[meteorite-list] Weathering Grades

From: Walter Branch <branchw_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:29 2004
Message-ID: <01c301c2095c$01848b20$21d93fd0_at_cc516468a>

Hello Bob and List,

>I would assume the "skew toward the lower grades" that
>Walter mentioned is the result of -
>the more a meteorite is weathered -
>the more it looks like a terrestrial rock -
>the the harder it would be to recognize it -
>the FEWER there will be found.

Perhaps worldwide, but I would not assume this for the NWA region,
considering the number of terrestrial rocks to be found in this region.

Here would be a good question for NWA meteorite dealers. How often are you
presented with terrestrial NWA meteorwrongs for purchase?

>But, here is a better reason -
>Since the weathering grade is based upon the interior
>condition of a stony meteorite, the high percentage of
>the total of meteorites that come from DRY (Hot &
>Cold) Deserts will drive this "skew toward the lower
>grades".

Yes, which is why I was so interested in the comparison statistics (although
I do note a slight lower weathering bias in the non-NWA statistics, which
may be explained by your observations above).

There may not enough data to make meaningful conclusions but I would REALLY
LIKE to see a breakdown by world geographical regions. For example,
climates more conducive to the preservation (e.g., southwest US) of
meteorite vs. those less favorable to meteorite preservation (e.g.,
southeast US).

Bernd?

-Walter



-----------------------------------------------
Walter Branch, Ph.D.
Branch Meteorites
322 Stephenson Ave., Suite B
Savannah, GA 31405 USA
www.branchmeteorites.com
----- Original Message -----
From: "Robert Verish" <bolidechaser_at_yahoo.com>
To: "Meteorite-list Meteoritecentral" <meteorite-list_at_meteoritecentral.com>
Sent: Friday, May 31, 2002 8:30 PM
Subject: [meteorite-list] Weathering Grades


> meteorite-list] Weathering Degrees - Breakdown II
>
> Bernd Pauli HD bernd.pauli_at_lehrer1.rz.uni-karlsruhe.de
>
> Fri, 31 May 2002 21:50:13 +0200
>
> Walter Branch wrote:
>
> > Thanks for the breakdown. The weathering grades
> appear not to take a normal distribution, rather there
> appears to be a skew toward the lower grades. For
> comparison, can you produce that same chart but
> excluding the NWA meteorites in your database?
> >- Walter
>
> Survey of weathering grades of meteorites excluding
> NWA:
>
> W0 or W0/1: 054
> W1 or W1/2: 241
> W2 or W2/3: 520
> W3 or W3/4: 448
> W4 or W4/5: 178
> W5 or W5/6: 013
> W6: 002
>
> Just like before, this survey does not yet include
> Met.Bull. 86! But it does include all the other Hot
> Desert meteorites like DaG's, HaH's etc.
>
> Cheers,
>
> Bernd
> ********** End of Original Message ***********
>
> Hello Bernd and Walter,
>
> I would assume the "skew toward the lower grades" that
> Walter mentioned is the result of -
> the more a meteorite is weathered -
> the more it looks like a terrestrial rock -
> the the harder it would be to recognize it -
> the FEWER there will be found.
>
> But, here is a better reason -
> Since the weathering grade is based upon the interior
> condition of a stony meteorite, the high percentage of
> the total of meteorites that come from DRY (Hot &
> Cold) Deserts will drive this "skew toward the lower
> grades".
>
> Once again, the weathering grade is irregardless of
> the condition of the exterior of a meteorite.
>
> Maybe this is a good opportunity to review how
> weathering grade is determined (for the recent
> collector or for those new to the List):
>
> Weathering Grade (Wlotzka, 1993)
>
> W0-no visible oxidation of metal or sulfide but a
> limonitic staining may be noticeable in transmitted
> light. Fresh falls are usually of this grade, although
> some are already W1.
> W1-minor oxide rims around metal and troilite and
> minor oxide veins.
> W2-moderate oxidation of metal, about 20-60%
> being affected.
> W3-heavy oxidation of metal and troilite, 60-95%
> being replaced.
> W4-complete (>95%) oxidation of metal and
> troilite, but no alteration of silicates.
> W5-beginning alteration of mafic silicates,
> mainly along cracks.
> W6-massive replacement of silicates by clay
> minerals and oxides.
>
>
>
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Received on Sat 01 Jun 2002 07:03:48 AM PDT