Hello Walter and List,
Now these are the kinds of threads that I love on this
list!
I have comments or responses on three of the
questions.
The first is the color of diogenites. My tatahouine
is definitely green, and my Bilanga is definitely not.
I first got really interested in meteorites because
at the time I wanted to know why olivine on Earth is
green, but in pallasites it is yellow or brown.
Actually, as later discovered, between pallasites such
as Albin, Imilac, Ahumada, and Acomita, you will see
pallasitic olivine that is green, yellow, brown,
black, and red. The answer, according to my college
mineralogy professor, is trace elements. Some
minerals have characteristic colors. The pyroxene
diopside is green, for example. Depending upon the
mineral, some are more suseptible to color variations
than others based on trace elements. Jewelry stores
are selling pink sapphires these days - years ago I'd
have said sapphires are blue and there's no such thing
as pink ones. The differences, maybe not in all cases
but in many, are trace elements. Now, is there a
meteorite mineralogist out there that can tell us
which trace elements color olivines and pyroxenes
certain ways in meteorites?
Next, Meteor Crater. Walter's question was why does
the shape appear differently in different
photographs/angles. There was a reply that states the
crater it is square and one that talks about karst
topography. I'm not sure either addresses Walter's
question. Actually, I think the crater is in the
Cocconino (spelling?) sandstone formation, but similar
to typically karst limestone topography, the sandstone
formation is fairly heavily jointed in a rectangular
pattern, and the shape of the crater was influenced
accordingly from the time of impact. I think the
difference in the appearance is a function of the
angle of the photo relative to the "corners" as well
as the elevation above the surface - almost an optical
illusion. If you are looking at a corner, as opposed
to one of the sides of the square, particularly from a
more shallow angle (closer to the ground), I believe
that it makes the crater look more circular. I've
seen photos where it appears circular and photos that
clearly show it's square-ish shape. I've always
wanted to fly around Meteor Crater in a small plane.
Maybe one of these days . . . .
Finally, Walter, if you ever find someone who can give
you a good answer to your question about determining a
reliable and defensible cosmic ray exposure age for a
meteorite, which by definition has suffered ablation
but of unknown extent, and may have suffered one or
more fragmentation events prior to ablation ceasing,
please pass the explanation on to me. I've been
wondering the same thing for years.
Thank you Walter for your post! Pondering questions
like this got me interested in the first place!
Frank Prochaska
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