The meteorite weathering grade scale W0-W6 now commonly used was first described by F. Wlotzka of the Department of Cosmochemistry at the Max Planck Institute in Mainz, Germany in a short article "A Weathering Scale for Common Chondrites" published in Meteoritics, July 1993. This article is available online in NASA's ADS database:
http://adsabs.harvard.edu/abstract_service.html
Here is Wlotzka's short description of the weathering grades:
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.
Best wishes to all,
Piper Hollier Amsterdam NL
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