Those structures do not consist of discrete molecules, and so for them, the term formula unit is used.
[3] The formula unit is used as an independent entity for stoichiometric calculations.
[4][5] Examples of formula units, include ionic compounds such as NaCl and K2O and covalent networks such as SiO2 and C (as diamond or graphite).
For example, the ionic compounds potassium persulfate (K2S2O8), mercury(I) nitrate Hg2(NO3)2, and sodium peroxide Na2O2, have empirical formulas of KSO4, HgNO3, and NaO, respectively, being presented in the simplest whole number ratios.
[7] In mineralogy, as minerals are almost exclusively either ionic or network solids, the formula unit is used.