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Raman spectroscopy investigation in the NWA 3118 meteorite: Implications for planet
formation
Bhuwan Ghimire, A. Dall’Asén, J. Gerton, I. Ivans, B. Bromley
A hierarchical model for the early stages of planet formation involves the
coagulation of micron-scale dust particles orbiting the sun into larger, centimeter- or meter-scale objects. Computational simulations can produce
planetary distributions that agree with astronomical data, but only when
they start with planetesimals of ~1 km or more in size. Further investigation
is needed to understand how these planetesimals arose from the adhesion
of dust aggregates. One possibility is that the small-scale structure of interstellar dust enhanced adhesion. At very least, this structure, as preserved
in stony meteorites commonly known as chondrites, may provide clues
to the physical processes during this critical juncture in planet formation.
Confocal Raman microscopy was used to investigate microstructures on
the NWA 3118 meteorite. It is a carbonaceous type CV3 meteorite composed
of micrometer scaled chondrules and inclusions embedded in the matrix
material. Olivines and graphitic structures were identified in the NWA 3118.
Raman mapping of meteorite samples was carried out, with a focus on the
interfaces between chondrules and the matrix.
T
he formation of terrestrial planets
and the icy cores of gas giants is
driven by coagulation and accretion.
The dust seen in the protoplanetary disks
around many young stars is concentrated
into larger planetesimals, which in turn
merge to form full-fledged planets. While
this scenario is compelling, it is incomplete
in one regard: The growth of kilometersized planetesimals from micrometer
sized protoplanetary dust particles remains
an unsolved problem. Several theories
have been proposed, including simple
electrostatic adhesion of dust aggregates
and the gravitational collapse of the
material concentrated in turbulent eddies in
the sun’s protoplanetary disk.
To find clues for discriminating
between these models, researchers have
used Raman spectroscopy of chondrules
– small grains formed in the early solar
system found in several meteorites like
Allende, Axtell & Murchison, NWA 3118,
Nakhla, and Vaca Muerta.1,2,3 Raman
scattering produces an optical spectrum,
which when compared to a reference
spectrum, can yield information on specific
minerals present in the chondrules and
the matrix, the material in meteorites that
binds constituents together. To focus on
the dust-to-planetesimal problem, this
study concentrates on processes by which
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meteorites were assembled. Specifically,
we examine the interface between the
chondrules and matrix of a type CV3
meteorite is examined in this review. The
interface between the chondrules and the
matrix may hold the answer to the question
of whether kilometer-sized planetesimals
were formed by collisional growth or
gravitational instabilities. This study
focuses on a sample from one meteorite,
NWA 3118, and presents Raman spectra
with the corresponding images of identified
olivines and graphitic structures within
this sample. The findings complement
and corroborate the recent findings on this
meteorite, and represents first efforts in a
more detailed analysis.
Chrondrites
Chondrites are stony meteorites primarily
composed of chondrules, rare refractory
inclusions and the matrix material within
which the chondrules and the inclusions
are embedded. The millimeter-sized
mineral aggregates of chondrites are the
“chondrules.” These are made from several
kinds of minerals, mainly olivine and
pyroxenes, and are the deposits of dust
aggregates from the early solar system.4
Refractory inclusions are composed of
either minerals like calcium and aluminium
or amoeboid aggregates of the matrix
minerals, such as olivine with proportions
of metals like iron, nickel, or both.
The matrix of the chondrites is
full of fine-grained silicate minerals like
forsterite, anorthite, and pyroxene. These
minerals play a vital role in the formation of
chondrites. They aid to fill the gap between
the chondrules coating them, acting as an
interface between them and the inclusion.4
Understanding of these components with
their formation will be simpler through
the thermodynamic equilibrium diagram
for solar nebula. Scott (2007) discusses
mainly two types of minerals at a pressure
of 10-3 bar: minerals that are stable below
1400 K, which are mainly present in
chondrules and the matrix material, and
the ones that are stable above 1400 K that
are found in the refractory inclusions.
The thermal processing of this silicate
dust in the solar nebula into the refractory
inclusions, chondrules in chondrites, and
sub-micrometer silicate crystals present
in chondrite matrices and comets needs to
be understood. These materials aid in the
accretion of asteroids, comets, and planets.4
The thermal and chemical history
of meteoritic material reflects the conditions
and processes that gave rise to the solar
system. The formation of planetesimals,
the kilometer-size building blocks of
planets, begins with growth of fractal
dust aggregates, followed by compaction
processes; with the increase in the size of the
dust aggregates, the mean collision velocity
increases, which in turn stops the growth
as the dust fragments reach decimeters
in size.5 The question is: what succeeds
this stage? Several hypotheses for further
growth have been proposed, including but
not limited to sticky materials, enhanced
growth at snow line (the region outside of
Mars’ orbit where refractory material can
condense into ices), and cumulative dust
effects with gravitational instability.4,6,7
These theoretical ideas may be realistic,
but they need to be tested in some way. The
approach here is to seek evidence of these
theoretical scenarios in the small-scale
structure of meteorites.
Experimental
Material
The NWA 3118 was obtained from Erfoud,
Morocco. It was first found in 2003. It is a
type CV3 meteorite. CV3 is named after the
type Carbonaceous ‘Vigarano’ meteorite
with petrologic type 3. Here, the type
refers to the degree of the effects of thermal
metamorphism and aqueous alterations,
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Fig. 1 (a) Raman spectra for olivine (forsterite) with the characteristic Fermi doublet at 821 and 852 cm-1 observed in the NWA 3118. (b)
Raman spectra of forsterite on Martian rocks (Sharma 2002).
with 3 generally given to the most pristine
chondrite with minimal aqueous alteration
and low metamorphism.
