Different Directions

Different Directions

Stony-Iron Meteorites

As a son of a paleontologist, I was brought up looking at how different types of stone were made and the different forces involved in the construction.

So, looking at Stony-Irons, we look at the mantles of planets and planetary parent bodies.


Excerpt from Wikipedia

Stony-Iron meteorites are mixtures of broken pieces of mantle material, mixtures of nickel-iron and silicate minerals, and these mixed meteorites have been melted and recrystallized and because of this violent mixing they are beautiful. And such beauty is sought after, especially when rare.

Stony-iron meteorites account for less than 1% of observed meteorite falls and about about 1.8% of the entire mass of all known meteorites.

When we look at their composition, we see it is roughly equal amounts by weight of silicate minerals and nickel-iron.
Modern meteoritics considers the stony-irons to consist of just two groups,

  • Mesosiderites
  • Pallasites

However, in the early days of the science of meteorites, the class of stony-irons represented a wider group of meteorites.

It comprised several chemically and genetically unrelated classes of meteorites that had just one thing in common approximately equal parts of nickel-iron metal and different types of stony components.

As a result, several groups of chondrites and achondrites fit neatly into this older definition, e.g. the bencubbinites, the lodranites, and several silicated irons were regarded as true stony-irons.

However, modern meteoritics assigns just two groups to this heterogeneous class, the pallasites and the mesosiderites.

Pallasites


Meteorit Brahin
Photo by Oliver Schwarzbach.

A cut and polished slab of the pallasite, Brahin.

Pallasites contain big, beautiful olive-green crystals, a form of magnesium-iron silicate called olivine , embedded entirely in metal. Pallasites can show big variations. Sometimes the olivine does not occur as a single crystal but as a cluster and elsewhere it can create a pattern of veins through solid metal.

Pallasites are thought to be samples of the boundary between the metal core and the silicate, olivine-rich mantle around it. As a result, pallasites can potentially tell us a lot about the formation of the Earth and other terrestrial planets because they all have a similar structure.

Mesosiderites

Mesosiderite
Photo by Mila Zinkova

The Vaca Muerta mesosiderite formed after a catastrophic collision between two asteroids.

Mesosiderites are breccias - coarse fragments cemented together by a finer material. The fragments are centimetre-sized and contain a mixture of an igneous (solidified) silicate and metal clasts (rocks made of pieces of older rocks).

Mesosiderites form when debris from a collision between two asteroids is mixed together. In the crash, molten metal mix with solid fragments of silicate rocks. Mesosiderites may, therefore, record the history of both meteorites.

Together with the stony achondrite meteorites, stony-iron meteorites may reveal how some of the asteroids melted.

As of June 2009 only 145 mesosiderites are known (of which 44 come from Antarctica) and only 7 of these are observed falls.

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