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Angrites are a rare group of
D'Orbigny Angrite
- Meteorites Australia
Angrite Micro Visions
- Thin section pictures of angrites.
Angrites are a rare group of 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 consisting mostly of Al-Ti bearing diopside
Diopside is a monoclinic pyroxene mineral with composition . It forms complete solid solution series with hedenbergite () and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull ...
, hedenbergite
Hedenbergite, Ca Fe Si2 O6, is the iron rich end member of the pyroxene group having a monoclinic crystal system. The mineral is extremely rarely found as a pure substance, and usually has to be synthesized in a lab. It was named in 1819 after M ...
, olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
, anorthite and troilite
Troilite is a rare iron sulfide mineral with the simple formula of FeS. It is the iron-rich endmember of the pyrrhotite group. Pyrrhotite has the formula Fe(1-x)S (x = 0 to 0.2) which is iron deficient. As troilite lacks the iron deficiency whic ...
with minor traces of phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phosph ...
and metals. The group is named for the Angra dos Reis meteorite. They are the oldest igneous rocks, with crystallization ages of around 4.56 billion years. Angrites are subdivided into two main groups, the quenched and plutonic angrites. The quenched angrites cooled rapidly upon the surface of the angrite parent body (APB), whereas the plutonic
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March ...
angrites cooled slower, deeper in the crust. The APB is thought to have been a similar size to the asteroid 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, t ...
.
Origin
Angrite meteorites are distinct from other meteoritic groups based on theiroxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
isotopic compositions. Based on their Mn-Fe ratios in pyroxene and other isotopic compositions, the source of angrites is constrained to the inner Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
. However, recent studies have suggested that the APB experienced mixing of multiple sources during its history.
By comparing the reflectance
The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
spectra of the angrites to that of several main belt asteroids, two potential parent bodies have been identified: 289 Nenetta
Nenetta ( minor planet designation: 289 Nenetta) is a typical A-type asteroid. It was discovered by Auguste Charlois on 10 March 1890 in Nice, France.
The spectrum of 289 Nenetta reveals the strong presence of the mineral Olivine
The mineral ...
and 3819 Robinson. Other scientists have suggested that angrites could represent ejecta from Mercury, however later work has cast significant doubt upon these claims.
Based on the lack of an asteroid matching the spectra of angrite meteorites it is thought that the APB was catastrophically disrupted and subsequently destroyed.
Notable meteorites
There are currently over 30 meteorites classified as angrites. The type specimen, the Angra dos Reis meteorite, was an observed fall in 1869 and weighed . Most of it has been lost over time; the largest remaining piece, weighing 101 grams, is kept at the Museum of Natural History in Rio de Janeiro. NWA 10463 has been suggested to represent an intermediate stage between the quenched and plutonic angrite meteorites. NWA 8535 has been suggested to represent aDunite
Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, with ...
.
Asuka-12209; Asuka-88371 and NWA 12320 demonstrate an oxygen isotopic disequilibrium indicative of planetary mixing.
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 ...
External links
D'Orbigny Angrite
- Meteorites Australia
Angrite Micro Visions
- Thin section pictures of angrites.
References
{{Meteorites Asteroidal achondrites