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Magnetite Sphere

This is a tapelift sample from a shop where welding, torch cutting, and high-speed grinding of iron is conducted. The magnetite sphere shows the ring of light on the outside of the particle when it is viewed with transmitted crossed linear polarized light. Note the dark areas are missing where the tangent to the sphere is parallel to the polarizing directions of the polarizing filters because circular polarized light is being used. The reflected darkfield illuminator is visible in the reflected ring of light in the center of the particle and it shows the irregularities caused by the characteristic texture on the surface of the sphere.

Transmitted Off Crossed Circular Polarized Light and Reflected Darkfield Illumination


Magnetite is Fe3O4 and is cubic in crystal habit all though octahedral {111} parting may be evident. Magnetite spheres may be created by welding, torch cutting, or high-speed abrasive grinding. The spheres tend to be hollow shells. This significantly reduces the bulk density of these particles and explains why they become so widely dispersed in the environment near a site where ironwork is or has been occurring.

Significance in the Environment:

The presence of magnetite spheres generally indicates that welding, iron torch-cutting, or high-speed abrasive forming (grinding) of iron is occurring in the nearby environment. They are also formed in much smaller quantity by a flint-type lighter.

Characteristic Features:

Magnetite spheres are opaque but because of their electrical conductivity they are surrounded by a bright ring broken by dark spaces where the tangent to the sphere is parallel to the polarizing filters. When viewed with reflected darkfield illumination the ring of light from the darkfield illuminator is reflected back from the sphere. The reflected ring of light is slightly diffuse because of the "hieroglyphic" texture of the surface of the sphere. Soot, cenospheres, toner, spores, and other non-conductive black spheres don't show the bright ring of light at the edge of the particle when viewed with transmitted crossed polarized light.

Associated Particles:

Weld slag, abrasives, hematite spheres, heat-treat scale, and rust (limonite, goethite, lepidocrocite, and hematite) are common associates near the source of the magnetite spheres. Magnetite spheres tend to travel farther from the source than these particles so the associates become less common the further from the source.