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CLASSIFICATION OF METEORITES;
Meteorites are classified into three main categories:
stones, stony-irons and irons, depending on their dominant composition. Stones are similar to common terrestrial rocks in that their mineral composition is dominated by silicates, by far the most prevalent rock-forming minerals on our planet. Irons are mostly metallic in composition; they consist of alloys of iron (Fe) and nickel (Ni), in varying proportions. Stony-irons are combinations of both; they contain silicate and metallic phases in approximately equal amounts.
Stones are subdivided into two classes: chondrites
and achondrites. Chondrites get their name from the fact that they all (with some
exceptions) contain chondrules, tiny mineral spherules made mostly of silicates. Although
some may be as large as a few millimeters in diameter, most chondrules are less than 1 mm
across. In chondrites, chondrules are bound within a consolidated and fine-grained
background matrix. Chondrites are the most primitive meteorites known. That is, they are
the most ancient ones in terms of when their constituents came together to form a rock,
and the most unprocessed ones in terms of how little their materials have been altered
since this rock formed. Achondrites, on the other hand, lack chondrules and represent more
processed materials.
Earth's surface rocks would be achondrites were they
meteorites; they lack chondrules and are the result of extensive geological processing
(melting, for instance).
Chondrites, achondrites, stony-irons and irons are subdivided into groups and subgroups. These will be presented in more detail below.
Falls are meteorites whose arrival on Earth was witnessed and recorded. Their time of fall is thus relatively precisely known. These meteorites were usually recovered shortly after their arrival, although often enough in the case of showers, additional fragments from a given fall may be recovered a long time after the fall occurred. When all falls exclusively are considered, a reasonably good estimate of the general population of meteorites reaching the Earth may be made. The vast majority of falls are stones (92.8%), most of which turn out to be chondrites (85.7% of all falls). Irons are rare (5.7% of all falls); stony-irons rarer still (1.5%). In other words, most meteorites falling on Earth are by far chondrites.
Finds are meteorites that were not seen to fall but were subsequently discovered on the ground, often long after they landed. Their arrival on Earth (time, circumstances) is thus not well documented. The vast majority of meteorites in museum and private collections around the world are finds, not falls. Because stones tend to look like ordinary terrestrial rocks, especially if they were subjected to weathering, they are easily overlooked. Stone finds are therefore rare in spite of the commonness of stones among falls. Meteorite collections are instead dominated by irons, which not only have a distinctive appearance and are therefore easier to spot, but they resist longer than stones to weathering and are particularly amenable to being found by metal detectors. Stony-irons would also be common among finds if it weren't for their lesser resistance to weathering compared to irons and, more importantly, for their extreme rarity among falls in the first place.
Meteorites, whether falls or finds, are usually given the name of the locality (post office, if any) nearest the site where they were recovered. In cases where many meteorites representing several falls are found within a relative small area (individual blue ice fields in Antarctica for instance), the meteorites are designated by an abbreviated locality name (the same name for all meteorites from that area) followed by a number giving the year of recovery and a serial number. ALH81005, for instance, is meteorite number 5 among those recovered in the Allan Hills area of Antarctica during the 1981-1982 field season (Note: number 5 does not necessarily mean that this meteorite was the fifth one recovered).
CHONDRITES
In addition to the silicaceous chondrules,
chondrites contain variable amounts of free metal (Fe, Ni).
Chondrites are subdivided into 3 main groups. In order
of decreasing degree of oxidation of the iron (Fe) that they contain, these are:
carbonaceous chondrites, ordinary chondrites, and enstatite chondrites.
Within each of these groups, chondrites are also classified according to their petrologic type; that is, on the basis of the degree of alteration (or alteration grade) to which they have been subjected on their parent body prior to arriving on Earth. This degree of alteration ranges from 1 to 6, grade 3 corresponding to the least altered state. The grade decreases from 3 to 1 as aqueous alteration (alteration by liquid water) intensifies, while the grade increases from 3 to 6 as thermal metamorphism (alteration by heating) increases. At grade 6, chondrites have undergone such intense heating and attendant recrystallization that chondrules may be almost completely obliterated. Carbonaceous chondrites have mostly undergone aqueous alteration (none are of grade higher than 4), while most ordinary and enstatite chondrites have undergone thermal metamorphism (none are of grade lower than 3).
