Weathering grade
The weathering grade is a code, usually W0 to W6, that records how much a meteorite has rusted and decayed since landing on Earth.
As soon as a meteorite lands, air and water start attacking it. Iron-nickel metal and the iron sulfide troilite rust first, and later the silicate minerals break down. To describe this, the researcher Frank Wlotzka proposed a scale in 1993 for ordinary chondrites, judged from polished sections.
W0 shows no visible oxidation of metal or sulfide, though slight staining may occur. W1 has minor oxide rims and veins. W2 shows moderate oxidation of metal, very roughly 20–60 per cent. W3 has heavy oxidation, about 60–95 per cent. W4 means metal and sulfide are almost completely oxidised. W5 marks the start of silicate alteration, and W6 massive replacement of silicates by clays and oxides. Antarctic meteorites are often described instead with hand-specimen letters A, B and C for increasing rustiness, plus "e" if evaporite minerals are present.
Fresh falls such as Winchcombe, which fell in England in 2021 and was collected within days, are essentially unweathered, while many hot-desert finds are W2 or W3. You can judge weathering roughly by eye: bright metal flecks suggest low grades, while brown-orange staining through the whole rock suggests high ones.
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