[meteorite-list] Red(dish) Fusion Crust

From: Michael Farmer <mike_at_meteoritecentral.com>
Date: Wed, 29 May 2013 08:05:01 -0700
Message-ID: <203FDC48-5A59-4C3E-B68C-B9525E311378_at_meteoriteguy.com>

This is seen on many stones, I have a Bensour that is almost red.
I have many Chelyabinsk stones also with brown to red crust, and some iridescent in every color in the rainbow. This is seen in many L and LL falls, but generally only before exposure to water. However I found my first Chelyabinsk which was 503 grams, perfect pyramid with one red crusted side, all other sides extremely black.
Michael Farmer

Sent from my iPad

On May 29, 2013, at 8:00 AM, Jason Utas <meteoritekid at gmail.com> wrote:

> Hello All,
> And the red crust isn't just found on trailing faces of stones:
>
> http://www.ebay.com/itm/meteorite-Chelyabinsk-chondrite-LL5-complete-stone-14-65-g-recent-fall-Russia-/161029553312?pt=LH_DefaultDomain_0&hash=item257e1bfca0&nma=true&si=jHrsL50utK2qqpfbNFqr9%252BcmQSM%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557
>
> It's been seen on stones from just about every reasonably-sized L and
> LL multiple-stone fall I can think of, and has been discussed on the
> list as far back as 2007, if not earlier. Similar stones have been
> noted from Breja, Bensour, Battle Mountain, Ash Creek, Mifflin, etc.
> This list seems to have a short memory.
>
> For those who are curious, "magnetite" content is a bit vague. The
> difference in fusion crust coloration is most likely caused by the
> oxidative state of the iron in the fusion crust.
>
> http://en.wikipedia.org/wiki/Iron_oxide
>
> If we assume that water is not abundant in the fusion crust due to the
> high heat necessary to form a fusion crust (perhaps wrong, but
> simplifies things), we have three oxides to work with:
>
> ----------
>> From above:
>
> W?stite (FeO) is a mineral form of iron (II) oxide found with
> meteorites and native iron. *It has a gray color with a greenish tint
> in reflected light.*
>
> Magnetite is a mineral, one of the two common naturally occurring iron
> oxides (chemical formula Fe3O4). Magnetite has been very important in
> understanding the conditions under which rocks form. Magnetite reacts
> with oxygen to produce hematite, and the mineral pair forms a buffer
> that can control oxygen fugacity. *Generally black or silvery, can
> have a brownish tint.*
>
> Iron (III) oxide or ferric oxide is the inorganic compound with the
> formula Fe2O3. We'd most likely be dealing with alpha-phase ferric
> oxide because it is the most stable Fe2O3 phase over ~500?C. This
> one's also called hematite. *Fe2O3 is dark red.*
> ----------
>
> The wikipedia page above links to nice summaries of the hydrous oxides
> as well, if you want to check them out.
>
> The variables we have to work with are: the amount of iron in the
> meteorite, plus abundances of other minerals that could affect oxide
> or other mineral formation in the crust. Fragment shape and
> orientation probably control oxygen flow to given areas (see link
> below) but also --
>
> ...the entry speed/angle and breakup height would probably help to
> determine the rate of ablation/deceleration of given fragments (e.g.
> the point at which fusion crust will remain on the surface of the
> meteorite versus ablating away), which would also affect the
> temperature at which the remaining fusion crust formed (a potential
> variable controlling the oxidative state of iron?). Either way, since
> access to oxygen seems to determine the "redness" of the fusion crust,
> altitude of fragmentation is probably quite important.
>
> http://www.ebay.com/itm/Chelyabinsk-Meteorite-Fall-from-Feb-15th-2013-in-Russia-7-098-grams-/111073775576?pt=LH_DefaultDomain_0&hash=item19dc834fd8
>
> ^One of the better examples currently on ebay, with topographically
> low areas that clearly show reddening/browning.
>
> In short, yes, hematite is red, so hematite content is a good
> candidate for the 'reddening agent.'
>
> But, then...why don't H chondrites usually form such red fusion
> crusts? It might be due to the higher iron content in H-chondrites
> and the ratio of iron to oxygen in the above three oxides. Fe2O3
> (hematite) has the lowest Fe to O ratio of the above three minerals
> (1:1 vs. 3:4 vs. 2:3), so a meteorite that is higher in iron might be
> less likely to form a "lower-iron" oxide (hematite) in the same
> conditions. But this seems somewhat unlikely, as this hypothesized
> cutoff for hematite formation in the crust would depend on the
> difference in the modal abundance of Fe in L's versus H's, and that's
> not a clear boundary. One would have to look at the metal content of
> various larger multiple falls and examine large numbers of pristine
> stones from each in order to reach a well-supported answer to that
> question.
>
> Chelyabinsk does support this general hypothesis, though. It broke up
> at a lower altitude than most bolides do, so fragments should have
> been exposed to a thicker atmosphere/more oxygen in their final
> ablative stages of flight. Because of this, we'd expect to see more
> iron oxides with higher ratios of oxygen to iron in the fusion crust
> (e.g. our red hematite) . Lo and behold, we're seeing more stones
> with reddish fusion crusts than usual. This could be a coincidence,
> but...perhaps not.
>
> One should also note that many Chelyabinsks aren't just black or
> reddish. Many are an unusual lighter brown/grey color:
>
> http://www.ebay.com/itm/meteorite-Chelyabinsk-chondrite-LL5-complete-stone-13-14-g-recent-fall-Russia-/161034404036?pt=LH_DefaultDomain_0&hash=item257e6600c4
>
> That's a color I've never seen before on an OC, but many Chelyabinsks
> show it. Could higher levels of (grey/metallic) magnetite be the
> cause? I wonder...and if that's the case, I'd be curious to know why
> this is specifically happening with Chelyabinsk and not other L/LL
> chondrites. Either way, something novel is going on.
>
> Regards,
> Jason
>
>
> www.fallsandfinds.com
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Received on Wed 29 May 2013 11:05:01 AM PDT