  Articles and gems - Exsolution in a meteorite
Frank Beunk - geochemist at the Vrije Universiteit in Amsterdam -
was impressed when he saw this huge meteorite.
He encountered it at the Naturhistoriska
Riksmuseet in Stockholm, Sweden and asked
Andrius Rimsa of the museum to take pictures for him.
The museum showcase gives you an indication of the scale: over one meter wide. See the
flashbulb reflections in the glass? Photograph: Andrius Rimsa,
Naturhistoriska Riksmuseet Stockholm, Dept. Isotope Geology.
These pictures show an iron meteorite - found in Greenland - that has been cut and etched. The etching has the effect of
revealing the so-called Widmanstätten patterns. These patterns develop as a result of
subsolidus exsolution
of the mineral taenite.
Close-up of the Widmanstätten pattern. Photograph: Andrius Rimsa,
Naturhistoriska Riksmuseet Stockholm, Dept. Isotope Geology.
Taenite is a solid solution. The composition
that was stable at high temperatures is not stable at low temperatures. If such a mineral cools
slowly enough, the atoms diffuse (migrate) to combine into different compositions that are
stable. The lamellae consist of
taenite
with more nickel and less iron and
kamacite with
more iron and less nickel (compared with taenite).
The cooling rate can be calculated from the diffusion rates of nickel and iron within these
crystal lattices and the width of the lamellae. Typical cooling rates vary from less than
one degree Celsius per thousand years to twenty degrees Celsius per thousand years. For an
asteroid, fairly normal.
The brownish blobs consist of troilite. Photograph: Andrius Rimsa,
Naturhistoriska Riksmuseet Stockholm, Dept. Isotope Geology.
The yellowish-brown droplets are troilite (FeS). The specific weight of such meteorites does not
match with a composition of 100% iron. It is about 10% less.
The "something" that makes these
meteorites lighter includes the sulfur in the troilite.
Close-up of the troilite droplets. Photograph: Andrius Rimsa,
Naturhistoriska Riksmuseet Stockholm, Dept. Isotope Geology.
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