View source for Earth:Meteorite weathering

{{Short description|Terrestrial alteration of a meteorite}}
[[File:Willamette Meteorite, AMNH, New York Times, 1911.JPG|thumb|Two children are sitting within the corrosion grooves of the Willamette meteorite. Considerable mass has been lost to terrestrial weathering.]]
[[File:Météorite Sikhote-Alin (Russie) 1.JPG|thumb|A Sikhote-Alin meteorite with visible rust.]]
'''Meteorite weathering''' is the [[Earth:Weathering|terrestrial alteration]] of a [[Astronomy:Meteorite|meteorite]]. Most meteorites date from the oldest times in the [[Astronomy:Solar System|Solar System]] and are by far the oldest material available on our planet. Despite their age, they are vulnerable to the terrestrial environment. [[Chemistry:Water|Water]], [[Chemistry:Chlorine|chlorine]] and [[Chemistry:Oxygen|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.<ref name=weatheringofchondritic>P. A. Bland, M. E. Zolensky, G. K. Benedix, M. A. Sephton. ''"[http://www.lpi.usra.edu/books/MESSII/9041.pdf Weathering of Chondritic Meteorites]"''</ref>

The most known weathering scale is based upon the effects seen in polished [[Earth:Thin section|thin section]]s of chondritic meteorites and it ranges from W0 (pristine) to W6 (heavy alteration). It was proposed by Jull A. J. T. et al.<ref>{{cite journal | bibcode=1991LPI....22..667J | title=Terrestrial Ages and Petrologic Description of Roosevelt County Meteorites | author1=Jull | first1=A. J. T. | last2=Wlotzka | first2=F. | last3=Donahue | first3=D. J. | volume=22 | date=1991 | pages=667 | journal=Abstracts of the Lunar and Planetary Science Conference}}</ref> (1991) and updated by Wlotzka<ref>{{cite journal | bibcode=1993Metic..28Q.460W | title=A Weathering Scale for the Ordinary Chondrites | last=Wlotzka | first1=F. | volume=28 | date=1993 | pages=460 | journal=Meteoritics}}</ref><ref>{{cite journal | bibcode=1995mfch.work...72W | title=Carbon-14 Terrestrial Ages of Meteorites from Acfer, Algeria | author1=Wlotzka | first1=F. | last2=Jull | first2=A. J. T. | last3=Donahue | first3=D. J. | date=1995 | pages=72 | journal=Workshop on Meteorites from Cold and Hot Deserts}}</ref>(1993) and Al-Kathiri et al.<ref>{{cite journal | doi = 10.1111/j.1945-5100.2005.tb00185.x | title = Weathering of meteorites from Oman: Correlation of chemical and mineralogical weathering proxies with14C terrestrial ages and the influence of soil chemistry | date = 2005 | last1 = Al-Kathiri | first1 = A. | last2 = Hofmann | first2 = B. A. | last3 = Jull | first3 = A. J. T. | last4 = Gnos | first4 = E. | journal = Meteoritics & Planetary Science | volume = 40 | issue = 8 | pages = 1215–1239|bibcode = 2005M&PS...40.1215A | doi-access = free }}</ref>(2005).
* W0: no visible oxidation of metal or [[Chemistry:Troilite|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.

The Meteorite Working Group at the [[Organization:Johnson Space Center|Johnson Space Center]] uses weathering categories ''A''
''B'', ''C'' and ''E'' to denote the alteration of antarctic meteorites.<ref name=weatheringofchondritic />
Their official definitions are:<ref>{{cite web|url=http://curator.jsc.nasa.gov/antmet/amn/weather.htm|title=Meteorite Weathering Categories|website=curator.jsc.nasa.gov}}</ref>
* A: Minor rustiness; rust haloes on metal particles and rust stains along fractures are minor.
* B: Moderate rustiness; large rust haloes occur on metal particles and rust stains on internal fractures are extensive.
* C: Severe rustiness;  metal particles have been mostly stained by rust throughout.
* E: Evaporite minerals visible to the naked eye.

==See also==
*[[Astronomy:Glossary of meteoritics|Glossary of meteoritics]]
*Meteorite classification
*[[Earth:Weathering|Weathering]]

==References==
{{Reflist}}


[[Category:Meteorite mineralogy and petrology]]

{{Sourceattribution|Meteorite weathering}}