The Chemical Analysis of Meteorites

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The Chemical Analysis of Meteorites
Slice of meteorite on display at the Vanderbilt
Museum, New York.
Chem 151, R. Corn. Fall 2006.
From Wikipedia.
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The Chemical Analysis of Meteorites
Meteorite types About 86 of the meteorites that
fall on Earth are chondrites, which are named for
the small, round particles they contain. These
particles, or chondrules, are composed mostly of
silicate minerals that appear to have been melted
while they were free-floating objects in space.
Chondrites also contain small amounts of organic
matter, including amino acids, and presolar
grains. Chondrites are typically about 4.55
billion years old and are thought to represent
material from the asteroid belt that never formed
into large bodies. Like comets, chondritic
asteroids are some of the oldest and most
primitive materials in the solar system.
Chondrites are often considered to be "the
building blocks of the planets. About 8 of the
meteorites that fall on Earth are achondrites,
some of which appear to be similar to terrestrial
mafic igneous rocks. Most achondrites are also
ancient rocks, and are thought to represent
crustal material of asteroids. One large family
of achondrites may have originated on the
asteroid 4 Vesta. Others derive from different
asteroids. Two small groups of achondrites are
special, as they are younger and do not appear to
come from the asteroid belt. One of these groups
comes from the Moon, and includes rocks similar
to those brought back to Earth by Apollo and Luna
programs. The other group is almost certainly
from Mars and are the only materials from other
planets ever recovered by man. About 5 of
meteorites that fall are iron meteorites with
intergrowths of iron-nickel alloys, such as
kamacite and taenite. Most iron meteorites are
thought to come from the core of a number of
asteroids that were once molten. As on Earth, the
denser metal separated from silicate material and
sank toward the center of the asteroid, forming a
core. After the asteroid solidified, it broke up
in a collision with another asteroid. Stony-iron
meteorites constitute the remaining 1. They are
a mixture of iron-nickel metal and silicate
minerals. One type, called pallasites, is thought
to have originated in the boundary zone above the
core regions where iron meteorites originated.
The other major type of stony-iron meteorites is
the mesosiderites.
From Wikipedia.
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Chondrites
Chondrites are stony meteorites that have not
been modified due to melting or differentiation
of the parent body. They formed when various
types of dust and small grains that were present
in the early solar system accreted to form
primitive asteroids. Prominent among the
components present in chondrites are the
enigmatic chondrules, millimeter-sized objects
that originated as freely floating, molten or
partially molten droplets in space most
chondrules are rich in the silicate minerals
olivine and pyroxene. Chondrites also contain
refractory inclusions (including Ca-Al
Inclusions), which are among the oldest objects
to form in the solar system, particles rich in
metallic Fe-Ni and sulfides, and isolated grains
of silicate minerals. The remainder of chondrites
consists of fine-grained (micrometer-sized or
smaller) dust, which may either be present as the
matrix of the rock or may form rims or mantles
around individual chondrules and refractory
inclusions. Embedded in this dust are presolar
grains, which predate the formation of our solar
system and originated elsewhere in the galaxy.
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Ordinary Chondrites
Ordinary chondrites are by far the most common
type of meteorite to fall on Earth about 80 of
all meteorites and over 90 of chondrites are
ordinary chondrites. They contain abundant
chondrules, sparse matrix (10-15 of the rock),
few refractory inclusions, and variable amounts
of Fe-Ni metal and troilite (FeS). Their
chondrules are generally in the range of 0.5 to 1
mm in diameter. Ordinary chondrites are
distinguished chemically by their depletions in
refractory lithophile elements, such as Ca, Al,
Ti, and rare earths, relative to Si, and
isotopically by their unusually high 17O/16O
ratios relative to 18O/16O compared to Earth
rocks. Most, but not all, ordinary chondrites
have experienced significant degrees of
metamorphism, having reached temperatures well
above 500C on the parent asteroids. They are
divided into three groups, which have different
amounts of metal and different amounts of total
iron H chondrites have High iron contents,
and smaller chondrules than L and LL chondrites.
40 of ordinary chondrite falls belong to this
group. L chondrites have Low iron contents.
About half of ordinary chondrite falls are L
chondrites, which makes them the most common type
of meteorite to fall on Earth. LL chondrites
have Low iron and Low metal contents. Only 1 in
10 ordinary chondrites is LL.
From Wikipedia.
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E. Jarosewich, Meteoritics 25, 323-327
(1990).Chemical analysis of meteorites A
compilation of stony and iron meteorite analyses.
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E. Jarosewich, Meteoritics 25, 323-327 (1990).
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E. Jarosewich, Meteoritics 25, 323-327 (1990).
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Determining whether a rock is a meteorite
I show plots here of concentrations or ratios of
concentrations of several chemical elements in
meteorites compared to rocks people have had
analyzed by Actlabs or some other lab. The
horizontal axis of all the plots is "Fe2O3(T)
MgO." Actlabs and most labs that analyze rocks
reports total iron as Fe2O3 because in Earth
rock, much or most of the iron occurs as Fe(III),
that is ferrous iron. There is little or no
Fe(III) in freshly fallen meteorites it's all
Fe(II), ferric iron, and Fe(0), iron metal. For
convenience, however, I use Fe2O3 in the plots.
If you had an analysis done, just add the Fe2O3
and MgO values together for comparison.
