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MessierScott
sage


Reged: 06/18/07

why irons have not rusted away new
      #5807493 - 04/18/13 06:57 PM

I had this question posed to me the other night, and it kind of made me go hmmmmmmmm

If a iron meteorite fell thousands of years ago, why hasn't it rusted away by now?

I kinda of guessed saying that it was becasue it was buried and not exposed to weathering or atmosphere. Yes, it is exposed to ground water, but water alone doesn't cause rusting, does it?

Did I answer this question correctly? The person who asked seemed to think that I kwew what I was talking about.


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Glassthrower
Vendor - Galactic Stone & Ironworks
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Reged: 04/07/05

Loc: Oort Cloud 9
Re: why irons have not rusted away [Re: MessierScott]
      #5808694 - 04/19/13 01:30 PM

Hi Scott,

Some do.

It depends on where the iron falls, the climate in that area, and the composition of the iron.

There are many types of iron meteorite and very few of them are homogenous. They contain silicates, graphite, and varying amounts of nickel alloys. There can be cracks and voids that allow weathering effects to deeply penetrate the metal.

Generally speaking, the more rust-resistant iron meteorites are high in nickel, although this is trumped by local climate. Solid iron meteorites known for a uniform texture and composition tend to resist weathering better than silicated types or stony irons like pallasites.

However, the climate is the key. Meteorites are preserved in areas with low precipitation and humidity - dry deserts (hot or cold), dry lake beds, and salt flats. Here, they can last for many thousands of years, sometimes hundreds of thousands of years, before they oxidize completely.

Sometimes, irons will develop an oxidation rind which will seal the inner core of the meteorite and protect it from further rusting. This rind is made of oxide products and acts as a protective shell that resists moisture penetration.

Even under the best of conditions, irons are problematic from a conservation standpoint. Earth is a toxic environment to meteorites, especially irons or types high in iron. The best way to preserve iron meteorites in your collection is to store them in a low-humidity sealed container and handle them very rarely.

Best regards and clear skies,

MikeG


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