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NUMBER 19 97
from the Maryland Academy of Sciences through
the courtesy of Paul S. Watson. The specimen
weighed 23.4 grams when it was received at the
Smithsonian Institution. It was photographed, a
model was made, and a 2.9 gram piece was removed
for analysis. A polished section and two polished
thin sections were prepared for optical and
elecron-microprobe examination (NMNH 5423),
and the remaining material was used for a bulk
chemical analysis. A 19.7 gram piece was returned
to the Maryland Academy of Sciences.
Texture
The St. Mary's County meteorite is a black,
matrix-rich chondrite of low petrographic type
(Figure 1). In thin section (0.03 mm), the matrix
is opaque and contains small angular silicate
crystals and finely dispersed metal and sulfide
particles. The predominant inclusions are broken
and intact olivine and/or low-Ca pyroxene chondrules,
which range up to 0.6 mm in diameter.
These frequently contain primary albitic glass and
metal droplets. In addition, several metal-sulfide
chondrules were observed, but these phases usually
occur in chondrule rims and as small inclusions in
chondrules and in the matrix.
FIGURE 1.—Overall chondritic texture of the St. Mary's
County (LL3) meteorite; length of specimen, 19 mm.
Chemistry
The bulk chemical analysis of the St. Mary's
County meteorite is presented in Table 1. It was
performed on a 1.5 gram sample that appeared to
be reasonably representative of the 23.4 gram specimen.
The procedures of Jarosewich (1966) were
used with the modification that the portion taken
for major constituent analysis was completely dissolved,
rather than analyzing an acid-soluble and
acid-insoluble fraction. The carbon content is high
compared to other unequilibrated ordinary chondrites;
only Sharps is reported to have a higher
carbon concentration of 0.95% (Fredriksson,
Jarosewich, and Nelen, 1969). The SiO2/MgO ratio
of 1.64 is typical of ordinary chondrites and the
Fe/SiO2 and Fe°/Fe ratios of 0.50 and 0.14 are
within the range recognized for LL-group chondrites
(Van Schmus and Wood, 1967).
TABLE 1.—Chemical analyses of the St. Mary's County
chondrite
Constituent A
Fe
Ni
Co
FeS
2.75
1.14
0.04
6.53
SiO2 40.45
TiO2 0.14
A12O3
FeO
2.08
17.28
MnO
MgO
0.33
24.71
CaO 1.86
Na2O 0.97
K2O
P2O5
H2O(+)
H2O(-)
Cr2O3
C
0.10
0.16
N.D.
0.27
0.55
Total
v
0.57
99.93
Total Fe 20.33
Constituent B
H
C
0
Na
Mg
Al
Si
P
S
K
Ca
Ti
Cr
Mn
Fe
Co
Ni
Total
0.03
0.57
37.58
0.72
14.90
1.10
18.89
0.07
2.38
0.08
1.33
0 = 08
0.38
0.26
20.33
0.04
1.14
99.88
A = Bulk chemical analysis by E. Jarosewich
B = Elemental composition
C=Volatile-free (less 0, C, H and S)
Fe°/Fe=metallic Fe/total Fe.
Mineralogy
Constituent C
Na
Mg
Al
Si
P
K
Ca
Ti
Cr
Mn
Fe
Co
Ni
Total
1.21
25.12
1.85
31.84
0.12
0.13
2.24
0.13
0.64
0.44
34.27
0.07
1.92
99.98
CHEMICAL PARAMETERS
SiO2/MgO =1.64
Fe/SiO2 = 0.50
Fe°/Fe = 0.14
The analyses of the St. Mary's County silicates
were made with the ARL microprobe model SEMQ,
utilizing an accelerating potential of 15 KV and a
beam current of 0.15 \iA. Results have been corrected
using procedures outlined by Bence and Albee
(1968) and the correction factors of Albee and
Ray (1970). Two spots separated by a 1-2 micron
spacing were counted for 10 seconds and averaged
for each analyzed area. An advantage of our microprobe
is that nine elements can be determined
simultaneously, thus providing minor element data
from spots that are analyzed for major elements.