PDF (Hi-Res) - Smithsonian Institution Libraries

108
Sampling
Inclusion 1 was taken from the surface of El
Taco. A piece of 3.5 grams was used for chemical
analysis; from another piece, polished thin section
(PTS) 2 was made. Inclusions 2 and 3 were obtained
from the research slice B, and their locations
and the sampling are shown in Figure 3. PTS
6 was made from a small inclusion (No. 6, Figure
3). Additional thin sections were made from inclusions
that were exposed during the cutting
of slices A and B. These are: location A 1-5/6:
inclusion 4, PTS 10; location A-15/18: inclusion 5,
PTS 5; location A 1-4/7: inclusion 8, PTS 8; location
B III-II/7: inclusion 12, PTS 12; and location
not known: inclusion 11, PTS 11.
Microscopic Texture
Between the metal host and the silicate inclusion,
a border zone consisting of schreibersite, troilite,
and graphite is found. Figure 4 shows an example:
schreibersite usually forms a thin band against the
metal, followed and partly replaced by troilite and
graphite. In some places a very fine-grained eutectoid
intergrowth of metal and troilite and/or
sphalerite is found. The same type of border zone
was described by Park, Bunch, and Massalski (1966)
for the El Toba silicate inclusion.
The silicate portions of the inclusions differ
considerably in grain size and distribution of
opaques and each inclusion has its own texture.
Two main types of textures can be distinguished:
(1) large silicate crystals set in a matrix of graphite
or sometimes troilite and metal (Figure 5), and
(2) allotriomorphic intergrowths of smaller, equigranular
grains (Figure 6).
The textural relationships of the opaque minerals
graphite, troilite, and metal with one another
and with the silicates are more complex and will
be described in a separate paper (Wlotzka, in prep.).
The conclusions of this study are that graphite is
a late addition in these inclusions and replaces
metal, troilite, and silicates in the texture of type
1 mentioned above. In type 2, graphite (if present)
occurs as small flakes dispersed throughout the silicates
and does not show tendencies to form veins
like in type 1 (Figure 7).
In contrast to mesosiderites and many chondrites
and achondrites no brecciation structure is found.
The silicates have an igneous texture, and the equi-
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES
FIGURE 4.—Border zone between metal host and silicate inclusion
8, reflected light. (M = metal, S = schreibersite, T =
troilite, G = graphite, with some rounded silicate grains in
the lower part, width of image = 1.5 mm.)
granular and allotriomorphic intergrowth suggests
a metamorphic recrystallization. Graphite and sometimes
metal veins fill cracks in the silicate fragment
in some places. Only in one thin section a straight
boundary between two different kinds of structures
or "fragments" was found; all other sections studied
appear to be homogeneous throughout, except for
the development of a coarser texture towards the
metal host, with euhedral silicate crystals in a
graphite or troilite matrix.
Table 1 gives the main minerals found in three
different inclusions and their relative amounts as
determined by point counting. Noteworthy are the
large variations in the relative amounts of the
opaque minerals graphite, troilite, and metal.
Where graphite is high, troilite is low, and vice
versa. Thus the sum of the opaques and conse-