[en] With recent literature demonstrating enhancement of the thermoelectric performance of nanoscale materials relative to their corresponding bulk materials, methods to synthesize low-dimensional nanomaterials in large scale at low cost are needed. We demonstrate a method for preparing nanostructured dimagnesium silicide (Mg₂Si) thermoelectric materials that are nanocomposites with MgO by the reduction of diatomaceous earth (diatoms) using a gas-displacement solid state reaction with magnesium vapor. The resulting semiconducting Mg₂Si preserves the general morphology of the original diatoms and their nanosized grains at least down to the size of 30 nm. This reaction represents a possible method for the production of large quantities of low-cost nanoscale thermoelectric materials with potential for enhanced thermoelectric performance. - Graphical abstract: A nanostructured Mg₂Si and MgO nanocomposite thermoelectric material is synthesized in the Mg gas-displacement solid state reduction of SiO₂ from diatomaceous earth. The resulting semiconducting Mg₂Si nanostructures preserve the original diatom morphology, with nanosized grains at least down to the size of 30 nm