Carbonaceous chondrites
Carbonaceous chondrites represent only 5.7% of all
falls (just about like the irons). They are chemically the most oxidized of all
chondrites. They contain virtually no free metal; all the Fe in them is oxidized. That is
not to say that carbonaceous chondrites appear rusty. These meteorites typically display a
very dark matrix, black to gray in color, containing relatively large amounts of carbon
and other organic matter, including amino acids, the building blocks of proteins and thus
of life on Earth. Their matrix also contains whitish irregular-shaped specks known as
calcium-aluminum-rich inclusions (CAIs). The CAIs consist of minerals uncommon on Earth,
with high concentrations of refractory elements such as titanium (Ti). Grains of
interstellar material, including microscopic diamonds, have also been found in the matrix
and chondrules of carbonaceous chondrites. The chondrules in carbonaceous chondrites are
usually well-defined, but they may, in some (rare) cases, be altogether absent.
Carbonaceous chondrites are further subdivided into four subgroups, in two different ways: 1) with respect to elemental composition; b) by petrologic type. In the first case, the subdivision is based on differences in the abundance of so-called minor and trace elements (for instance calcium, potassium, iridium and zinc). The resulting four subgroups are designated CI, CM, CO and CV, after their typical representatives, the carbonaceous chondrites I, Murchison, Orgueil and V, respectively. In the second subdivision scheme, the carbonaceous chondrites are classified on a petrological (as opposed to compositional) basis more specifically according to their state of alteration. The resulting four subgroups are designated C1, C2, C3 and C4. As described earlier, grade 3 is the least altered of all. It should be emphasized that the alteration processes involved here took place on the parent bodies of the meteorites, not after their arrival on Earth. The fact that aqueous alteration has affected some carbonaceous chondrites is of fundamental importance: it implies that liquid water was available on their parent worlds. There is no simple correspondence between the compositional (elemental) subdivision and the petrologic subgroups.
Carbonaceous chondrites being relatively fragile, most of the ones known are falls. Allende and Murchison are particularly famous because they fell in relatively large numbers (due to fragmentation during transit through the Earth's atmosphere) and in very recent times. They have been studied extensively, with modern techniques and before the onset of any significant weathering alteration.
Carbonaceous chondrites might come from the most
primitive asteroids known, the C and/or D-type asteroids. Most C and D-type asteroids are
located near the outer reaches of the asteroid belt and may, therefore, be the most remote
sources of meteorites available. Interestingly, however, Phobos and Deimos, the two small
moons of Mars, are also C and D-type objects (respectively) and are much closer to the
Earth. They might once have been rogue asteroids which were captured by Mars.
Some carbonaceous chondrites could conceivably have
come from the martian moons. Because of the presence of organic matter of extraterrestrial
(although likely not biogenic) origin in carbonaceous chondrites, these meteorites are
believed to hold fundamental clues to the origin of life on Earth.
Ordinary chondrites
Ordinary chondrites represent 79% of all falls. They
are subdivided into 3 subgroups on the basis of content in free metal (Fe, Ni): H (high),
L (low), LL (low low, i.e. very low). All ordinary chondrites are rich in the mineral
olivine.
H chondrites
H chondrites are characterized by relatively high iron,
nickel and sulfide contents (16-22% by weight). The sulfide is essentially iron sulfide
(FeS), a mineral known as troilite. Because bronzite is the dominant form of pyroxene in
these meteorites, H chondrites are also known as olivine-bronzite chondrites. H chondrites
may be identified by the fact that their matrix displays abundant iron flakes and facets,
which are often oxidized. Alteration grades for H chondrites range from 3 to 6. (There are
no H1 or H2 chondrites). As described earlier, H6 chondrites have undergone significant
thermal metamorphism, where H3 chondrites have not.
Most H chondrites are of the H5 petrologic type.
