Primitive achondrites are a subdivision of
meteorites. They are
classified 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 p ...
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 recrystalli ...
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 recrystalli ...
s that have retained much of their original
chondritic
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 ...
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 p ...
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 p ...
s. There are similarities in trace element concentrations between primitive achondrites and chondrites. A textural indicator can be observed in
petrographic thin sections. 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
Ureilite is a rare type of stony meteorite that has a unique mineralogical composition very different from that of other stony meteorites. This dark grey or brownish meteorite type is named after the village Novy Urey ( Cyrillic: ''Новый У ...
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