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Stony-meteorites

 

 

Achondrites

 

Achondrites

PAC-group (Primitive Achondrites)

Acapulcoites

Lodranites

Brachinites

Winonaites

Primitive Enstatite Achondrites (Zaklodzie, ITQIY)

Angrites

Aubrites

Ureilites

HED-group ("Vestameteorites")

Diogenites

Eucrites

Howardites

LUN-group ("Moonmeteorites")

Anorthositic Regolith-Highlandbreccias

Fragmental Highlandbreccias

Impact-melt Breccias

Marebasalts

Maregabbros

SNC-group ("Marsmeteorites")

Shergottites

Nakhlites

Chassignites

Orthopyroxenites

 

 

 

Achondrites represent a very heterogeneous class of meteorites. They differ from the more frequent Chondrites substantially, first of all by the missing of chondrules, but we have already seen that there are also Chondrites, that possess no chondrules at all, and just like that there is at least one kind of Achondrite that has chondrules. These exceptions mark transitions between both classes that become understandible if we bear in mind, that the Chondrites represent almost unchanged premeval matter from of the origin of our Solar system. Through melting- and recrystallization processes on some asteroids and planets out of the chondric premeval matter emerged differentiated rock, that delivered the basic substance of the Achondrites.

Achondrites therefore are samples of other differentiated worlds and resemble in this earthly stone. Nevertheless, most Achondrites are quite primitive - i.e. are little differentiated - and consequently are very old. They come mostly from the remote time of the origin of our solar system, a time that reaches back between 4,6 and 4,2 billions of years. The reason is, that they come from smaller mother bodies, asteroids, who quickly cooled off after their emergence and therefore became geologically inactive.

Only few Achondrites provable come from larger mother bodies, what expresses also in a substantially younger origin. Planets like our neighbour Mars were (or are) geological active until now, and so it isn't surprising that Mars meteorites were found, that - compared to other meteorites -are astonishingly young with an estimated origin age of only few hundred millions years. These meteorites are correspondingly highly advanced e.g. strongly differentiated and resemble the earthly rock the most.

 

PAC-group (Primitive Achondrites)

The symbol "PAC" stands for "Primitive Achondrites" and comprises different groups of meteorite that are quite similar to their chondritic precursors in chemistry and structure. They are little differentiated and emerged probably on very small asteroids, that became melted by impact happenings and then recrystalized very fast.

 

Acapulcoites

This small class was named after a fall that happened in 1976 in the environment of Acapulco, Mexico. Originally this meteorite was classified as an anomalous Chondrite, but after in the following decades some similar meteorites were found, one recognized that it concerns a class of primitive Achondrites, that marks the transition between chondritic basic substance and differentiated rock. That fourteen until now known Acapulcoites consist mainly of Olivine and Pyroxene and slighter shares of plagioclase, metallic nickel iron and troilite. In lots of Acapulcoites also intact chondrules were found, what underlines once again that this group represent a scarcely differentiated mother body and is to be regarded as the maybe most primitive class of Achondrites.

 

Lodranites

Lodranites, named after the fall at Lodran, Pakistan in 1868, were originally coinsidered as a small group of Stony-iron meteorites because they consist approximately to same parts of Olivine and Pyroxene as well as finecrainy nickel iron. Only the discovery of the Acapulcoites led, based on great chemical and isotopic resemblance, to incorporate them into the group of the Primitive Achondrites. Probably Acapulcoites and Lodranites come from the same little differentiated mother body, a little asteroid, who's chemistry shows some relations with the mother bodies of the E- and H-Chondrites.

 

Brachinites

The Brachinites are a very small group of olivine rich, Primitive Achondrites and were named after their find at Brachina, Australia. This meteorite was first of all classified as a Chassignite, a rare type of Mars meteorite ,but as one examined it and further six similar finds more exactly, one assessed that it concerns one characteristic class of Primitive Achondrites, that resemble the Chassignites only in their mineralogical composition. In comparison to the relatively young Mars rock, the Brachinites are with an origin age of 4,5 billions of years very old and show an entire other pattern in their trace elements, that show a typically chondritic distribution.

