[meteorite-list] Meteorite Questions

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Thu, 30 Aug 2007 01:14:27 -0700
Message-ID: <93aaac890708300114q3f48ae60g2df7933ebea5b11_at_mail.gmail.com>

Hello Walter, All,
I'll tkae this apart bit by bit.

> For example, why does the rim of meteor crater appear "squared" in some
> photos, while in others it appears very round? Perspective? Lighting?
> Extremely highly localized tectonic shifting (back and forth)?

It has eroded into a somewhat rectangular shape over the 50,000 years
that it's spent sitting there in the desert...it's current shape is
probably due somewhat to stress fractures that exist either because
they were created in the underlying rock by tectonic movement or
possibly from the impact itself, so the angle of impact might have
something to do with the orientation of the 'sides,' but you're
dealing with advanced geology and physics there, in both of which I'm
something of a dilettante.

> Also, why is Tatahouine so green? Olivine? Krylon?

Diogenites like Tatahouine are composed primarily of granoblastic
orthopyroxines, and their colour is derived from this. They all
started out as green crystals just like Tatahouine.

> I am looking at a slice of NWA 4664 right now (thank you Eric Olson) and I
> don't see any much green. Maybe that one is a bad example because NWA 4664
> doesn't even look like at Diogenite!

Diogenites like Johnstown and Bilanga have a different colour because
although they started out looking like Tatahouine and the other, more
green, unbrecciated diogenites, the individual crystals have since
been shattered, creating a light coloured powder between the
still-green pieces of orthopyroxine. If terrestrial weathering occurs
and there's any iron in the diogenite, it will turn yellow or possibly
even brown.

What ticks me off is when dealers try to pass off those small black
specks on tiny fragments of Tatahouine as fusion crust - they're not.
The little shiny black things are chromite crystals....learn that
already.

> Also, I have read that some meteoroids travel through space in streams and
> impact the Earth simultaneously (i.e., they have already broken up before
> they hit the Earth's atmosphere). How can this be? I would think that once
> a meteoroid has broken in space (most likely due to impact), minute
> deviations of the individual pieces in the initial trajectory would
> translate into ever increasing deviations in the individual piece's
> trajectory, over time. Unless two pieces were traveling in EXACTLY parallel
> lines, over time the pieces would be widely dispersed in space.

If two large asteroids collide in space, what's to say that a number
of fragments won't go in a similar direction? It might be rather
unlikely, but why do you consider it impossible for there to be
'streams?' I admit that the concept is probably ill-named, but we
aren't necessarily talking about small swarms or trains of meteorites
going along - the concept is used to generally describe fragments of
the same asteroid that wind up in similar orbits around the sun that
we might come across coming from the same general direction at a
similar velocity, etc, possibly years apart from each other.

(Skipping a paragraph in here because it's already been addressed.)

> Traveling over eons to make it to the inner solar system, how can a
> meteoroid stream stay intact enough to cause a tiny strewnfield on the
> Earth? I would not think that the Earth's gravitational field would be
> strong enough to do what Jupiter did.

That's the point - they don't stay together - a similar orbit is all
we're talking about...

> Also, I know I have asked this before but I still don't understand how
> researchers can determine cosmic ray exposure ages for a meteorite which
> ablated a significant portion of the material that absorbed most of the
> cosmic rays and which may have fragmented in flight through the Earth's
> atmosphere.

I've heard 'Ne' as well, as well as Krypton...I don't know the
procedure on that, but surely there's a scientific mind on here who
could shed some light on the subject without having to do the
sleuthing that I would to find it...

With regard to the colour of olivines in Pallasites...
Unweathered pallasites will generally have green-to-black olivines,
depending on the trace elements within them, but other silicates
within irons can be mistaken for olivines like the silicates in IIE
irons, etc - these can range all the way to red, which seems only
logical when one takes into account that small spinel crystals have
been found in that group of meteorites, as well as in others.
The yellower or more red the olivines in a typical pallasite are, the
more weathered the pallasite is, in general. Some weathered olivines
seem to hold their colour well, but in general, the older the
pallasite is, the more fractured and rust-stained the olivines within
become.
It's hard to single a single pallasite out for having such stained
crystals, as most have more-and-less weathered sections (I've seen
green-to-red Albin, Brahin, Brenham, Eagle Station, Ahumada, Admire,
Pallasovka, etc). The only pallasites with crystals of a reliable
colour are, to my knowledge, falls such as Marjalati and then Esquel
(green, but some external pieces have lost this hue), Imilac (yellow,
if there's any olivine there at all), Fukang (green if an internal
specimen), Quijingue (very dark, typically warm colours)...generally.

Regards,
Jason


On 8/29/07, Walter Branch <waltbranch at bellsouth.net> wrote:
> Hello Everyone,
>
> I have had plenty of time recently to ponder things such as meteorites. I
> am also alone at home at present and am bored. Would some kind,
> more-knowledgeable-than-me soul help me with some meteoritical questions.
>
> For example, why does the rim of meteor crater appear "squared" in some
> photos, while in others it appears very round? Perspective? Lighting?
> Extremely highly localized tectonic shifting (back and forth)?
>
> Also, why is Tatahouine so green? Olivine? Krylon?
>
> I am looking at a slice of NWA 4664 right now (thank you Eric Olson) and I
> don't see any much green. Maybe that one is a bad example because NWA 4664
> doesn't even look like at Diogenite!
>
> Also, I have read that some meteoroids travel through space in streams and
> impact the Earth simultaneously (i.e., they have already broken up before
> they hit the Earth's atmosphere). How can this be? I would think that once
> a meteoroid has broken in space (most likely due to impact), minute
> deviations of the individual pieces in the initial trajectory would
> translate into ever increasing deviations in the individual piece's
> trajectory, over time. Unless two pieces were traveling in EXACTLY parallel
> lines, over time the pieces would be widely dispersed in space.
>
> Remember comet Shoemaker-Levy 9? It was broken apart by gravitational
> forces from Jupiter only a year prior to impact, yet by the time it had
> encountered the Jovian atmosphere the separation between the pieces was
> wider than the diameter of the Earth! After only a year.
>
> Traveling over eons to make it to the inner solar system, how can a
> meteoroid stream stay intact enough to cause a tiny strewnfield on the
> Earth? I would not think that the Earth's gravitational field would be
> strong enough to do what Jupiter did.
>
> Also, I know I have asked this before but I still don't understand how
> researchers can determine cosmic ray exposure ages for a meteorite which
> ablated a significant portion of the material that absorbed most of the
> cosmic rays and which may have fragmented in flight through the Earth's
> atmosphere.
>
> Anyone?
>
> -Walter Branch
> ________________________
>
>
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Received on Thu 30 Aug 2007 04:14:27 AM PDT