NUMBER 19 101 these olivines is normal, with Mg-rich cores and relatively Fe-rich rims. One single crystal metal-poor chondrule, however, was observed with a core of Fa 6.2 and a rim of Fa 0.2. PYROXENE Pyroxene was analyzed for Si, Al, Fe, Mg, Ca, K, Mn, Ti, and Cr. The primary mineral standards used for pyroxene analyses were Johnstown hypersthene (Si, 25.06%; Fe, 12.14%; Mg, 16.40%) and an omphacite standard (Al, 4.71%; Ca, 9.83%). The more abundant minor elements, manganese and chromium, were determined using Rockport fayal- 1 -OCU 0.20 0.00 0.20 0.40 NT. PCT. MNO ite (Mn, 1.30%) and a chromite standard (Cr, 40.00%). Johnstown hypersthene was analyzed every twenty samples to provide a check on instrumental drift, which was negligible. Iron concentrations of the low-Ca (
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.
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