O --- H4SiO4 (silicic acid) + 2 H2

A very small amount of web searching will show you that forsterite can
form via solid phase reactions.
"It is presumed that the formation of forsterite proceeds according to
the solid phase reaction: 2MgO+SiO2 - Mg2 SiO4"
http://www.freepatentsonline.com/6899768.html
or gas phase
"Let us consider the formation of forsterite (Mg2SiO4) from the gas
phase in. a thermodynamic equilibrium state."
http://www.ita.uni-heidelberg.de/~gail/prepr/dalg-rev.ps

Jonathan Silverlight wrote:
In message . com, John
Curtis writes
Andrew Yee wrote:

The varied
compositions of the GEMS make it difficult to arrive at a consensus
regarding their origin and formation process.

The glassy substance in GEMS is forsterite, Mg2SiO4. The SiO4
portion, as well as the entire salt, requires water for its formation.
Si* + 4 H2O --- H4SiO4 (silicic acid) + 2 H2

A very small amount of web searching will show you that forsterite can
form via solid phase reactions.
"It is presumed that the formation of forsterite proceeds according to
the solid phase reaction: 2MgO+SiO2 - Mg2 SiO4"
http://www.freepatentsonline.com/6899768.html
or gas phase
"Let us consider the formation of forsterite (Mg2SiO4) from the gas
phase in. a thermodynamic equilibrium state."
http://www.ita.uni-heidelberg.de/~gail/prepr/dalg-rev.ps

Formation of an oxyanion such as silicate (SiO4) without using
water is a challenge. Formation of Mg salt of said oxyanion without
usig water is a double challenge.
This extra effort is not really necessary because the atmospheres
of late-type stars are awash with water:
http://www.edpsciences.org/articles/....pdf?access=ok
John Curtis