Primitive achondrites are a subdivision of
meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object en ...
s. They are
classified
Classified may refer to:
General
*Classified information, material that a government body deems to be sensitive
*Classified advertising or "classifieds"
Music
*Classified (rapper) (born 1977), Canadian rapper
*The Classified, a 1980s American roc ...
on the same rank (historically called "Class") and lying between
chondrite
A chondrite is a stony (non- metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form pri ...
s and
achondrite
An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystall ...
s. They are called primitive because they are
achondrite
An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystall ...
s that have retained much of their original
chondritic properties. Very characteristic are relic chondrules and chemical compositions close to the composition of
chondrite
A chondrite is a stony (non- metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form pri ...
s. These observations are explained as melt residues, partial melting, or extensive recrystallization.
History
The concept of primitive achondrites was first summarized in 1983.
In 2006 a classification was published that assigned 7 groups to the primitive achondrites, but the classification remains controversial. The authors define primitive achondrites as meteorites "''that exceeded their solidus temperature on the parent body''" and thus would partially melt. Meteorites that have been fully melted are included if they did not reach isotopic equilibrium on their parent body.
Description
There are many indicators why some achondrites are deemed primitive. Some contain relic chondrules (e.g.
Acapulcoite,
Lodranite) and some have oxygen isotope ratios that are similar to
chondrite
A chondrite is a stony (non- metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form pri ...
s. There are similarities in trace element concentrations between primitive achondrites and chondrites. A textural indicator can be observed in
petrographic thin section
In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and elect ...
s. If an achondrite is primitive it formed through recrystallization and will have many 120° grain boundaries. Normal achondrites form by crystallizing from a magma and will show magmatic textures.
Lodranites are for example seen as residues of partial melting, while acapulcoites are seen as partial melts, but both were produced by partial melting. Ureileites and brachinites remain uncertain and could either be explained as residues or cumulates.
Subdivisions
The following groups are assigned to the primitive achondrites:
*
Ureilite group
*
Brachinite group
*
Acapulcoite group
*
Lodranite group
*
Winonaite group
*
IAB group
*
IIICD group
See also
*
Glossary of meteoritics
This is a glossary of terms used in meteoritics, the science of meteorites.
#
* 2 Pallas – an asteroid from the asteroid belt and one of the likely parent bodies of the CR meteorites.
* 4 Vesta – second-largest asteroid in the asteroid b ...
References
{{meteorites
Achondrite meteorites