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,
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
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
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. |
