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Mineralogical and Chemical Compositions of
Silicate Inclusions in the El Taco,
Campo del Cielo, Iron Meteorite
ABSTRACT
The silicate minerals in these inclusions (olivine,
orthopyroxene, diopside and a sodic plagioclase)
have the same composition as silicates in chondrites
except for a lower FeO-content of the mafic
minerals and a lower K-content of the feldspar.
Small grains of K-feldspar occur with the plagioclase.
Olivine contains 3.5% to 4.5% FeO, orthopyroxene
3.8% to 4.6% FeO. The range of compositions
within one inclusion is smaller, however, and
distinct differences in mean FeO-content occur
between different inclusions; the fayalite to ferrosilite
ratio varies from inclusion to inclusion.
The outer zone of olivine and pyroxene grains is
depleted in iron oxide compared to the central part.
Thus three types of disequilibria occur: (1) disequilibrium
between different inclusions, (2)
disequilibrium between olivine and pyroxene in a
given inclusion, and (3) disequilibrium within
single olivine and pyroxene grains. Both the mineral
composition and the bulk chemistry of the
silicate part of the inclusions are close to normal
chondrites, if compared on an iron-free basis.
Variations that do not seem to be caused by
nonrepresentative sampling are found between different
inclusions. The variations affect mainly the
elements Ca, Al, and the alkalies. They can be
explained by the removal of diopside and/or feldspar
from a chondritic material, perhaps through
partial melting. Other features suggest that the
lower iron content of the mafic minerals was produced
by reduction of chondritic silicates.
Frank Wlotzka, Max-Planck-Institut fur Chemie, Mainz,
Germany. Eugene Jarosewich, Department of Mineral
Sciences, National Museum of Natural History, Smithsonian
Institution, Washington, D.C. 20560.
Frank Wlotzka and Eugene Jarosewich
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Introduction
The El Taco meteorite is one of the Campo del
Cielo, Argentina, irons. It was recovered by an
expedition described by Cassidy, et al. (1965). The
El Taco specimen, which weighed 2000 kg, was cut
at the Max-Planck-lnstitut fur Chemie at Mainz,
Germany (Hintenberger, Schultz, and Weber, 1969).
Three cuts were made parallel to the longest axis
of the meteorite, so that two slices of about 2 cm
thickness and two end pieces resulted. One of the
slices (slice A, Figure 1) is on exhibit in the
Meteorite Hall of the National Museum of Natural
History, and the other (slice B) was made
available for research by the Max-Planck-Institut
fiir Chemie, Mainz.
Slice A revealed a large number of silicate inclusions
(Figure 1). One inclusion of this kind had
already been found in the El Toba specimen of
Campo del Cielo and described by Park, Bunch,
and Massalski (1966). Silicate inclusions are known
from other iron meteorites, for instance, Linwood
(Henderson and Perry, 1949), Woodbine (Mason,
1967), Netschaevo (Olsen and Jarosewich, 1971),
and Odessa (Marshall and Keil, 1965). A thorough
description of the mineral compositions of the
silicate inclusions from most of the iron meteorites
known to contain them was given by Bunch, Keil,
and Olsen (1970). All these iron meteorites (except
Netchaevo) belong to the same chemical group IAB
ofWasson(1970).
In continuation of earlier work (Wlotzka and
Jarosewich, 1969), this paper describes the mineralogy
and the bulk chemical composition of a