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www.meteorite-recon.com
Meteorite Fusion Crust
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An illustrated introduction
Frothing and Bubbling
Meteoroids with stable flights develop a high
vacuum at their trailing side. The vacuum favors intensive
degassing of volatile elements which, in turn, produces vesicles
and sometimes larger bubbles or splash craters in
the outer fusion crust. The longer the
meteoroid maintains a stable flight, the more intense
the degassing process and the resulting frothing formation.
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Frothing on the flank of a Noktat Addagmar LL5 chondrite |
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Though most common on the trailing edge of meteorites
with stable flight attitudes, the forming of bubbles
and larger vesicles is not limited to those. The violent
release, for example, of S from troilite, of H2O from
serpentinite (in carbonaceous chondrites) or the release
of CO2 during the combustion of graphite can lead to bubble
formation on areas where one would expect it less; e.g.,
on the breast side or flanks of a specimen.
Contraction Cracks
While continuing its fall toward the lower regions of our atmosphere,
the meteoroid encounters low temperatures, often well below freezing
point. This leads to a further contraction of the just-cooled fusion
crust. Frost forms on the meteoroid; under special conditions it may
even develop a layer of ice. The thermal stresses induced by low temperetaures
often lead to tiny weblike cracks in the fusion rind. These
delicate structures resemble the crazing on fired ware and are called contraction cracks (see last image on this page).
Color
The color of fusion crust on stone meteorites is
generally black. However, the quality
ranges from a glassy jet black on basaltic achondrites
(with vesicles as on crusts of lunar mare basalts) to a dull
greyish black with shades of rich or lighter brown on ordinary chondrites.
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Glossy fusion crust on the basaltic eucrite NWA 5787
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Frothy, transparent, olive-green crusts, as on anorthositic lunar highland
meteorites, or bright whitish and ash-colored fusion crusts as, for example,
on aubrites, are rare exceptions. The thin fusion crusts on irons show a
blueish black often changing in tempered colors. In general, the color of the
fusion rind is mainly a function of the
iron content, the mineralogic composition of the meteorite and the range of melting temperatures
and subsequent viscosity of the melt on the surface of the meteorite (Schneider, et al. 2000).
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Blueish black fusion crust on a Sikhote-Alin IIAB iron meteorite
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Often, freshly-fallen chondrites display rather black fusion
crust on one side and a more brown color of the crust on the opposite
side. This is not a weathering effect. The color differences are due
to variations in the magnetite contents in the crusts of opposite
sides. These, in turn, are controlled by the supply of atmospheric
oxygen and the temperature of the oxidation process.
A high vacuum on the trailing side,
for example, blocks the supply of atmospheric oxygen to the reaction.
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Difference in the color of the fusion crust between leading (left) and trailing surface (right) of this freshly fallen H4 chondrite
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Degree of Crust
Meteoroids, particularly those of stone and of stony iron
composition, were not built to withstand the enormous mechanical
stress of atmospheric entry. They often break apart into clouds of fragments
continuing their individual trajectories. A considerable percentage of
the body’s mass is turned into meteoritic dust during these breakups.
Fragmentation events are not limited to the hot phase of the flight
and can occur subsequently during the complete trajectory until impact.
Specimens crossing earth’s atmosphere in one piece develop a
rather thick fusion crust and a high degree of surface ablation.
Fragmentations which occurred at later stages of the hot flight
produce specimens that show a far lesser degree of ablation. These
are often only coated incompletely, or
with an extremely thin layer of crust. Sometimes they only
display a moderate sooting.
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Thick (primary) fusion crust (upper half) and thin fusion rind (lower half) on a lesser ablated surface originating
from a later stage fragmetation. The light colored patches are limestone dust from the meteorite's impact. Tamdakht, H5 chondrite
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Hairfine contraction cracks on primary (upper half) and later stage fusion crust (lower half) of a Nuevo Mercurio H5 chondrite |
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A multiple fall arriving in the shape of
hundreds or thousands of specimens on the ground shows a
variety of transient and intermediate forms. Collectors and
researchers alike try to categorize the differing degrees
of fusion crust by adressing them as primary, secondary and tertiary
crust. This makes perfect sense if several degrees of crust are
observed on several surfaces of one specimen. But one has to be
aware that successive fragmentations produce a fluid spectrum of
gradually-decreasing manifestations of fusion crust. If comparing
specimens of a multiple falls we suggest addressing a certain
degree of fusion crust by referring to the time the crust
formed relative to the progression of the meteorite’s flight.
Inner and Outer Crust
As Ramdohr (1967) and, later, Genge and Grady (1999) have demonstrated,
fusion crust is not a homogenous layer but can be
separated into an outer crust and an inner layer.
While the outer crust of stone meteorites is composed of
entirely melted products of the meteorite’s original lithology,
the inner layer (named substrate in Genge’s nomenclature) has experienced
lower degrees of melting and is dominated by silicate glass veins
and sulphide droplets. Morphologically the outer crust contains a
high amount of very small vesicles of which the outermost are
opening toward the surface. These vesicles can make up to 50 percent
of the outer crust. The inner layer, or substrate, is rather compact
and intersected by metal veins and droplets. Under the substrate the
meteorite’s mineralogy
is unaltered by the heat generated during atmospheric passage.
Because of their better heat conductivity iron meteorites
usually possess a zone displaying a thermally altered crystal
structure under their fusion crusts.
This heat affected zone is called α2-zone and may
reach as deep as 2 cm into the meteorite.
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