L chondrites
L chondrites are characterized by a low free metal and
sulfide content (7-16% by weight). Because hypersthene is the dominant form of pyroxene in
these meteorites, L chondrites are also known as olivine-hypersthene chondrites.
Alternation grades for L chondrites range from 3 to 6. Most L chondrites are of the L6
petrologic type.
LL chondrites
LL chondrites have the lowest free metal and sulfide
content (less than 7% by weight). As for the L chondrites, hypersthene is their dominant
form of pyroxene. To distinguish the LLs from the L chondrites, LL chondrites are
sometimes referred to as amphoterites. The matrix of LL chondrites displays very little
visible iron. Alteration grades for amphoterites range from 3 to 6. Most LL are of the LL6
type.
Enstatite chondrites
Enstatite chondrites (or E chondrites) represent only
1.0% of all falls. They have the highest free metal and sulfide content (23-35% by weight)
and the lowest oxidation state of all chondrites. Their silicate phase is almost purely
enstatite (MgSiO3), an iron-free form of pyroxene. A distinction is sometimes made between
EH and EL chondrites on the basis of their free metal and sulfide contents (H for high, L
for low). Quite distinctively, all the iron in E chondrites is visible as free metal.
Alteration grades for E chondrites range from 3 to 6. Most E chondrites are of the E6
petrologic type.
E chondrites are of the E6 petrologic type.
B Chondrites- New proposed group
that include some meteorites of the CR (see below). Currently they are only Bencubbin,
Weatherford, HaH 237, and
GRO95551. They have a metal-silicate chondritic
composition with reduced silicates and more than 50% of FeNi. Chondrules are present, and
the are CAIs in HaH 237.
R Chondrites- This includes prior
known Carlisle Lake meteorite, but was created after the only fall of the group, Rumuruti.
The group is highly oxidized, olivine-rich,
and metal-poor. Compared with the ordinary,
carbonaceous and enstatite chondrites they have a very different oxidation state, oxygen
isotope composition, and mineralogy.
ACHONDRITES
Achondrites are a grab-bag category of meteorites
which are:
1.Rich in silicates
2.Lack chondrules
3.Have undergone relatively advanced geological
processing on their parent bodies.
They are generally more coarse-grained than the chondrites and are generally closer to terrestrial igneous rocks in terms of chemistry, mineralogy, and structure than the chondrites. The near-absence of metal phases (Fe, Ni) in achondrites suggests that they are differentiated rocks; that is, that they were produced by melting, decantation and then cooling of a more primitive material (possibly chondritic originally). In this picture, achondrites would be rocks that have crystallized as part of near-surface magmas on their parent worlds, whereas iron meteorites would represent material that sank to their interior to form a core. Stony-irons would then be samples from the boundary between the achondritic upper layers and the core. Achondrites are classified into several groups, each believed to represent either a distinct parent body or, in some cases, a particular location on a common parent body. Some groups may be subdivided into subgroups.
HED group stands for
Howardite-Eucrite-Diogenite meteorites. These are three subgroups of achondrites which,
although different in mineral composition, share a number of other characteristics (such
as isotopic chemistry) which suggest they are probably related. The HED meteorites are
believed to come from the same parent body, specifically from the large asteroid Vesta.
Because of Vesta's very unusual composition among asteroids and the relatively good match
between its composition and that of the HED meteorites, the relationship is thought to be
relatively well established.
Eucrites
Eucrites are calcium-rich basaltic achondrites. They
are fine-grained volcanic rocks, samples of lava flows from the surface of another world!
Eucrites are nevertheless very different from terrestrial basalts in that:
1.they contain sodium-rich plagioclase
2.they have pigeonite as their dominant pyroxene
3.they contain no water at all (no hydrous
minerals) 4.they have a reduced oxidation state.
Diogenites
Diogenites are calcium-poor basaltic achondrites,
consisting almost entirely of the Mg-rich pyroxene, hypersthene. They contain only minor
mounts of plagioclase and olivine. The mineralogy and oxidation state of diogenites
are close enough to those of eucrites to suggest a common parent body. Because diogenites
have a coarse-grained texture with large interlocking crystals, however, they must have
cooled more slowly than the eucrites. Diogenites probably crystallized from a magma at
some depth:. They are plutonic rocks (as opposed to volcanic ones).
