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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.
98 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES OLIVINE Olivine was analyzed for Si, Al, Fe, Mg, Ca, K, Na, Cr, and Mn. The primary mineral standard used for major element determinations was Springwater olivine with 13.26% Fe. The minor elements calcium and manganese were determined using An 80 and Rockport fayalite with 11.64% Ca and 1.30% Mn, respectively. Springwater olivine was analyzed routinely every twenty samples to provide an internal check on instrument stability. Iron concentrations of those analyses with oxide summations between 98%—101% were applied to a cal- 0.7CU 0.60 0.50 0.00 10 15 20 25 30 MOLE PCT. FRYRLITE culated calibration curve to establish the fayalite content of each well-analyzed grain. A total of 269 olivine analyses were made, which includes analyses of crystals from 137 separate chondrules and 82 matrix crystals. For all olivine analyses, the average iron content (Fe) is 10.4%, the average fayalite content (Fa) is 14.4 (range, Fa, 0.2-76.6) and the percent mean deviation (%MD) is 51.9. (The %MD is the parameter used by Dodd, Van Schmus, and Koffman, 1967, as an index of olivine and pyroxene heterogeneity.) These results compare well with the interpretation of an x-ray diffractometer trace of the meteorite in which the CI XTLS IN METRL-RICH CHOND A XTLS IN METRL-POOR CHOND + MRTRIX CRYSTRLS - ZONED CRYSTRLS 35 40 45 50 FIGURE 2.—Plot of CaO concentration vs fayalite content of olivine (lines join compositions in zoned crystals). Note particularly the high to low CaO values of the Mg-rich olivine in metalrich chondrules, possibly representing a separate chondrule fraction, and the relatively Fe-rich fragments from the matrix. Only one olivine fragment from the matrix with the composition Fa 76.6 and containing 0.14% CaO is not represented in the plot.
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SMITHSONIAN CONTRIBUTIONS TO THE EA
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Contents MINERALOGY COMPOSITION OF
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Constituent SiO TiO Al 0 fl 2 3 Cr
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PER CENT CBLCIUM FIGURE 3.—The ca
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Constituent TiO,, 2 2 3 FeO* MnO Mg
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20 Cfl 40 .60 SMITHSONIAN CONTRIBUT
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35'50' I L 6.. 3.. 2.. r FOKT DEFIA
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12 SMITHSONIAN CONTRIBUTIONS TO THE
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14 20 SMITHSONIAN CONTRIBUTIONS TO
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16 Constituent Si0 2 TiO2 A12O3 Cr
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18 20 40 £0 SMITHSONIAN CONTRIBUTI
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20 40 20 SMITHSONIAN CONTRIBUTIONS
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Abyssal Basaltic Glasses as Indicat
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24 Electron-M icroprobe Analysis An
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30 Hill in the NE Atlantic, and fro
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32 cores the arrangement is stratig
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36 ATLANTIC OCEAN LAT. LONG. DEPTH
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