Fig. 6. Comparison of the bond-valence constraints between polymerization units [SiO₄]⁴⁻ and [M⁶⁺N₄]⁶⁻; in addition to matching the prolific potential for polymerization of silicate minerals, ^[4]M⁶⁺ nitridometalates necessarily form harder compounds, and may be tuned via external factors.