[meteorite-list] mars iron classification

[Beda Hofmann]

I agree with Jeff Pringle that the MER microscopic images show what 
is basically a Widmannstaetten pattern. There are obviously taenite 
lamellae in what must be kamacite. So "Heat shield rock" HSR clearly 
is an octahedrite. Based on the images the bandwidth of kamacite 
lamellae is approximately 1.5 mm wich yields to the structural 
classification Og (coarse octahedrite). Several chemical groups are 
possible, but IAB seems the most likely based on abundance 
statistics. Possible inclusions of silicates and large (burned out) 
troilites, typical for IAB, would also explain some of the 
irregularities seen in the surface of HSR.

I wonder what the reddish dark coating is on HSR (visible on color 
images constructed from Sol 352 Pancam images). I first thought it 
might be fusion crust, bue the microscopic images show a rather 
irregular transition from the metal to that crust, and the reddish 
color suggest weathering. It might be a weathering crust from an 
earlier, more oxidizing climatic period, later on being partially 
removed by sandblasting.

Beda