BRACHINITE
Brachinites are rare, ultramafic, olivine-rich,
achondrite meteorites - about 38 have been reported. They represent
pieces of asteroidal dunite. Very few recovered meteorites have
been classified as brachinites. The rock shown below is a sample from the
NWA 3151 Meteorite, the largest-ever identified brachinite. It was found
in northwestern Africa in 2005.
This rock has an ~equigranular crystalline texture and
is overwhelmingly dominated by olivine crystals (about 95% of the rock volume).
Published chemical analysis has shown that the olivine is forsteritic (as is
all brachinite olivine). Minor reported components include clinopyroxene,
altered Fe-Ni metal, troilite (FeS), chromite, Na-plagioclase feldspar, and
orthopyroxene.
Brachinites have been inferred to be residual mantle
rocks or possibly recrystallized igneous cumulate rocks. They appear to
have come from a differentiated planetary body in the asteroid belt that
has since broken apart. Research has indicated that the Nenetta
Asteroid may be the parent body of brachinites. This brachinite shown
below does not yet have a published age, but other brachinites date to 4.56
billion years.
Brachinite - front & back of partial slice (1.3 cm across) of the NWA 3151
Meteorite.
Brachinite, tilted in the light to better show crystalline texture & olivine
crystal shapes.
(More info. on the
NWA 3151 Meteorite)