 

Winonaites

The Winonaites represent a further rare class of primitive Achondrites and were named after an unusual find. The meteorite Winona was discovered 1928 in archaeological excavations in the prehistoric Elden-Pueblo, Arizona, in a wall niche chiseled into the stone. Obviously it had served the indians living here as a sanctuary after perhaps they even had observed its fall. And also for science, Winona became somewhat entirely special, the prototype of a new meteorites class, that differs in its isotopic signature very clearly from all other Achondrites. A narrow relationship exists to the Iron meteorites of the chemical class IAB. Many IAB-irons contain Silikate inclusions, that resemble a lot the Winonaites, and probably come both from one and the same mother body.

 

Primitive Enstatite Achondrites (Zaklodzies, ITQIYs)

Next to the Acapulcoites/Lodranites,the Brachinites and Winonaites there is another series of further Primitive Achondrites, that however are each more or less individual finds and therefore don't justify the set up of a new class. Examples for this are two lately found, enstatite rich Achondrites: Zaklodzies out of Poland and ITQIYs out of North Africa. Both are unique in chemistry and mineral substance and consist mainly of Enstatite and metallic nickel iron. Somehow Zaklodzies remind in their structure the Acapulcoites, while the ITQIYs structurally are similar to the Lodranites. Possibly Zaklodzies and ITQIYs are descendants of one and the same, little differentiated mother body, whose development was similar to the mother body of the Acapulcoites and Lodranites. Further investigations and new finds will probably give more information in the future about this question.

 

Angrites

The Angrites, named after the fall at Angra dos rice, Brazil, represent a very small class of differentiated Achondrites with only six members, that consist of pyroxene, olivine and plagioclase. In contrast to the Chondrites and Primitive Achondrites, these minerals however exist in forms, that are typical for a magmatic origin: the pyroxene exists mainly in form of the mineral fassait, that olivine contains next to iron and magnesium also calcium, and the plagioclase exists almost exclusively as a calcium rich anorthite. Also the general structure as well as frequently appearing round vesicals, that are considered as hardened gaz bubbles, let the Angrite appear as typical discharge rock, i.e. as a basalt magmatic origin, that are quite similar to earthly basalts. With the difference that the Angrites with an origin age of about 4,56 billions of years are very old! The origin of this primeval basalt however is now as ever a riddle. Based on certain isotopic resemblances one assumed at times, that the Angrites like the meteorites of the HED-group came from the asteroid 4 Vesta. But based on their high age and certain differences in the chemistry one assumes today that the Angrites must come from a separate mother body, not yet identified so far.

 

Aubrites

The Aubrites were named after the fall at Aubres, France, where in 1836 an about 800 g heavy meteorite went down. Based on its peculiar mineralogical structure, the Aubrites are considered also as Enstatite Achondrites, because to the large part they consist out of this almost iron free, magnesium rich Pyroxene. Next to Enstatite the strongly brecciated Aubrites contain also different parts of reduced nickel iron, Olivine, Troilite and some rare minerals, that let assume on a magmatical origin under reducing conditions. In spite of a chemical resemblance to the Enstatite Chondrites and Primitive Enstatite Achondrites, the Aubrites certainly come from another mother body, that endured a substantially stronger differentiation. Spectrological investigations have identified the asteroid 44 Nysa and some of its companions as possible mother bodies, whereby above all an outsider out of this asteroids family, the erdnear asteroid 3103, moved in point of view. Possibly this nameless asteroid with about 1,5 kms of diameter is the origin of all Aubrites existing on Earth. Forty-six of these rare meteorites were described until now.

 

Ureilites

The Ureilites, named after the fall at Novo Urei, Russia in the year 1886, are the probably most mysterious Achondrites in general. They consist mainly of Olivine and Pyroxene in a carbon rich matrix out of graphite, diamond, reduced nickel iron and Troilite. Both the chemical and the isotopical investigations of the Ureilites led to contradictory results: some facts let the Ureilites appear as a highly differenciated kind of rock while other results move these meteorites more into the region of the Primitive Achondrites. Based on these contradictions, until now there is no theory generally acknowledged about the emergence and the origin of the Ureilites. Nevertheless most researchers agree, that the high carbon content suggests some relations to the Carbonaceous Chondrites, and that the Ureilites probably come from a differentiated asteroid of the C-class. Until now ninety-five of these mysterious meteorites were found, what makes the Ureilites to one of the "more frequent" Achondrites classes.

