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Antarctica - Part 5b

Smithsonian Contributions to the Earth Sciences, Number 23

Catalog of Antarctic Meteorites, 1977-1978

Initial Processing and Description (cont.):

Pebble-sized meteorites, which generally are those specimens weighing
less than 100 grams and which were returned several to a collection bag,
were initially processed in a different manner. These meteorites were
allowed to thaw overnight in the processing cabinet, and all processing
operations were done with the specimens at room temperature. Three
specimens at a time from the same collection bag were processed
together. Only three orthogonal photographs were made of all three
specimens together. No chips for cold storage were taken from pebbles.
Most of the pebbles were not sawed, but were subdivided by chipping.
The four iron meteorites in the collection were handled in yet another
manner. The specimens were allowed to thaw and dry for 48 hours in a
nitrogen cabinet. They were then placed on a laminar flow bench,
weighed, photographed, and sealed in a teflon bag. The irons were sent
to the Smithsonian Institution where they were sawed and subdivided,
using the standard procedures of that institution.
All photography of meteorites was made with Kodak VPS color film in a
large format camera, which produced 4 x 5 inch negatives. The six
orthogonal views taken of the larger meteorites record many details of
the surface features (ablation marks, fusion crust, coloration, etc.) as
they appeared before the specimens were divided and distributed.
Photographs also record features on sawed faces of the meteorites.
Generally, the sizes and shapes of chondrules and clasts, the extent of
internal fracturing, and the degree of oxidation of the interior metal
can be readily discerned in these color photographs. The lettered cube
shown in the photographs (T, N, E, etc.) indicates relative orientation
of the specimens as assigned during processing and does not necessarily
indicate the meteorite's orientation on the Antarctic ice cap.
Sawing of the stony meteorites in the Curatorial Facility was done with
a cleaned band saw, which was contained in a nitrogen cabinet and which
was furnished with a diamond-encrusted, stainless blade either 0.010 or
0.020 inch thick. Sawing was done dry; no lubricants were used. Between
specimens, the blade and sample stage was cleaned and the entire saw
cabinet was flushed with freon to remove meteorite dust. A portion of
the meteorite dust produced during sawing was swept up, labeled as
bandsaw fines, and retained. The fine dust that was washed from the saw
cabinet was discarded with the freon washings.

Allocations to Scientific Investigators

The meteorites were subdivided according to guidelines prepared by the
Meteorite Working Group. By prior agreement between the Division of
Polar Programs of the National Science Foundation and the National
Institute of Polar Research of Japan, each specimen collected by joint
U.S.-Japanese field parties would be shared equally between the two
nations. Thus, after the preliminary examinations and descriptions, each
of the larger stones was either cleaved or sawed in half for division
with the Japanese. The U.S. portion of the specimen was placed in a
stainless steel chipping bowl in a nitrogen-processing cabinet.
Allocations for requested material were filled by chipping off
appropriate samples with a chisel.
Attempts were made to obtain samples according to specific requirements
of investigators. Chipping rather than sawing was used as much as was
practical, to minimize possible contamination. In the case of the
achondrites, several grams of interior material were crushed to provide
representative samples to investigators performing chemical analyses. In
the case of allocations from carbonaceous chondrites, the final freon
washings of cleaned processing tools were checked to determine levels of
residual hydrocarbons. The relative positions of the samples were
usually documented. In cases where several samples of documented
subsurface depths were requested (e.g., for particle track studies),
these depths were either measured when the samples were taken or a
single piece of meteorite containing a wide range of depths was
allocated so that the investigator could extract his own samples. Most
polished thin sections allocated to investigators were made in the
thin-section laboratory of the Curatorial Facility. Sections of a given
meteorite were usually made from an interior chip, which was not the
chip from which library thin sections were made as part of the initial
survey.


Cheers,

Bernd

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