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102 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
of more calcic pyroxene (1-18% CaO), which occurs
both as fragments in the matrix and as rims on
chondritic low-Ca pyroxene. The average iron
content (Fe) of the low-Ca pyroxene is 5.2%, the
average ferrosilite content (Fs) is 9.9 (range, Fs
0.5-37.5) and the percent mean deviation (%MD) is
67.3, which is a significantly higher value than has
been reported for other unequilibrated ordinary
chondrites (Dodd, Van Schmus, and Koffman, 1967).
As with the olivine analyses, iron determinations
are useful for classification purposes and comparative
studies, but minor element data may help distinguish
possible pyroxene fractions.
Binary oxide plots were prepared of A12O3, CaO,
MnO, and Cr2O3 values against ferrosilite content
of low-Ca pyroxene. When MnO concentrations
are plotted against ferrosilite content (Figure 4),
it becomes evident that pyroxene from metal-rich
chondrules is Mg-rich and low in MnO relative to
both fragments in the matrix and metal-poor
chondrule pyroxene. But unlike the olivine, both
the Mg-rich fraction and the pyroxene in metalpoor
chondrules and the matrix overlap to a large
measure and show a continuous positive correlation
of MnO with ferrosilite. Therefore, the
pyroxene data provides no compelling evidence to
suggest that metal-rich chondrules and metal-poor
chondrules represent two distinct chondrule fractions
although that possibility may be supported
by the olivine data. Also of interest in the MnO vs
Fs plot is the extensive mixing of points representing
matrix analyses (Fs = 9.8) with pyroxene
analyses from both metal-rich (Fs = 3.0) and metalpoor
(Fs = 12.6) chondrules. The pyroxene fragments
in the matrix clearly could be fragments of
pyroxene derived from chondrules similar to the
compositions studied.
Of the other minor elements determined, a
Cr2O3 vs MnO binary plot shows that a positive
correlation exists (Figure 5), both elements apparrently
increasing in concentration with Fe. No clear
trends are apparent for Al or Cr against ferrosilite
content. However, Cr2O3 against CaO and A12O3
plots show a subtle positive correlation suggesting
that calcium and aluminum also increase in concentration
to some degree with increasing iron.
LESS ABUNDANT MINERALS
The less abundant minerals identified in the St.
Mary's County meteorite include small amounts of
taenite, kamacite, troilite, and chromite, and minor
amounts of plagioclase, pentlandite, copper, spinel,
and a phosphate mineral, probably whitlockite.
The feldspathic fraction occurs both as primary
albitic glass, which is commonly found in ordinary
chondrites, and as crystalline calcic plagioclase.
Two abraded fragments were found to contain
twinned plagioclase with compositions in the range
An 80-83, which occur with low-Ca, Fe-Mg silicates.
Both fragments appear equally medium-grained;
however, one occurs with equilibrated olivine (Fa
13) and pyroxene (Fs 12) while the other contains
less equilibrated and higher iron silicates with compositions
in the range Fa 23-25 and Fs 15-18 for
olivine and pyroxene, respectively. One plagioclase
matrix fragment was found with a composition
An 26.
The most commonly occurring opaque minerals
are troilite, taenite, and kamacite. Typical of most
unequilibrated ordinary chondrites and some carbonaceous
chondrites, globular metal grains or
chondrules rimmed with troilite were observed.
The taenite in these features contains from 30%-
50% Ni and frequently displays the familiar Mshaped
Ni-concentration profile when in contact
with kamacite. Two pentlandite grains with 16%
Ni were found in troilite with taenite and kamacite
and presumably represent disequilibrium assemblages,
which are not often found in unequilibrated
ordinary chondrites (Dodd, Van Schmus, and Koffman,
1967). In addition to the large inclusions,
metal also occurs in small, occasionally sulfiderimmed
droplets in glassy chondrules. Scanningelectron
pictures of Na, Ca, and P distribution in
these 10 micron and less diameter features suggests
that phosphate nucleation, probably whitlockite,
has occurred at the metal-silicate interface, or, if
rimmed with sulfide, at the metal-sulfide interface.
Literature Cited
Albee, Arden L., and L. Ray
1970. Correction Factors for Electron Probe Microanalysis
of Silicates, Oxides, Carbonates, Phosphates, and
Sulfates. Analytical Chemistry, 42:1408-1414.
Bence, A. E., and A. L. Albee
1968. Empirical Correction Factors for the Electron
Microanalysis of Silicates and Oxides. Journal of
Geology, 76:382-403.
Cecil, Francis D.
1944. The St. Mary's Meteorite of 1919. Sky and Telescope,
3(12):9.