  The hydrothermal synthesis of novel materials typically relies on both
knowledge of the redox activities of all cations present in the reaction
solution and a small toolset of so-called mineralizers to tune the solution's
overall chemical potential. Upon the use of a less conventional mineralizer
species, SiO$_2$, we show the stabilization of spiroffite-type
Co$_2$Te$_3$O$_8$ under less forceful hydrothermal conditions than in previous
reports. When synthesized in the presence of both SiO$_2$ and each respective
alkali carbonate as a secondary mineralizer, silicon substitution in place of
tellurium in the host structure becomes apparent, and the corresponding
introduced disorder gives rise to enhanced low-temperature ferromagnetism. Our
results highlight the complexities of underutilized and combined mineralizer
species in the stabilization and tuning of complex magnetic ground states via
hydrothermal synthesis techniques.
