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Re: Meteorite Classification

At 12:17 PM 4/7/98 -0600, you wrote:
>Hello Jeff,
>
>Thank you very much for the classification list.
>
>I do have a couple of questions.  First, regarding the following entry:
>
>>Achondrites
>>             Primitive (from undifferentiated bodies)
>>                   Angrites
>>                   Acapulcoites
>>                   Brachinites
>>                   Lodranites
>>                   Ungrouped (e.g., the E-chondrite melt rocks,
>>                              Ilafegh 009 and Happy Canyon)
>
>What's going on with the "Ungrouped (e.g., the E-chondrite melt rocks,...)
>where a specimen such as Happy Canyon can be both listed as a chondrite and
>as a primitive achondrite? Is the classification scheme be breaking down
>here?
>
>Also, I know we (some members of this list) have discussed the concept of a
>primitive meteorite before, but that chat was centered mostly on the nature
>of carbonaceous chondrites.  How does the term "primitive" apply to
>achondrites, or in other words, what makes an achondrite a primitive one?
>
>Sincerely,
>
>Martin
>
>
>

First off, achondrites are simply those meteorites that completely
lack chondritic structure (excluding the metal-rich types that
are classified separately).  

"Primitive" achondrites can be thought of as deriving from asteroids 
that have not completely differentiated.  Thus, the whole body is still 
in a more-or-less primitive (i.e., chondritic) state, both physically 
and chemically.  The achondrites from these bodies can be melts 
produced locally, partial melts, or the residues of partial melting.  
Impact melting may play a role in the genesis of some primitive 
achondrites.  Others may come from asteroids that didn't quite get
hot enough to experience extensive melting, but did partially melt
to a low degree.

All of the other achondrites come from fully differentiated bodies.  
In the case of asteroidal meteorites, like the HED group or aubrites, 
the parent bodies presumably have (or had before they were destroyed) 
metal-rich cores, silicate-rich mantles, and basaltic crusts.  In the 
case of meteorites from Mars or the Moon... well you know about them.  

Some meteorites have been classified as impact-melt rocks, like
Ilafegh 009 and Happy Canyon. Many of these objects have absolutely
no chondritic structure, and formed from total melts.  These are
technically "primitive achondrites," although we know pretty well
that they came from a certain kind of chondritic asteroid, and formed
in a certain way.

Having said all of this, I probably made an error in this morning's
posting.  Angrites probably belong as their own class of
achondrites, and are probably not primitive achondrites.  Some
people might also like to add "winonaites" as an additional class of 
primitive achondrites.

jeff