Aubrites
Aubrites are calcium-poor achondrites consisting mostly
of enstatite as their pyroxene. They are sometimes referred to as enstatite achondrites
and might somehow be related to the enstatite chondrites. Aubrites are believed to come
from E-type asteroids, although it has also been suggested that they are what meteorites
from Mercury might look like. Less than twenty distinct Aubrites are known.
Ureilites
Ureilites are calcium-poor achondrites, consisting
mainly of olivine, pigeonite (their dominant form of pyroxene) and carbon (2.2%). The
carbon is in the form of either graphite, diamond, or lonsdaleite (a rare pure carbon
mineral like diamond, but with a different crystal structure). The ureilites are the only
achondrites containing significant amounts of free metal (Fe, Ni) (5%). They also contain
the highest proportions of heavy rare gases such as argon, krypton and xenon, among all
meteorites. The origin of ureilites remains an enigmatic. They appear to result from a
complex igneous history, involving perhaps carbonaceous chondrite-like material as a
starting point.
Lunar meteorites
Twelve meteorites from the Moon have been found to this
day. All except one, Calcalong Creek (Australia), were recovered in Antarctica. All lunar
meteorites are impact breccias, rocks formed from the rewelding during energetic impact
events of loose fragments once part of the lunar soil. Some lunar meteorites are
dominantly basaltic in composition and thus come from the lunar mare (the dark, concealed
flood lavas that occupy large impact basins on the Moon, especially on its near side);
others are composed dominantly of the mineral anorthosite, a sodium-rich plagioclase,
suggesting these meteorites come from the lunar highlands (the brighter and more heavily
cratered terrains on the Moon). Lunar meteorites are of great scientific importance
because they come from areas of the Moon that were likely not sampled by the Apollo or
Luna missions. On statistical grounds, it is estimated that at least one of the lunar
meteorites found so far must have originated on the far side of the Moon. Lunar meteorites
are believed to have been blasted off the Moon in the form of high-speed ejecta during
impact events.
Lunar meteorites may be identified by a fusion crust
with slightly greenish hues and by a grayish interior with angular clasts (inclusions) of
often brighter materials.
Martian meteorites
Twelve meteorites are apparently samples from Mars.
They are commonly referred to as the SNC meteorites (after Shergotty, Nakhla, and
Chassigny, the representatives of the first three subgroups known). These martian
meteorite subgroups are distinguished on the basis of mineralogy, but they all share
isotopic signatures, petrologic characteristics and for some, relatively young
crystallization ages (less than 1.4 billion years), which together point to a martian
origin. About half of the known martian meteorites were found in Antarctica. The other are
falls and finds from elsewhere.
Shergottites
The shergottite subgroup is named after Shergotty, an
achondrite which fell in India in 1865. Shergottites are pigeonite and augite-dominated
basalts, samples of lavas that once flowed on their parent world. Aside from Shergotty,
shergottites include Zagami (Nigeria 1962) and the antarctic meteorites ALH77005 and
EET79001.
Nakhlites
The nakhlite subgroup is named after Nakhla, an
achondrite which landed in Egypt in 1911. Nakhlites are augite-rich achondrites. Aside
from Nakhla, nakhlites include Lafayette (Indiana find) and Governador Valadares (Brazil
find).
Chassignites
The chassignite subgroup is named after Chassigny, an
achondrite which fell in France in 1815. Chassignites are olivine-rich achondrites. Aside
from Chassigny, chassignites include Brachina (Australia find).
Others
Recently, the antarctic meteorite ALH84001 was
identified as having originated on Mars. The meteorite, however, does not fit into any of
the SNC subgroups and must be classified into a subgroup of its own. ALH84001 crystallized
(solidified as a rock) more than 4 billion years ago. It is believed it came from the
heavily cratered highlands of Mars, which would make it the only sample from these
terrains found so far.
