Real Dirt and Magnetic Particles

[nachoman]

But the real question is how necessary is it to get those metal particles out before using the stuff on the layout.

Gary, how much and what size of magnetic material do you expect? I wrote about the turning that resulted from milling locomotive frames etc. They are a problem you should avoid but what kind of particles do you expect and how do you get them in the dirt on the streets of Houston?

Only a few naturally occurring mineral are magnetic - primarily one called magnetite. I'm not exactly sure if there are secondary reactions that form or concentrate these minerals within the soil, but within whole rocks magnetic minerals are pretty rare in most places. I suspect in roadside "dirt" the magnetic particles are primarily from cars and trucks wearing out (probably mostly brake and clutch dust). Sifted railroad ballast would be the same way. Any water running from the roadway would carry these particles into nearby streams, and from there could travel quite a long way. That would be an interesting study if someone used magnetic particles as an "indicator" of sorts. Lead compounds from years of burning leaded gasoline can be found in most soil, especially along highways, and I've seen that used as an indicator. Certain radioactive isotopes are also present in modern soil as a result of nuclear testing. When scientists report a radiocarbon date it is always reported as years prior to 1950, because nuclear weapons testing disrupted the natural carbon ratio in the atmosphere. If one is trying to determine the age of a sedimentary deposit, presence of materials like lead and radioactive isotopes help pinpoint the age.

EDIT: It seems that people are already using magnetic particles as pollution tracers:
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