Meteorite weathering
Encyclopedia
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. However, despite their age, they are pretty vulnerable to terrestrial environment: water
, chlorine
, oxygen
attack the meteorites as soon they reach the ground.
The most known weathering scale is based upon the effects seen in polished thin section
s and it ranges from W0 (pristine) to W6 (heavy alteration). It was proposed by Jull A. J. T. et al. (1991) and updated by Wlotzka(1993) and Al-Kathiri et al.(2005).
The Meteorite Working Group at the Johnson Space Center uses weathering categories A
B, C and E to denote the alteration of antarctic meteorites.
Here are the official definitions:
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
of a meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
. Most meteorites date from the oldest times in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
and are by far the oldest material available on our planet. However, despite their age, they are pretty vulnerable to terrestrial environment: water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
attack the meteorites as soon 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 desertic samples.The most known weathering scale is based upon the effects seen in polished thin section
Thin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
s and it ranges from W0 (pristine) to W6 (heavy alteration). It was proposed by Jull A. J. T. et al. (1991) and updated by Wlotzka(1993) and Al-Kathiri et al.(2005).
- W0: no visible oxidation of metal or troiliteTroiliteTroilite is a rare iron sulfide mineral with the simple formula of FeS. It is the iron rich endmember of the pyrrhotite group. Pyrrhotite has the formula FeS which is iron deficient...
, but may be noticeable in transmitted light a limonitic staining. Usually recently fallen meteoritesMeteorite fallsMeteorite falls, also called observed falls, are those meteorites that were witnessed by people or automated devices as they moved through the atmosphere or hit the Earth, and were subsequently collected. All other meteorites are called "finds"...
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 Johnson Space Center uses weathering categories A
B, C and E to denote the alteration of antarctic meteorites.
Here are the official definitions:
- 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.