SHERGOTTITES
About 164 rocks are known that demonstrably come from
the Planet Mars.
Meteorite researchers and collectors generally refer to the Martian rocks as the
SNC meteorites - the shergottites, the nakhlites, and the chassignites.
Most of these Martian rocks are shergottites.
Shergottites are a group of Martian rocks named after the Shergotty Meteorite, the
type example. Below is a shergottite that was found and identified in
2004.
Shergottite (NWA 2373) (6 mm across) - this is a small sample from the NWA 2373
Meteorite (NWA = "Northwest Africa"). The light brown-colored
material is the outer weathered surface of the rock. The greenish and
black speckled surface shows the crystal & mineral make-up of the rock
itself. Mineral analysis performed
by Theodore Bunch and James Wittke at Northern Arizona University has shown the
rock is principally composed of olivine, pigeonite pyroxene, augite pyroxene,
plagioclase feldspar glass (maskelynite), chromite, Ti-magnetite, chlorapatite,
and trace amounts of other minerals. It looks like an ultramafic
rock, but it's apparently a basaltic shergottite (also regarded as a picritic
shergottite).
Based on chemical and physical similarities, the NWA
2373 has been paired with the NWA 1068 Meteorite. Available isotopic
dates on the NWA 1068 Meteorite show it formed 185 million years ago (late
Amazonian, equivalent to Earth's Early Jurassic), and was ejected from the
Martian surface about 2.2 million years ago (information based on cosmogenic
isotope analysis).