[meteorite-list] Let's talk about meteorites

[Alan Rubin]

There are three principal goups of silicated iron meteorites: the IAB-IIICD, 
IIE and IVA groups.  Recent work seems to indicate that the IAB and IIICD 
groups are related.  Iron meteorites that are thought to have formed by 
fractional crystallization processes have certain slopes on element-element 
diagrams (e.g., Ir-Ni) that match those expected by fractional 
crystallization, presumably located in cores.  The IVA irons exhibit such 
trends and so are presumably from the core of a differentiated asteroid.  A 
few IVA irons contain small grains of silica.  The IAB-IIICD irons do not 
exhibit trends on element-element diagrams consistent with fractional 
crystallization.  Some researchers (e.g., John Wasson) believe that they are 
not from cores but are rather from impact pools on chondritic asteroids. 
Their silicates are basically chondritic in bulk composition and even 
contain the so-called "planetary gases" as are found in chondrites but not 
differentiated meteorites. The IIE irons have alkali-rich silicates.  These 
silicates are not chondritic in composition but do have planetary gases. 
The metal portion of these iron meteorites also do not conform to the slopes 
expected for fractional crystallization.  They also may have formed as 
impact pools on chondritic asteroids.  The alkali-rich silicates, I believe, 
formed because feldspar has a low impedance to shock compression and hence 
melts first during shock events.  The IIE silicates are similar in 
composition to impact-melt pockets in ordinary chondrites.
    Mesosiderites are differentiated meteorites.  Their metal may have been 
derived from a core.  Their silicates are basically eucrite and diogenite 
material.  I modeled mesosiderites as having formed via the collision of a 
core (with some overlying mantle) to the basaltic surface of another 
asteroid.
Alan Rubin