Instrumental
This study used a laser excitation
wavelength at 488 nm to investigate
carbon structures and different meteoritic
compounds present on the NWA 3118
meteorite. The analysis system used was
a WITec alpha 300 S scanning near-field
optical microscope. The detection gear
included a WITec UHTS 300 Spectroscopy
system equipped with a thermoelectriccooled, back-illuminated CCD detector
chip. The grating used in the spectrometer
was 600 g/mm and the spectral resolution
was around 3 cm-1. Lenses 10x and 20x
were used as objectives. Data acquisition
was evaluated through the in-built WITec
Project software. Spectral analysis was
performed through Matlab 7.11.
Results and Discussion
The olivine micro-structures were verified
on the NWA 3118 chondrite with the
findings of Fries & Steele (2008). A strong
signature of olivine detected from Raman
bands at 821 and 852 cm-1 is seen in the
spectrum of the NWA 3118 sample (Fig.
1a). Olivines of forsterite (Mg,Fe)2SiO4
have been identified to exhibit this
doublet as a fingerprint in Martian surface
minerals.8 Fig. 1c represents the optical
image of the surface with fragments of
olivine chondrule under 20x objective lens,
and Fig. 1d represents the Raman map of
an olivine chondrule fragment.
Several line scans around the
chondrule rims were taken. A plot for a
Fig. 1 (c) Photomicrograph of an olivine chondrule fragment taken with a 20x objective
lens. (d) A sum filtered Raman image of the marked region in Fig.1 (c) with the olivine
fragment at the bottom left .
line scan with spatial separation of 10
μm around the chondrule rim generated
through Matlab is shown in Fig. 1e. The
Olivine fingerprints show strong peaks at
821 and 852 cm-1 with a lot of background.
The background was from space weathered
materials and highly reflecting particles.
Graphite
Graphite-like
phases,
lonsdaleitehexagonal diamond, and graphite whiskers
have been predicted and recorded on the
NWA 3118, marking it as a carbonaceous
chondrite.2,9 Fig. 2a gives a clear spectrum
of the presence of bulk carbon in the form
of imperfect graphite near an edge of the
NWA 3118 chondrite. The most intense
features were identified as the D peak
at ~1350 cm-1, the G peak at ~1580 cm-1
and a weak G’ peak at ~2682 cm-1. The G’
peak should be the second most intense
peak in graphitic samples. It is the second
order of zone-boundary phonons; but since
Raman fundamental selection rules are not
satisfied by zone-boundary phonons, they
are usually not seen in first order Raman
spectra of defect-free graphite. However,
these phonons give a rise to a peak at ~1580
cm-1, known as the D peak in defected
graphite.10
Graphene was not identified in
the structures, as a D peak should not
be observed on pure graphene layers. A
significant change in shape and intensity
should be found in the G and G’ peaks to
indicate any graphene presence compared
to the spectrum of defected graphite.11
This study corroborates the findings of
Karczemska et al. (2006) and Fries &
Steele (2008). The photomicrographs of
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Fig. 1 (e) A plot of the line scan around
the chondrule rims represented by the
black line in the Raman image. The spatial
separation is 10 µm.
Fig. 2 (a) Raman Spectrum of defected
graphite with peaks at 1350, 1582 and 2682
cm-1 observed in the NWA 3118. A doublet
indicating the presence of olivine in around
816-855 cm-1 wavenumbers.
and the matrix with the amoeboid micro
and nanostructures discussed by Scott
(2007) explain the possible coagulation of
pre-solar grains and dust. However, it is
not yet clear if or how this process leads
to growth of kilometer-sized structures
like planetesimals. The interface between
the chondrules and the matrix may yield
the answer to the question of whether
kilometer-sized planetesimals were formed
by collisional growth or gravitational
instabilities. Until it does, the dust-toplanetesimal problem will remain “one
of the great unsolved problems of planet
formation.”4
Acknowledgement
We would like to thank Randy Polson from
The University of Utah for his valuable
technical support with the facilities.
References
Fig. 2 (b) Photomicrograph of bulk carbon in defected graphite form. (c) Raman image of
the spot with brighter regions representing carbon structures.
the spots of bulk carbon and Raman image
of various defected graphite structures are
shown in Figure 2b.
Conclusions
This study presents a comparative Raman
spectroscopic study of a carbonaceous
meteorite NWA 3118 in this review. The
findings corroborate the recent discoveries:
graphite-like phases, lonsdaleite-hexagonal
diamond and graphite whiskers on the
NWA 3118.2,9 We also confirm that our
technique and equipment is capable of
making valuable analytical measurements
on the scales necessary to glean information
about the interfaces between constituents
of meteorites.
Olivines were identified in the
form of forsterites (Mg, Fe)2SiO4 on the
NWA 3118 in the form of micrometer
chondrules embedded in the matrix.
Defective graphites were identified near
the edges of the sample specimen. These
microstructures may answer some of the
questions raised on planet formation. It
is critical to understand the processes
with which planetesimals are formed.
Several theories have been proposed
including sticky properties of the grains,
dust aggregate collisions and gravitational
instability. The coating of the chondrules
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Bhuwan Ghimire was born in Kathmandu, Nepal, and now studies physics at
Westminster College in Missouri. He is particularly interested in observational
cosmology and instrumentation, and after graduation plans to research galactic
archeology and cosmology. In his spare time he likes to stay active, and particularly enjoys playing soccer and tennis.
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