What is Structural formula, Mg number?

The structural formula of a mineral gives, for a fixed number of oxygen atoms, the number of atoms of each element and the site each occupies, the ions sharing a site written together: olivine $\mathrm{(Mg, Fe)_2SiO_4}$, a plagioclase $\mathrm{(Na_{0.5}Ca_{0.5})(Al_{1.5}Si_{2.5})O_8}$. The Mg number of a mineral or a rock is

the numbers of atoms being counted in moles (or per formula unit).