Multiwall carbon nanotubes have been grown on montmorillonite clay catalysts through anchoring on FeCo nanoparticles. The starting clay is a commercial sodium-rich montmorillonite in which the intercalated sodium ion was exchanged for cobalt(II) and iron(III) ions via mechanical agitation or sonication, both with and without subsequent centrifugation. The cobalt-iron intercalate clay was used as a catalyst for the synthesis of carbon nanotubes via decomposition of ethylene at 700 degrees C. The largest carbon deposit was obtained for catalysts prepared with 3 or 4 cation exchange equivalents. X-ray diffraction indicates both that the basal spacing of the clay increases from 12.43 angstrom to 16.4 angstrom upon intercalation of cobalt and iron. Atomic absorption analysis of the catalysts indicates that virtually all of the sodium ions originally present in the clay have been replaced by iron(III) and cobalt(II). Transmission electron micrographs show the presence of multiwall carbon nanotubes with inner and outer diameters of ca. 10 nm and 20 nm grown on metal particles present on the plates of catalysts. The iron-57 Mossbauer spectra indicate that the intercalated clay contains iron(III) in octahedral and tetrahedral sites and iron(II) in octahedral sites, the catalysts contain an extensive amount of small superparamagnetic particles of alpha-Fe2O3 and that the carbon-nanotube catalyst composites show the presence of iron(II) and iron(III) paramagnetic doublets, characteristic of a reduced montmorillonite, and of sextets that are characteristic of an FeCo alloy and of Fe3C cementite. The Mossbauer spectra indicate that the carbon nanotubes grow on FeCo metallic nanoparticles and bond to these particles through the formation of cementite.