PDF (Hi-Res) - Smithsonian Institution Libraries

NUMBER 19
TABLE 5.—Average CaO, Na2O, and K2O content of plagioclase in different inclusions
as measured by electron microprobe
Inclusion: Section'
2:
2:
2:
2:
1:
3:
4:
5:
6:
8:
11:
12.1*:
12.1*:
Average
3.1
3.2
4
7
9.1
9.2
2
1
10.1
10.2
5.1
5.2
6.1
6.2
8.1
8.2
11
12.1
12.2
n
14
14
56
13
12
19
13
17
15
17
9
18
20
22
15
38
13
11
11
CaO
A
2.93 -
2.95 :
3.05 :
3.00 :
H 0.06
I- 0.08
\- 0.15
f 0.08
2.91 " f 0.06
2.92 \ f 0.07
2.97 -
3.18 :
2.94 :
2.96 -
3.00 '-
3.00 :
3.50 :
3.36 :
3.12 -
3.29 :
2.91 :
¥ 0.09
t- 0.10
t- 0.09
t- 0.08
i- 0.06
h 0.08
h 0.31
i- 0.26
H 0.11
t- 0.45
h 0.13
2.99 Hh
0.09
3.16 Hh
0.16
Na2O
10.0
10.4 •
11.1
10.0
10.1
10.3
10.1
9.8
10.3
10.0
10.2
9.8
8.5
9.9
10.0
10.0
10.0
9.9
9.8
K2O
0.57
0.57
0.56
0.58
0.55
0.54
A
+ 0.06
+ 0.05
+ 0.06
+ 0.04
+ 0.07
+ 0.06
0.57 + 0.07
0.53 + 0.05
0.59 + 0.06
0.58 + 0.06
0.56 + 0.07
0.57 + 0.06
0.51 + 0.06
0.51 + 0.05
0.58 + 0.05
0.57 + 0.07
0.57 + 0.06
0.57 + 0.08
0.60 + 0.09
An
14.4
15.0
14.7
14.3
14.5
15.6
14.5
14.7
16.8
15.9
14.3
14.7
15.5
15.0
Ab
82.7
82.2
82.4
83.0
82.6
81.7
82.6
82.5
80.6
81.2
82.8
82.5
81.5
+The suffix .1 or .2 following a thin section number means that two different areas in the
same section were probed.
n = number of grains analyzed.
A = values giving the standard deviation calculated from the distribution of the n measurements',
not from counting statistics.
An, Ab, Or = mole percent anorthite, albite, and orthoclase, respectively. For the calculation
of these values only the CaO and K£0 measurements were used, because evaporation of Na
under the electron beam makes the Na figures less reliable.
•Inclusions 12.1 and 12.2 are two different pieces on section 12.
7. Because the carbon, troilite, and metal content
of these inclusions varies, the analyses were recalculated
on a carbon, troilite, and metal-free basis in
Table 8 and compared with bulk analyses of other
silicates from iron meteorites and with chondrites.
The chondrite values for Tables 7 and 8 were calculated
from an average of 19 H- and 27 L-group
chondrite analyses by one of us (Jarosewich). Only
meteorites showing no signs of alteration by
weathering were chosen for the calculation of the
averages.
Discussion
COMPARISON TO CHONDRITES
Mason (1967) first noted that the minerals of the
silicate inclusions in iron meteorites are essentially
those of chondrites: olivine, orthopyroxene, diopside,
and an albitic plagioclase. This distinguishes
them clearly from achondrites and mesosiderite
silicates, which contain anorthitic plagioclase. The
main difference from normal chondrites is the FeO
82.2
Or
2.9
2.8
2.9
2.7
2.9
2.7
2.9
2.8
2.6
2.9
2.9
2.9
3.0
2.85
115
content of olivine and pyroxene, which is intermediate
between enstatite and bronzite chondrites
(Table 2).
The bulk chemical composition of whole inclusions
does not match that of chondrites. As the
amounts of metal, troilite, and graphite are highly
variable (Table 1), only the bulk compositions
of the silicates are compared here. Some of the
metal and troilite and most of the graphite are
probably secondary in origin, i.e., they were introduced
later into the silicate fragments.
Table 8 gives the bulk chemical composition of
the chemically analyzed inclusions 1, 2, and 3,
recalculated for the silicate portion only. Included
in Table 8 are calculated bulk chemical values for
inclusion 1 and for the additional inclusions 4 and
5. These values were calculated from the modal
composition of these inclusions (Table 1) and the
chemical composition of the minerals as determined
by the electron microprobe. The agreement between
the chemical analysis and that calculated from the
modal composition of inclusion 1 shows that these
two samples from the same inclusion are similar.