Meteorites Inc.


Stony-irons are mixtures of silicates and free metal (iron-nickel) in approximately equal proportions. They are rare among falls but somewhat more common among finds because of the ease with which they can be distinguished. The stony-irons are divided into three main groups: pallasites, mesosiderites, and lodranites. Other groups exist, but they comprise unusual and rare specimens and will not be described further here.

The pallasites consist of olivine crystals that may be as large as 1 cm across embedded in a Fe-Ni alloy matrix. Esthetically, pallasites are easily the most beautiful meteorites, especially when cut and polished. When etched, the metal phase may exhibit a Widmanstatten pattern (see "Irons" below). Pallasites are thought to have formed at the core-mantle boundary within a differentiated asteroid; the iron would be derived from the asteroid's core, while the olivine crystals would come from the base of its mantle.

The silicate phase in mesosiderites consists mostly of hypersthene pyroxene and plagioclase feldspar (as opposed to olivine as in the pallasites), and often includes abundant troilite (FeS). Unlike in the pallasites also, the metal phase in mesosiderites does not form a continuous matrix of free metal. Instead, the mixing between the silicate and metal phases is more intimate.

Lodranites consist of approximately equal amounts of metal, olivine, and pyroxene. Lodranites are sometimes classified as primitive achondrites - "primitive" because they seem to have a chondritic major element bulk chemistry and "achondrites" because they may exhibit achondritic textures. Because all known lodranites are similar to one another in terms of major element chemistry and mineralogy, they are believed to have formed on the same parent body. The differences that do exist among lodranites are attributed to varying degrees of melting.
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