 

HED-group ("Vestameteorites")

The HED-group consist of three different classes of closely related Achondrites, that Howardites, Eucrites and Diogenites, whose initials form the symbol "HED". From their mineralogical composition and origin these meteorites are quite different, but their almost identical chemical and isotopical structure show very clearly that all three must come from one and the same mother body. Comparisons of the reflection spectrums of these meteorites with the spectrums of various asteroids yielded that this mother body concerns probably of the main belt-asteroid 4 Vesta, one of the largest asteroid in our solar system (see origin). For this reason the meteorites of the HED-group are sometimes also namrd "Vestameteorites".

 

Diogenites

The Diogenites were named after the Greek philosopher Diogenes, that already in the fifth century before Christ assumed that meteorites don't come from Earth, but rather from the cosmos. Mineralogically the Diogenites, of which ninety-six falls and finds are known, are of differentiated depths rock, that consist above all of magnesium rich, calcium scarce orthopyroxene. They contain beside it also smaller quantities of olivine and plagioclase, but the main part of the mass are the often large pyroxene crystals, that again are typical for plutonic rock that formed itself in magma chambers in the coat of a planet or asteroid, and were able to grow up by slow cooling to such a size. The Diogenites are therefore samples of magmatic processes that happened about 4,4 billions years ago within their mother body Vesta!

 

Eucrites

The term Eucrite comes of the Greek word eukritos that means something like "to distinguish easily". The old meteorites science lent the Eucrites, that form the numerically largest Achondrites class with more than two hundred falls and finds, this name out of two reasons: first because they can be distinguished by their appearance easily from the Chondrites, and secondly, because they resemble certain earthly rock of volcanic origin, that were also named Eucrites at that time. Today this name is only common only for the meteorites of the same name although the Eucrites remind actually in various regard to the terrestric basalts. They consist of calcium rich plagioclase (anorthite), calcium scarce pyroxene and often contain shares of reduced nickel iron, what makes many Eucrites a bit magnetic. Often they contain also small vesicles, that are considered as hardened gas bubbles and underline the magmatic origin of the Eucrites as real discharge rock once again. Eucrites are the basalts of another world, comparably with the basalts, that formed and still form on Earth through volcanic activity.

 

Howardites

The Howardites were named after the British chemist Edward Howard, a pioneer of the meteorite science (see history), and today ninety-five falls and finds of this rare class are known. In certain regards the Howardites form a kind of link between Diogenites and Eucrites, that however - not as one could assume -emerged in the coat of the Vesta, but rather at its surface. The Howardites are, considered mineralogically, a Regolith rock: baked together "dust layer" from the surface of their mother body in which about same parts of diogenitic and eucritisc rock with chondric impact matter from space mix. And so one finds in the strongly brecciated Howardites also most different inclusions of smaller and larger meteorites, that fell in the course of the billions of years on Vesta and with by them set free and by the solar wind pulverized Eucrites and Diogenites combined to new rock. This natural variety makes the Howardites not only to some of the most interesting, but also to some of the prettiest Achondrites in general.

 

LUN-group ("Moonmeteorites")

The symbol "LUN" comes from "Lunaite" and represents a group of Achondrites, that come from our Earth satellite, the Moon (see also origin). If one exactly looks at the Moon in a starlit night, one can well imagine, how these meteorites arrived from the surface of the Moon on the Earth: the Earth satellite is strewn with large craters of impacts, that had enough power to eject rocks from its crust and accelerate them such that they got through the field of gravity of the Moon and landed in the cosmos. Some of these fragments got into an unstable orbit around our planet and after some time fell as Moon meteorites on the Earth. These meteorites are of course as different as the rocks on the Moon. Until now twenty-four Moon meteorites were found worldwide, that can be classified into five different classes:

 

Anorthositic regolith highlandland breccias

The Regolith Highlandbreccias are the majority of the well known Moon meteorites with more than ten finds. They come from the highlands of the Moon, that form the largest part of its surface, and dominate mainly the from the Earth invisible side. In their mineral consistence, they consist mainly of regolith that emerged through meteorite bombardement and solar wind, a baked together dust, containing numerous clasts of original plagioclase that as a calcium rich anorthosite is the primary rock forming mineral on the highlands of the moon. This composition gives the anorthositic regolith highland breccias their typical appearance with angular, white clasts (anorthite) in a carbonaceous-black matrix (regolith).

 

Fragmental highland breccias

The fragmental highland breccias, of which until now only two are known, also origine from the highlands of the moon. However they don't consist of regolith, but rather out of brecciated rock fragments from underneath the lunar surface. Thus they consist mineralogially mostly of anorthosite, the calcium rich plagioclase from the lunar highlands, as well as of some other extra minerals e.g. pyroxene and olivine.

