Achondrite

Achondrites (Greek ά (a ), without ' and χόνδρος ( chondros ), grain ') are stony meteorites that contain as opposed to the more common chondrites no or only a few chondrules. On the basis of elemental and isotopic ratios Achondrites be divided into groups. Differences are primitive achondrites, which differ from chondrites only in texture and may contain residues of chondrules and differentiated stony meteorites, which are depleted of iron and soluble iron elements. They were knocked out of the crust or the mantle of larger parent bodies that have differentiated during their formation in an iron core and shell. Three groups, which together account for about two-thirds of achondrites, probably derived from the dwarf planet ( 4) Vesta, some from the Moon and from Mars, others could not be assigned so far.

The magmatic origin of Achondrites was first recognized by the Berlin mineralogy professor Gustav Rose.

Appearance

Achondrites (apart from the dark crust ) often colored gray to white and are made of igneous rock, which is composed primarily of the silicate minerals olivine, pyroxene and plagioclase. Achondrites almost always have a much lower content of nickel iron as chondrites. There are also Achondrites which contain up to 80 % by volume of nickel-iron.

Classification

We distinguish the subclasses described below.

HED group

The stony meteorites of the HED group are the most common group, as the origin of asteroid ( 4) Vesta is assumed because the reflectance spectra of this group and these are very similar to each other Vesta.

All previously investigated diogenites have an exposure age of 22 or 36 million years, ie to these two points in time they must have been blown off by a collision of a parent body. As the parent body of the asteroid ( 237442 ) 1999 TA10 is discussed, which might even come from the mantle of (4 ) Vesta.

SNC- group

They must come from a relatively large body, most likely from Mars.

Angrite

The Angrite (named after the archaeological site of Angra dos Reis, Brazil) are differentiated achondrites, consisting of pyroxene, olivine and plagioclase. Unlike the chondrites and primitive achondrites these minerals are present in forms that indicate a magmatic origin. They often contain inclusions that are interpreted as a solidified gas bubbles. From their structure and chemical composition they resemble terrestrial basalts. The origin of the Angrite is unknown as yet. Obviously, they are descended from a separate parent body, which could not yet be identified.

Aubrite

The Aubrite ( after the site Aubres, France) containing the magnesium-rich mineral enstatite. In addition, varying amounts of reduced nickel - iron, the iron sulfide troilite, the silicate olivine and rare minerals are found, suggesting a magmatic origin. When comparing the reflectance spectra of asteroids is a match with the asteroid Nysa was found. Perhaps the Aubrite derived from this celestial body.

Ureilite

The Ureilite ( Novo Urei, Russia) consist mainly of olivine and pyroxene. A special feature of Ureilite is a carbon-rich, traversed by veins matrix graphite, diamond, nickel - iron and the spaces can contain filling silicates. Chemical and isotopic studies of the Ureilite lead to inconsistent results. A heterogeneous distribution of oxygen isotopes and a high percentage of noble gases in ureilites speak against far-reaching Diffenrenzierung of ureilites nut body. Simultaneously Ureilite but are depleted in siderophile and lithophilen elements, indicating the fractional crystallisation of a basaltic and a metallic component. So far there is no generally accepted theory about the origin and the origin of meteorites.

• 2008 TC3 was the first asteroid, for a collision with Earth was predicted incorrect. The meteorites found are labeled Almahata Sitta and are classified as Urelite.

Primitive achondrites

Acapulcoites (Acapulco, Mexico), fallow Destinite ( Brachina, Australia), Lodranite ( Lodran, India), Winonaite ( Winona, United States): These four classes of achondrites are summarized as primitive achondrites. The differentiation of these meteorites is incomplete, so that they have retained their primitive ( chondritic ) composition largely. The chondrites typical structure got lost, chondrules were destroyed.

Lunar meteorites or Lunaite, also LUN group

By comparison with the samples brought back from the moon can be clearly shown that they have come from the moon. They often contain lunar regolith from the surface of the moon. It is interesting that the moon meteorites on Earth, were found only after the moon landing, as only the analyzes of moon rocks allowed reliable detection of the origin of the moon. The first lunar meteorites were found in Antarctica. The first non - Antarctic meteorite moon was Calcalong Creek from Australia, described in 1991. Later, further identified among the thousands of meteorite finds from Africa and Oman. In this group we can distinguish different subclasses according to their origin, the anorthositic regolith Hochlandbrekzien, the Frag mental Hochlandbrekzien, impact - Schmelzbrekzien, Mare basalts and finally Maregabbros.