http://www.journalof911studies.com/volume/200704/JonesWTC911SciMethod.pdf

Similar spheres were also found by other investigators, including the R.J Lee company and the USGS.  But although the EPA considered "iron spheres" to be a potential signature component of the WTC dust, it rejected using them as an indicator, along with asbestos and PAHs, for unidentified reasons.  See link below.

http://www.epa.gov/wtc/panel/pdfs/SubGroupComments_110305.pdf

Such spheres would form in the molten state due to surface tension of the metal as it fell or was sprayed from a high temperature source.  The presence of iron spheres in the dust indicates temperatures of at least 1500 °C, much higher than anything NIST suggests even in reports of its computer modeling.  NIST suggested steel temperatures of up to 760°, but had physical evidence for only about 250 °C.  Temperatures as high as 1500 °C would mean that something other than simple jet fuel or office furnishings was present during the destruction of the WTC buildings.

My sample of WTC dust comes from a worker at ground zero, who obtained it near the remains of the north tower.  As Dr. Jones indicated, there is a substantial magnetic portion in the dust, and the spheres are quite prominent.  These photomicrographs were taken with a Nikon Labphot microscope and camera.

Metallic microsphere

Trapped microsphere

Additional images can be found here.  This is a powerpoint file, so it takes a few seconds to download.

http://www.ultruth.com/Photomicrographs%201.ppt