Achondrite, Martian Shergottite
18.1 kg Recovered Mass
Zagami is a fine-grained shergottite with a black shiny fusion crust. Like all Martian meteorites it is highly shocked. In the case of Zagami there is fine fracturing of the crystal grains and in the augite and feldspar there is mosaic extinction. In the following photograph the gray and white grain in the center shows what looks like undulatory extinction those it May be mosaic extinction.
The black areas in these crossed polar photos are maskelynite with is glassy and
therefore isotropic showing no birefringence. It would be clear in unpolarized
light. This is the reason for the flat gray color of a Zagami slide when seen
outside the microscope. The meteorite is a delight to the eyes however when in
the microscope. The long prismatic crystals of augite are arranged in mostly one
direction and exhibit strong polarization colors. Most shergottite are cumulates
which formed in magma chambers. They are close cousins to basalt found on earth.
Like all Martian meteorites their place of origin has been determined by
comparison of the gases trapped within them. These gases match the Martian
isotopic composition that our Viking probes measured in the 1970ís