Anomalous achondrites
There are a number of other achondrites that do
not fit into any of the preceding groups or subgroups. Some are the only specimens known
of their kind. More detailed descriptions of these rare meteorites is beyond the scope of
this review.
STONY-IRONS
Stony-irons are mixtures of silicates and free metal (iron-nickel) in approximately equal proportions. They are rare among falls but somewhat more common among finds because of the ease with which they can be distinguished. The stony-irons are divided into three main groups: pallasites, mesosiderites, and lodranites. Other groups exist, but they comprise unusual and rare specimens and will not be described further here.
Pallasites
The pallasites consist of olivine crystals that
may be as large as 1 cm across embedded in a Fe-Ni alloy matrix. Esthetically, pallasites
are easily the most beautiful meteorites, especially when cut and polished. When etched,
the metal phase may exhibit a Widmanstatten pattern (see "Irons" below).
Pallasites are thought to have formed at the core-mantle boundary within a differentiated
asteroid; the iron would be derived from the asteroid's core, while the olivine crystals
would come from the base of its mantle.
Mesosiderites
The silicate phase in mesosiderites consists
mostly of hypersthene pyroxene and plagioclase feldspar (as opposed to olivine as in the
pallasites), and often includes abundant troilite (FeS). Unlike in the pallasites also,
the metal phase in mesosiderites does not form a continuous matrix of free metal. Instead,
the mixing between the silicate and metal phases is more intimate.
Lodranites
Lodranites consist of approximately equal amounts of
metal, olivine, and pyroxene. Lodranites are sometimes classified as primitive achondrites
- "primitive" because they seem to have a chondritic major element bulk
chemistry and "achondrites" because they may exhibit achondritic textures.
Because all known lodranites are similar to one another in terms of major element
chemistry and mineralogy, they are believed to have formed on the same parent body. The
differences that do exist among lodranites are attributed to varying degrees of melting.
IRONS
Iron meteorites are classified according to both
structure and chemistry. Structure is determined by nickel content alone, while chemistry
depends not only on nickel content but also on gallium (Ga) and germanium (Ge) contents.
The structural classification usually serves as the primary framework. Chemistry is then
used for further subdivisions. There are three main structural groups for irons:
hexahedrites, octahedrites, and ataxites. Other groups exist, but they comprise unusual
and rare specimens and will not be described further here. Iron
meteorites represent samples from the very core of
their parent bodies. The parent bodies were presumably broken up in violent collisions
between asteroids, exposing their cores to further fragmentation by impacts.
Hexahedrites
Hexahedrites are the irons with the lowest nickel
content (4-6% by weight). They consist of large crystals of kamacite and contain no
taenite. Kamacite and taenite are both Fe-Ni alloys, but they differ by the relative
amounts of iron and nickel, and (consequently) have a different crystal structure;
kamacite forms a body-centered cubic lattice whereas taenite forms a face-centered one.
Because of the virtual absence of taenite in
hexahedrites, polished surfaces of these meteorites are featureless except for the
occasional presence of fine striations known as Neumann lines. Neumann lines are formed by
shock deformation of the metallic kamacite crystals during violent impacts.
Octahedrites
Octahedrites are characterized by an intermediate
nickel content (6-17% by weight) and contain both kamacite and taenite. These two metallic
minerals occur in a distinctive arrangement of bands intersecting in two, three or four
directions, which results in the characteristic Widmanstatten pattern. This
beautiful pattern appears conspicuously when a section of an octahedrite is polished and
etched in weak acid (usually nitric acid). The larger bands in a Widmanstatten pattern
consist of kamacite. This kamacite is surrounded by fine sheets of taenite, while the
interslices between the bands are made of plessite, a combination of kamacite and taenite.
Ataxites
Ataxites have the highest nickel content among iron
meteorites (more than 16% by weight). They consist almost entirely of taenite, with only
microscopic plates of kamacite. The largest meteorite known on Earth, the Hoba meteorite
in Namibia, is an ataxite. It weighs approximately 55 metric tons and still lies where it
fell.
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