Meteorite weathering is the
terrestrial alteration of a
meteorite. Most meteorites date from the oldest times in the
Solar System and are by far the oldest material available on our planet. Despite their age, they are vulnerable to the terrestrial environment.
Water,
chlorine and
oxygen attack meteorites as soon as they reach the ground.
Weathering scales
In order to quantify the degree of alteration that a meteorite experienced, several qualitative weathering indices have been applied to antarctic and desert samples.[1]
The most known weathering scale is based upon the effects seen in polished
thin sections of chondritic meteorites and it ranges from W0 (pristine) to W6 (heavy alteration). It was proposed by Jull A. J. T. et al.[2] (1991) and updated by Wlotzka[3][4](1993) and Al-Kathiri et al.[5](2005).
W0: no visible
oxidation of metal or
troilite, but may be noticeable in transmitted light a limonitic staining. Usually recently
fallen meteorites are of this grade, although some are already W1.
W1: small oxide rims around metal and troilite, small oxide veins.
W2: moderate oxidation of metal (about 20-60% replaced).
W3: heavy oxidation of metal and troilite (60-95% replaced).
W4: complete oxidation of metal and troilite (>95% replaced), but no alteration of silicates.
W5: beginning alteration of mafic silicates, mainly along cracks.
W6: heavy replacement of silicates by clay minerals and oxides.
^Jull, A. J. T.; Wlotzka, F.; Donahue, D. J. (1991). "Terrestrial Ages and Petrologic Description of Roosevelt County Meteorites". Abstracts of the Lunar and Planetary Science Conference. 22: 667.
Bibcode:
1991LPI....22..667J.
^Wlotzka, F. (1993). "A Weathering Scale for the Ordinary Chondrites". Meteoritics. 28: 460.
Bibcode:
1993Metic..28Q.460W.
^Wlotzka, F.; Jull, A. J. T.; Donahue, D. J. (1995). "Carbon-14 Terrestrial Ages of Meteorites from Acfer, Algeria". Workshop on Meteorites from Cold and Hot Deserts: 72.
Bibcode:
1995mfch.work...72W.