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NUMBER 19 99
olivine peak (130) indicates a broad range in composition
from Fa 0 to Fa 80 with a small peak at
Fa 12 (B. Mason, pers. comm.). The shape of the
peak shows that most of the olivine is Mg-rich, in
the range Fa 0-15, with diminishing amounts of
more Fe-rich material. Compared with olivine compositions
from other unequilibrated chondrites
(Dodd, Van Schmus, and Koffman, 1967), St. Mary's
County is clearly the least equilibrated of the
ordinary chondrites. Although the major element
analyses and x-ray diffraction data are useful for
classification purposes, they do not provide clues
as to the number or origin of olivine fractions
Q_
0.5C1
0.40..
0.30.-
0.20..
0.10..
0.00
10 15 20 25 30
MOLE PCT. FRYRLITE
that may exist in a particularly unequilibrated
chondrite like St. Mary's County, information
which may become apparent with minor element
data.
When CaO and MnO concentrations in olivine
from relatively metal-rich and metal-poor chondrules
and from silicate fragments in the matrix
are plotted against fayalite content (Figures 2 and
3), at least two important points become apparent.
Firstly, silicate inclusions in the matrix are not
simply fragments of crushed chondrules. The
olivine has a mean fayalite content of 21.2, which,
if the silicates are of primary composition, cannot
• XTLS IN METRL-RICH CHOND
A XTLS IN METRL-POOR CHOND
-I- MRTRIX CRYSTRLS
- ZONED CRYSTRLS
35 40 45 50
FIGURE 3.—Plot of MnO concentration vs fayalite content of olivine. Only one olivine fragment
from the matrix with the composition Fa 76.6 containing 0.46% MnO is not represented in
the plot.