The emerging trends in carbon nanotube applications make them exceptional functional materials of highly added value. Thermocatalytic CH decomposition is an effective pathway toward their production, forming H as the only byproduct. However, catalyst deactivation due to sintering and blockage of the active sites, together with their detachment from the support remains a challenge. In this work, nickel phyllosilicate is employed as a catalyst precursor for the formation of active and stable metal sites. Surprisingly, the particles remain attached to the support, switching from the typical tip-growth reported for state-of-the-art catalysts to a base growth mechanism. The nickel nanoparticles remain stable against sintering even under harsh conditions up to 750 °C. A combination of DFT calculations, in situ TEM, and in situ XRD studies reveals that the reduction of Ni─O bonds, particularly those involving silicon-bonded oxygen (Si─O─Ni; apical oxygen), requires high temperatures. Post-activation, the small, dispersed nickel nanoparticles catalyze CH decomposition into carbon nanotubes and H. Unlike prior reports, in situ XRD confirms no nickel carbide formation in the bulk. Additionally, in contrast to any known nickel-based catalyst, it is demonstrated that particles below 10 nm can effectively activate CH cracking, avoid encapsulation, and enable the base-growth of micrometer-long, narrow carbon nanotubes.