Eucrites are
achondritic stony
meteorites, many of which originate from the surface of the
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
and are part of the
HED meteorite
HED meteorites are a clan (subgroup) of achondrite meteorites. HED stands for "howardite– eucrite– diogenite".
These achondrites came from a differentiated parent body and experienced extensive igneous processing not much different from the ma ...
clan. They are the most common achondrite group with over 100 meteorites found.
Eucrites consist of
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
ic rock from the
crust of
4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
or a similar parent body. They are mostly composed of
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
-poor
pyroxene,
pigeonite
Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene group. It has a general formula of . The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations.
Pigeonite crystallizes in th ...
, and calcium-rich
plagioclase (
anorthite
Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon.
Mineralo ...
).
Based on differences of chemical composition and features of the component crystals, they are subdivided into several groups:
* Non-cumulate eucrites are the most common variety and can be subdivided further:
** Main series eucrites formed near the surface and are mostly
regolith breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
s
lithified
Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificati ...
under the pressure of overlying newer deposits.
** Stannern trend eucrites are a rare variety.
** Nuevo Laredo trend eucrites are thought to come from deeper layers of 4 Vesta's crust, and are a transition group towards the cumulate eucrites.
* Cumulate eucrites are rare types with oriented crystals, thought to have solidified in magma chambers deep within
4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta, the ...
's crust.
* Polymict eucrites are
regolith breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
s consisting of mostly eucrite fragments and less than one part in ten of
diogenite. They are less common.
Etymology
Eucrites get their name from the Greek word meaning "easily distinguished". This refers to the silicate minerals in them, which can be easily distinguished because of their relatively large grain size.
Eucrite is also a now obsolete term for
bytownite
Bytownite is a calcium rich member of the plagioclase solid solution series of feldspar minerals with composition between anorthite and labradorite. It is usually defined as having between 70 and 90% An (formula: (Ca0.7-0.9,Na0.3-0.1) l(Al,Si)Si2 ...
-
gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...
, an
igneous rock formed in the Earth's
crust. The term was used as a rock type name for some of the
Paleogene igneous rocks of
Scotland.
[Sutherland, D. S. (1982) ''Igneous Rocks of the British Isles'', Chichester, John Wiley, page 536.]
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
External links
Eucrite images- Meteorites Australia
{{Meteorites
Planetary science
Asteroidal achondrites
4 Vesta
it:Meteorite HED#Eucriti