From Randy L. Korotev, Washington University in
St. Louis, Department of Earth and Planetary
Sciences http//epsc.wustl.edu/admin/resources/moo
n_meteorites.html
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Potassium Sodium and potassium are both alkali
elements, and all alkali elements are in low
concentrations in meteorites compared to most
terrestrial rocks. Rocks with greater than 0.4
K2O are probably not meteorites.
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Sodium Most terrestrial rocks are richer in Na2O
than any meteorite. Rocks with gt2 Na2O are not
meteorites.
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Calcium Lunar meteorites also have high
concentrations of CaO compared to most
terrestrial rocks (except, of course, those rocks
rich in calcite, like the limestones that plot
around 50 CaO). The terrestrial rocks that plot
with the martian meteorites are probably basalts,
which have similar mineralogy to the martian
basalts
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Chromium One of the best elements for
distinguishing meteorites is Cr. All stony
meteorites have high concentrations of Cr
compared to most Earth rocks.
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Lunar Rocks
Mare basalt   Apollo 11 sample 10049 (left) and
Apollo 15 sample 15556 (right).  The Apollo 15
mare basalt is vesicular - it has holes which
were once gas bubbles.  Most mare basalts are not
vesicular.  The cube is for scale and is 1 inch
on each side. (From NASA photos S76-25456 and
S71-45240).
http//epsc.wustl.edu/admin/resources/moon_meteori
tes.html
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The Chemistry of Lunar Rocks Because of the
simplicity of lunar mineralogy, lunar rocks have
predictable chemical compositions.  Nearly all
the aluminum is in plagioclase and nearly all
the iron and magnesium are in pyroxene_at_, olivine,
and ilmenite.  Thus, on a plot of concentrations
of iron plus magnesium versus the concentration
of aluminum, all lunar meteorites and nearly all
Apollo lunar rocks plot along a line connecting
the composition of plagioclase and the average
composition of the three iron-bearing minerals
because these are the only four major minerals in
the rock.  Plagioclase (NaAlSi3O8 to
CaAl2Si2O8), where sodium and calcium atoms can
substitute for each other in the mineral's
crystal lattice structure. _at_Pyroxenes have the
general formula XY(Si,Al)2O6 (where X represents
calcium, sodium, iron(II) and magnesium.
http//epsc.wustl.edu/admin/resources/moon_meteori
tes.html
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The Chemistry of Lunar Rocks Because of the
simplicity of lunar mineralogy, lunar rocks have
predictable chemical compositions.  Nearly all
the aluminum is in plagioclase and nearly all
the iron and magnesium are in pyroxene_at_, olivine,
and ilmenite.  Thus, on a plot of concentrations
of iron plus magnesium versus the concentration
of aluminum, all lunar meteorites and nearly all
Apollo lunar rocks plot along a line connecting
the composition of plagioclase and the average
composition of the three iron-bearing minerals
because these are the only four major minerals in
the rock. 
http//epsc.wustl.edu/admin/resources/moon_meteori
tes.html
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Presolar Grains
Embedded in the fine-grained dust of chondrites
are presolar grains, which predate the formation
of our solar system and originated elsewhere in
the galaxy.
Larry R. Nittler, Earth and Planetary Science
Letters, 209 259-273 (2003). Presolar stardust
in meteorites recent advances and scientific
frontiers.
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Larry R. Nittler, Earth and Planetary Science
Letters, 209 259-273 (2003). Presolar stardust
in meteorites recent advances and scientific
frontiers.
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Nanodiamonds
Nanodiamonds (ca. 2.5 nm diameter) are the most
abundant, but least understood type of pre-solar
grains. They are identified as presolar on the
basis of containing highly unusual Xe and Te
isotopic ratios, which seem to reflect
nucleosynthetic processes in supernovae (SN).
However, their small size precludes isotopic
measurement of individual grains and the average
C isotopic composition (determined by
measurements of large aggregates of grains) is
indistinguishable from that of the solar system.
Making matters worse is the fact that the Xe
abundance is such that only about one in a
million diamond grains con- tains a single Xe
atom! Nitrogen is much more abundant and has a
15N/14N ratio some 35 lower than the terrestrial
atmosphere, apparently arguing for a presolar
origin as well. However, a recent measurement of
the N isotopic composition of Jupiter suggests
that the solar 15N/14N ratio is very similar to
that observed in the meteoritic nanodiamonds.
Thus, it is possible that most of the diamonds in
fact were formed in the solar system, with only a
tiny fraction having an origin in presolar SN
explosions.
Larry R. Nittler, Earth and Planetary Science
Letters, 209 259-273 (2003). Presolar stardust
in meteorites recent advances and scientific
frontiers.
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Presolar Oxide Grains
Oxygen isotopic ratios measured in presolar oxide
grains from meteorites.
O isotopic ratios span several orders of
magnitude in the presolar oxide grains (Fig. 4).
Many of the grains also show evidence for high
initial 26Al/27Al ratios when they formed and a
handful of grains have been analyzed for N, K
and/or Ti as well. The oxide grains have been
divided into four groups (labeled ellipses in
Fig. 4), on the basis of their O isotopic ratios
16.
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Resonance ionization mass spectrometry (RIMS)
RIMS is a new method that allows for the
measurement of ppm-level trace elements in
micron-sized grains.
Larry R. Nittler, Earth and Planetary Science
Letters, 209 259-273 (2003). Presolar stardust
in meteorites recent advances and scientific
frontiers.