Hello List
I rarely comment on anything, but I will throw my 2¢ in here. I think
Dr. Ebel makes a good point. That bit of learning was brought home to me
when Tim McCoy showed me the Smithsonian's spectacular Semarkona LL3.0
specimen. I exclaimed that it was "like a sedimentary rock." (I had seen a
few sedimentary rocks in my many years as a field geologist.) Dr. McCoy's
response was that it really was a sedimentary rock. I had to agree.
It is true that the Earth-oriented texts describe sedimentary rocks as
settling out of air or water. But what else besides air and water is there
on Earth? That misses the point. The operative process is the settling.
Indeed, the word sediment is derived from a Latin verb that means "to
settle." I would say that it is not the origin of the grains that counts,
but the process of deposition--the settling.
The chondrules, mineral grains and glass that make up a chondrite form
by various processes in space. They "settled" by gravity through the vacuum
of space to their asteroid destination. The processes that makes them
rock--induration as a geologist would say--are not that different that
induration of Earth's sedimentary rocks. From there these rocks were
metamorphosed (except Semarkona) and some were even melted. They became, in
turn, sedimentary, metamorphic and igneous rocks--the three basic kinds of
rocks we know here on Earth.
Eric Twelker
[EMAIL PROTECTED]
http://www.meteoritemarket.com
> From: [EMAIL PROTECTED]
> Date: Fri, 29 Aug 2003 01:43:18 +0000
> To: Mark Ferguson <[EMAIL PROTECTED]>
> Cc: [EMAIL PROTECTED]
> Subject: Re: [meteorite-list] Are chondrites sedimentary rocks?
>
> Mark,
>
> I totally agree with your thinking on this. I did not intend anyone to think
> otherwise. My comment last night was only trying to put some kind of spin on
> what could be the thought process when someone uses chondrites and
> sedimentary rocks in the same reference. So, I too agree that they are not
> sedimentary rocks as we know them. I'll let others try to argue if they
> should even be mentioned in the same context.
>
> My apologies to anyone who might have been confused by my comments. My
> personal belief is that chondrites are formed by a process that is still not
> completely understood by us earth dwellers. I doubt if it will be. Models are
> just that...but let's keep trying.
>
> John
>
>> Hi List
>>
>> I just have to comment on this. Sedimentary rock, in any definition found in
>> geology and petrology books is matter which is laid down by either wind or
>> water, then compacted over time into a durable rock. This is not how
>> chondrules formed. Chondrules most likely are a coalescing of minute liquid
>> droplets into larger droplets which is very different from the sedimentary
>> process. It would be more accurate to liken it to the formation of a
>> chondrule to that of a rain drop, but that might well be to simple a process
>> to adequately describe chondrule formation since there are some high
>> temperatures and very different pressures indicated . But to use
>> sedimentation is not even close. Totally different mechanisms are involved,
>> that much is sure. For anyone to use totally different processes and
>> mechanisms is wrong for that only corrupts a new persons attempt understand
>> meteoritics.
>>
>> Mark
>>
>> ----- Original Message -----
>> From: <[EMAIL PROTECTED]>
>> To: <[EMAIL PROTECTED]>
>> Sent: Wednesday, August 27, 2003 2:51 PM
>> Subject: [meteorite-list] Are chondrites sedimentary rocks?
>>
>>
>>> Martin wrote:
>>>
>>>> In the interview, Ebel makes two comments that made me wonder...
>>>> First, he mentions that "Most meteorites are pieces of asteroids.
>>>> A very few are comets."
>>>
>>>> My question is which "very few"? I figure the usual suspects are Orgueil
>>>> and Murchison, but some comet experts I have talked with discount them
>>>> and all other meteorites as being of cometary origin.
>>>
>>> .. do not forget Tagish Lake and to some extent Krymka (see David Weir's
>>> comments on Krymka on his website: " ... This material is enriched in
>> volatile
>>> siderophiles such as Ag, Tl, and Bi, and represents a late condensate from
>>> a metal-depleted region of the solar nebula, possibly related to cometary
>>> material."
>>>
>>>> The second thing that caught my eye was when Ebel said, "Chondrites are
>>>> really sedimentary rocks made up of dust and then chondrules, these
>> round
>>>> droplets that were once molten and now are little beads, many containing
>>>> glass, which were present in the solar system."
>>>
>>> He may have read O.R. Norton's comments in Joel Schiff's magazine:
>>>
>>> NORTON O.R. (1998) Are chondrites sedimentary
>>> rocks? (M! Feb. 1998, Vol. 4, No. 3, pp. 22-23).
>>>
>>>> My question here is if chondrites can
>>>> really be considered sedimentary rocks.
>>>
>>> The only references I have about sedimentary meteorites:
>>>
>>> TOMEOKA K. et al. (1997) Evidence for early sedimentary
>>> processes in a dark inclusion in the Vigarano CV3 chondrite
>>> (Meteoritics 32-4, 1997, A129).
>>>
>>> TOMEOKA K. et al. (1998) Arcuate band texture in a dark inclusion from
>>> the Vigarano CV3 chondrite: Possible evidence for early sedimentary
>>> processes (Meteoritics 33-3, 1998, 519-525).
>>>
>>> BRIDGES J.C. et al. (1998) Traces of Martian sediment in Nakhla
>>> and other SNC meteorites (Meteoritics 33-4, 1998, A023).
>>>
>>>
>>> Best wishes,
>>>
>>> Bernd
>>>
>>>
>>> To: [EMAIL PROTECTED]
>>> Cc: [EMAIL PROTECTED]
>>>
>>>
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>>
>>
>>
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