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Iron Meteorites and Heat

Hello all,

I've been looking at the gorgeous Sikhote-Alin specimens that have been
appearing on web sites recently. Walt's post last week on the care that must
be taken when cutting and drying iron meteorites to avoid generating too much
heat got me thinking. There may be some newer collectors on the list that do
not really understand the danger heat posses to irons.

The reason I bring this up is that iron meteorites with natural, pristine
fusion crust in gun-metal and rose tones are rather scarce animals. When they
start showing up by the wheelbarrow load there is a possibility that they have
been processed to remove oxidized crust and tone the metal surface. One way of
doing this is with heat and the recent spate of material from Russia may
indicate that a "factory" has been set up. If the specimens have been
processed by heat, collectors should be aware of two possible problems.

First, the Widmanstatten structure is quite sensitive to heat, as Walt pointed
out. If an iron meteorite has been heated to 210-220C (roughly 420F), the
Widmanstatten structure begins to break down and fade. By the time the
temperature reaches 340-350C (roughly 650F) the structure is gone. A good
portion of this temperature range can be reached in a cooking oven. (These
temps are from memory, if anyone has the specifics, please post them.)

The second problem caused by too much heat is the expansion and weakening of
the crystal boundaries. This can result in more rapid weathering and further
breakdown of the crystal boundaries until the specimen can separate along a
boundary.

So, if you're thinking about buying one of the new Sikhote specimens, think
through what you want to do. Discuss the specimen with the dealer; a good one
can tell the difference between pristine crust and toned metal. If you want an
eye-catching display specimen, you have some stunning meteorites to choose
from. If you were thinking about cutting and etching a window in the meteorite
to display the Widmanstatten structure, or if you are planning to section,
etch and sell slices you should be very careful about what you're buying.

If you do get one for display, check it carefully for cracks. It's not unusual
for a meteorite to have a few small stress fractures (especially something as
violent as the Sikhote fall), but cracks running deep into the specimen or
around a large portion of it could prove troublesome and you may want to have
a prearranged agreement to return such a specimen. 

Whenever possible, display the meteorite in an enclosed, dry environment and
your investment will last much longer.

Gene