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have studied the iron sulfide from iron meteorites and assumed
that the concentrations of elements in the iron sulfide enclosed
in the silicates were the same. There is no reason to believe
that this assumption is Justified.
In fact the amounts of the
elements in these trollite nodules in the iron meteorites can
probably be neglected in any avei^ge because they constitute
such a very small fraction of the iron meteorites and hence
of all meteoritic matter.
Another difficulty in computing the "average" composition
of the meteorites arises from our ignorance of the relative
amounts of the three meteoritic main phases. The meteorites
reaching the earth's surface cannot serve as a basis for an
estimate of these relative amounts because iron meteorites are
better preserved during their fall and on the surface of the
earth than are stony meteorites and pallasites.
The assumptions
made by different invesigators are summarized in Table 1.
TABLE I
Assumptions made on the average composition of meteoritic matter
by various authors
Parts by Weight
Author Metal Sulfide Silicate
Noddack and Noddack (1930) 68 9»8 100
Noddack and Noddack (1934) 14.6 6.7 100
Persman (1934) 20 • 4 100
Qoldschmidt (1937) 20 10 100
H. Brown (I949) 67 0 100
Urey (1952 a) 10.6 7 100