[meteorite-list] Weathering Grades

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:29 2004
Message-ID: <20020601003013.38969.qmail_at_web10401.mail.yahoo.com>

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 Fri 31 May 2002 08:30:13 PM PDT