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Re: Iron Meteorite Classification. Part II

	
	Today I will discuss the chemical classification of iron 
meteorites. It is based on a simple idea, but one that is hard to follow. 
After this writing, I hope that everyone will be able to look at the 
number designation of an iron group and be able to realize what it means.
	There are twelve groups of iron meteorites. Each group has 
members that are related to each other, but each group is NOT related to 
another group. This will become more apparent later.
	These twelve groups are so divided on the basis of bandwidths,the 
concentrations of the elements germanium(Ge),gallium(Ga) and iridium(Ir), 
and the amount of Ni. These features help best to distinguish between the 
major groups, because they can be measured with some confidence, and read 
sufficently different between the groups.
	The reason for the choice of elements to anaylze for, is due to 
the fact that when, lets say Ge-Ni compositions are plotted on a simple 
graph, a few groups overlap each other. But when you plot all three 
elements(Ga, Ge, and Ir) each group has it's own area on the graph. 
	I could keep writing, but the above is the classification scheme 
in a nut shell. I will put up a table that shows the major properties of 
the individual groups.

Group   Number in Group   Band Width(mm)     Ni%      Ga(ppm)     Ge(ppm)
IA            82             1.0-3.1       6.4-8.7    55-100      190-520
IB             8             .01-1.0       8.7-25     11-55       25-190
IC            10             <3            6.1-6.8    49-55       212-247
IIA           39             >50           5.3-5.7    57-62       170-185
IIB           13             5-15          5.7-6.4    46-59       107-183
IIC            7            .06-.07        9.3-11.5   37-39       88-114
IID           13             .4-.8         9.6-11.3   70-83       82-98
IIE           12             .7-2          7.5-9.7    21-28       62-75
IIIA         120             .9-1.3        7.1-9.3    17-23       32-47
IIIB          36             .6-1.3        8.4-10.5   16-21       27-46
IIIC           7             .2-.4         10-13      11-27       8-70
IIID           5             .01-.05       16-23      1.5-5.2     1.4-4
IIIE           8              1.3-1.6      8.2-9.0    17-19       34-37
IIIF           5              .5-1.5       6.8-7.8    6.3-7.2     .7-1.1
IVA           40              .25-.45      7.4-9.4    1.6-2.4     .09-.14
IVB           11              .006-.03     16-26      .17-.27     .03-.07
Table based on Scott(1975) table 2.
	Ir was not considered above, because it is such a small amount, 
measuring with any accuracy is difficult. Still today, Ir is difficult to 
measure, so I did not want to give information that may or may not be 
right. 
	Now you can see what the term fine and coarse octahedrite means. 
Coarse and fine relate to band width. Octahedrite refers to the crystal 
structure of the Ni and Fe. It is basicly a cube.
	The next time I write I will take a brief look at the different 
groups, and how they are related to asteroids. 

Frank Stroik
University Of Wyoming

Reference: Scott E.R.D. and Wasson J.t(1975) Classification and Properites
                            of Iron Meteorites. Reviews of Geophysics and
                            SPace Physics 13,4 pp527-546
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