 

Impact-melt breccias

The breccias, of which also only two finds are known, resemble in their mineralogical compound the other lunaites of the highlands with the difference that some minerals contained in them exist in strongly shocked form, i.e. became changed in their structure by an earlier impact event. Thus in these impact-melt breccias also frequently glass veins of shocked rock as well as high pressure minerals are found.

 

Marebasalts

These meteorites of which only five are known are samples of the lunar mares, the large, dark basins, that form the lowlands of the moon. Three of these five moon meteorites are regolith breccias, that show comparing them with the regolith highlandbreccias only fragments of the original basalts in a regolith-matrix, while in two of these meteorites it concerns of real samples of the lunar marebasalts. These consists mainly of the pyroxene Pigeonite and Augite as well as a smallerr share of plagioclas and olivine.
In contrast to the highland rock, the Mare basalts are by far younger rock, taking into account that the lunar mares came into existence only about a billion years after the about 4,5 billions year old highlands.

 

Mare gabbros

This class is represented only by one meteorite find from the Antarctica, the meteorite asuka-881757, that in its coarse-grained structure represents the only unbrecciated moon meteorite well known until now. Mineralogical it is a so-called gabbro, a rock, that consists primarily of the minerals plagioclase (in the form of Anorthite) and pyroxene (Pigeonite and Augite). It remains to be seen whether in the future further samples of this material are discovered and other until now not known lunar rocks in the form of meteorites!

 

SNC-group ("Marsmeteorites")

The rare SNC-meteorites, that were named after the initials of three historic falls: Shergotty, Nakhla and Chassigny, form a further group. These meteorites are summarized with some further falls and finds based on mineralogical and chemical resemblances into one group, and in the mean time it is rather certain that all these meteorites come from our neighbour planet Mars (see also origin). For this assumption indicates not only the comparably young age of most of the SNC-meteorites, that often is only few hundred millions years, but rather also a comparison of gases included in this meteorites with the results of the Viking-probes, which examined in the year 1976 the exact composition of the Mars atmosphere. In the mean time twenty-two of this "Mars meteorites" are known, that stem however probably out of only seventeen falls. Basically the SNC-meteorites subdivide themselves into four mineralogical classes :

 

Shergottites

The Shergottites, named after the fall at Shergotty, India, are among the Mars meteorites especially frequent and make out seventeen of the twenty-two well known SNC-meteorites. Mineralogically they split into two different types, into the basaltic and into the lherzolithic Shergottites. The basaltic Shergottites consist mainly of plagioclase and pyroxene and resemble earthly basalts of volcanic origin. The iherzolithic Shergottites consist of olivine and orthopyroxene and contain only slighter quantities of plagioclase. They are closely related to the basaltic Shergottites, but represent no discharge rock, but rather pluto depths rocks of the Mars, that came out of the same magma layers like their basaltic relatives. But there are also transitions between the basaltic and lherzolithic type, that for example are represented by the Mars meteorites out of the Dhar al Gani desert, Libya.

 

Nakhlites

The Nakhlites are named after the fall at Nakhla, Egypt, and only three of these finegrainy, greenish brown meteorites are known. In the mineralogical sense, they consist mainly of calcium rich pyroxene (augite) and a slight share of olivine. Interestingly enough, recently in the Nakhlites one discovered rare minerals that could only emerge in the presence of fluid water. Analyses yielded that these minerals must have emerged still being on Mars, what means that our red neighbour still possessed at least at the moment of origin of the Nakhlites, therefore about 1,5 billions years ago, fluid water and perhaps even oceans!

 

Chassignites

Closely related with the Nakhlites are the Chassignites that were named after the only representative of this class, the meteorite Chassigny, France. Mineralogically in Chassignites it concerns of a so-called dunite, a depths rock consisting almost purely out of olivines. Also in Chassignites, traces of minerals were found, that were able to emerge only in the presence of water, so for example the on Earth also well known amphibol.

 

Orthopyroxenites

Also this group is represented only by one representative, by the antarctic meteorite ALH84001. In contrast to the other Mars meteorites it consists almost exclusively of orthopyroxene and possesses a substantially higher age. Especially wellknown became this hottly discussed meteorite by numerous investigations of various inclusions, that indicate a - at least earlier - primitive life on our neighbour planet Mars (see sensations).

 

Chondrites