Natural microtektites versus industrial glass beads: an appraisal of contamination problems

Precise and convenient distinctive criteria are needed to avoid mistaking man-made micro glass beads for natural microtektites. The present study bases on both raw samples provided by two major manufacturers, and samples recovered from some of the main industrial products involving glass beads (i.e., reflective paints for road markings). Morphology of the industrial beads was studied by optical- and electron-microscopy. Chemical compositions were investigated by electron-probe microanalysis, using wavelength dispersive X-ray spectrometry. Although closely resembling natural microtektites in terms of both size and morphology, the standard industrial beads prove to consist of recycled, hard soda-lime silica glasses. These can be distinguished readily from most microtektites by their larger content in Na – and to a lesser extent – by their smaller Fe and Al contents. Other recycled glasses possibly involved in the manufacturing of non-standard microbeads, also can be recognized by their B and/or P, Ba, Pb, Zn, Cu, Ni, Co, or Mn contents, which are diagnostic. Accurate electron-probe microanalysis thus appears to be the most straightforward way to discrimitate natural and industrial glass beads, excepting the special case of samples with SiO2 > 90 wt%.