[meteorite-list] Red(dish) Fusion Crust
From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Wed, 29 May 2013 08:00:02 -0700
And the red crust isn't just found on trailing faces of stones:
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.
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:
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.
^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
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,
One should also note that many Chelyabinsks aren't just black or
reddish. Many are an unusual lighter brown/grey color:
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.
Received on Wed 29 May 2013 11:00:02 AM PDT