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NUMBER 19 107
crystals. About nine different primary taenite crystals
can be recognized in slice A (Figure 1). There
is no disturbance of the Widmanstatten structure,
i.e., flow or shock features, visible either around
the silicates or in other parts of the metal areas,
as has been reported for smaller Campo del Cielo
specimens by Bunch and Cassidy (1968). This indicates
clearly that these inclusions were incorporated
into the metal before the taenite-kamacite transformation
took place, or at temperatures above
about 750° C.
The outline of most of the silicate inclusions is
angular with usually straight margins. This is suggestive
of a fragmental nature of these inclusions.
It can be noted, however, that several of the
straight boundaries are parallel to kamacite laths
in the Widmanstatten pattern and may be second-
cutting lines
ary features. This point is difficult to prove, and
most observers fail to see it. It seems, however,
that some of the silicate-metal boundaries and
indentations in the silicate inclusions are controlled
by the Widmanstatten pattern or the growth directions
of the kamacite laths. Examples are shown
in Figure 2. The microscopic texture of the silicates
shows replacement of silicates by opaques
(mostly by graphite, which also seems to replace
troilite and metal) in many places, which could
explain such a reshaping of the silicate inclusions.
In contrast to other iron meteorites with silicate
inclusions, for instance, Woodbine (Mason, 1967),
no swathing kamacite is found around the inclusions
of El Taco. Macroscopically the inclusions
differ in the relative amounts of graphite and
opaques and in the grain size of the silicates.
FIGURE 3.—Line drawing of part of El Taco slice B. Silicate inclusions are hatched. Open bars
in inclusions 2 and 3 give locations of samples taken for chemical analysis, lines 1, 3, 4, 7,
and 9 give locations of the corresponding polished thin sections. Inclusion 6 was a veinlike
inclusion which was exposed along the indicated cutting line and from which section 6 was
made. Width of cut, 6 cm.