Stony-iron-meteorites

Pallasit

 


The class of Stony-iron meteorites is a loose category in the classicscience of meteorites, in which all meteorites were summarized that approximately consist of equal shares of metal e.g. nickel-iron and stone. Nowadays most researchers include only two groups of meteorites in the stony-irons, Pallasites and Mesosiderites, although there are enough reasons to expand this category with some groups that usually are listed as stony- or iron meteorites, as they fulfill all criteria, that make them to real Stony-irons. For this reason, these groups are also described further below.

The Stony-iron meteorites are, concerning frequency, the smallest class, because they only come to about 1,5% of all observed cases. And although they are, based on their high iron portion, quite heavy meteorites, until now only about 10 tons of them were found, what corresponds to a weight share of only 3,5% of the total mass of all well known meteorites. Stony-iron meteorites are therefore rare in each regard.

 

Stony-iron meteorites

Pallasites

Mesosiderites

Further groups

Bencubbinites

Lodranites

Siderophyres

 

 

Pallasites

Pallasites consist of olivinecrystals that are embedded in a networklike nickel-iron matrix. In old, weathered Pallasites, that already lie long on the earth, the olivinecrystals often are blackish discoloured or weathered out of the matrix, what gives these pieces an ugly, spongelike outward appearance. But fresh Pallasites with often centimeter-big, clear Olivine crystals and an intact matrix, cut in slices and polished, belong to the undoubtedly prettiest meteorites in general! The olive-green, often clear Olivines correspond to the earthly Peridotes and can be designated rightly as the only cosmic jewels. The Russian Czar house possessed magnificent gems of pallasitic Peridote, and also today occasionally exquisite jewels are made of this unique material.

By origin Pallasites are closely related to the Iiron meteorites. While the Iron meteorites correspond to the innermost core of differentiated asteroids, the Pallasites represent the internal coat, a zone in which the metallic core and the silikate rich crust of the asteroids meet and mix together. Probably there are also similar minerals on the Earth, however in such a depth, that we will probably never come to see them.

As for classification, Pallasites are divided into three chemically different groups, whereby each of these groups represents a characteristic mother body. The Pallasites of the main group, to which are counted most Pallasites found until now, show some narrow relations with the Iron meteorites of the IIIAB-group what suggests a common home. A similar relationship could also be proved for the Pallasites of the numerically weakly represented Eagle-station group and the Irons of the IIF-group. Unique on the other hand are the so-called Pyroxen-Pallasites, which like olivinecrystals also show pyroxenecrystals in a nickel-iron matrix. They obviously come from a mother body of which we have no further evidence. Apart from these, there are a few further, ungrouped Pallasites that show also no resemblances to other well known meteorites.

 

Mesosiderites

Mesosiderites represent a complex structure of often chaotic consistences of nickel-iron and breccia silicates, that consist of pyroxene, plagioclase and olivine. In some Mesosiderites are also found inclusions of Eucrites, Diogenites and other basaltic rocks, what lets one conclude that the home of the Mesosiderites has a eventful history in the past. One theory of the origin of the Mesosiderites is assuming that the mother body of the Mesosiderites is a large, differentiated asteroid, that had collided with another large asteroid, possibly with 4 Vesta, what would explain not only the powerful crater on Vesta, but perhaps also the occurrence of eucritic and diogenitic inclusions in the Mesosiderites. Yet there are also some other theories so that here the last word is not yet spoken.

 

Further Groups

 

Bencubbinites

This quite new, little group with only five members is incorporated by most researchers into the class of the Carbonaceous Chondrites, although these meteorites consist of approximately the same share of nickel-iron and stone. The rock component of the Bencubbinites is of chondrite nature and shows some narrow relations to the CR-clan, a subgroup of the Carbonaceous Chondrites,. Possibly therefore the Bencubbinites are a counterpart to the Pallasites and represent samples of the coat zone of a differentiated C-asteroid, that also is the mother body of the CR-Chondrites.

 

Lodranites

Also the Lodranites are a small group of which most members were found in the perpetual ice of the Antarctica. They mainly consist of a mixture of olivine, pyroxene and nickel-iron whereby chemically some relationship was assessed to the Chondrites of the E- and H-group. Based on the missing of chondrules, they however nowadays are usually put together with the closely related group of the finecrainy and also rare acapulcoites into the group of the Primitive Achondrites - an example where one again sees how outdated the classic distinction between Iron, Stony-iron and Stony meteorites actually is.

 

Siderophyres

This is a very old group that I would like to "revive" in the context with this description. It's main representative is the peculiar meteorite Steinbach, that was already mentioned in the category Iron meteorites as an unusual member of the IVA-group. It consists of an unique mixture of red-brown pyroxene in an IVA-nickle-iron matrix and possibly is a counterpart to the Pallasites, an example for the coat of the asteroid where the IVA-iron meteorites come from.

The case of Steinbach also illustrates however - like both prior groups - the basic deficiency of the classification system of the classic meteorite science, while it raises the question, where actually the dividing-line between silicate-rich Iron meteorites and the real Stony-iron meteorites is to be drawn. As long as this question is not sufficiently answered, for us nothing else remains, than to record certain meteorite types in several classes simultaneously. Therefore be not surprised, if you find the Bencubbinites and the Lodranites in the category Stony meteorites under a different header!