This article was written in 2012. A more up-to-date discussion of where the iron microspheres came from can be found at:
https://www.metabunk.org/making-iron-microspheres-grinding-impacts-welding-burning.t9533/
https://www.metabunk.org/making-iron-microspheres-grinding-impacts-welding-burning.t9533/
The iron microspheres (as evidence for thermite) were debunked years ago, but they keep coming up. The bottom line is:
- Iron Microspheres form from condensed vaporized iron or from molten iron
- You can melt iron by igniting it with a Bic lighter, if the pieces of iron are thin enough.
- There are several other sources of iron microspheres
- Iron microspheres were expected in the WTC dust

If you ignite some steel wool with a hydrocarbon flame, then you get lots of iron spheres, some of the same size as these microspheres. Note this is not from the flame melting the steel, but from the steel itself burning, and melting itself. This is only possible with a sufficiently large surface area to mass ratio - i.e. with very small or very thin particles.

The below debunking is by Dave Thomas of NMSR, JREF, and others. I'm collating it here to allow easier reference via Google, and so we don't have to keep going over the same ground.
http://www.nmsr.org/nmsr911.htm

The following is extracted from a JREF forum thread with extensive discussion of the objections. Please read at least the first three pages of that thread to see if your personal objections are covered.
http://www.internationalskeptics.com/forums/showthread.php?t=264234 (archive: http://archive.is/ypm04)
Burning some scrap primer painted steel

in a wood fire in a barrel:

made iron microspheres:

Iron Microspheres can also created from a high energy collision or friction between iron oxide (rust) and aluminum. As seen by this classic science experiment using two rusty iron cannonballs, one coated with aluminum foil.
Given the high energy of the collapsing building and the large amount of aluminum cladding in close contact with a large area of steel, then there would certainly be some microspheres created via this mechanism.
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Other in-thread references:
RJ Lee Reports:
- WTC Dust Signature Report - Composition and Morphology
- Expert Report - WTC Dust Signature
- WTC Dust Signature Report - Asbestos
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