[meteorite-list] Weathering Grade

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:46:24 2004
Message-ID: <3B02F0E4.D797380_at_lehrer1.rz.uni-karlsruhe.de>

entropydave inquired (in gentle decay):

> I wonder if you could help me regarding weathering grade ... I
> have a couple of rocks that are weathering grade 2 (Oum Rokba)
> and NWA073 with weathering grade 3.

WLOTZKA F. (1993) A weathering scale for the ordinary chondrites
(Meteoritics 28-3, 1993, A460):

Weathering categories A, B, and C are used by the Meteorite Working
Group at the NASA Johnson Space Center in Houston for Antarctic
meteorite finds, denoting minor, moderate, and severe rustiness of
hand specimens. A different scale can be setup from the weathering
effects seen in polished sections with the microscope. These weathering
effects finally lead to the disintegration of the meteorite; they are
important in connection with its terrestrial age and an estimate of
the true fall rate of meteorites. In order to avoid confusion with the
hand specimen classification A, B, C, the weathering grades determined
on polished sections were named Wl-W6. Weathering affects first the
metal grains, later troilite, and finally the silicates. The following
progressive stages can be distinguished:

W0: No visible oxidation of metal or sulfide. A limonitic staining may
       already 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; 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 (= of or pertaining to rocks rich in
       dark, ferromagnesian minerals) silicates, mainly along cracks.

W6: Massive replacement of silicates by clay minerals and oxides.

More or less massive veining with iron oxides can already be found in
stage W2. These veins develop independently from the weathering grade,
apparently in cracks that form through mechanical forces. Broad cracks
are often filled with carbonates. Grades W5 and W6 are rare. The
silicate alteration first affects the olivines; it starts inside the
grains, not from the rim. In stage W6 intact chondrules were found,
where olivines were completely replaced by a mixture of clay minerals
and iron oxides, with the feldspathic mesostasis being unaffected.

Best Regards,

Received on Wed 16 May 2001 05:28:04 PM PDT

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