meteorite-list] Weathering Degrees - Breakdown II
Bernd Pauli HD [EMAIL PROTECTED]
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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