What are the formula weight and gram-formula unit (gfu)?
$M_i$ is the molar mass of the ith element. Dividing $w_i$ by $M_i$ yields the number of moles of that element in the sample.

There are two primary ways of looking at abundances. One is by weight (or mass), the other is by number of atoms. Consider beryllium and uranium. Their abundances are very similar when looked at in terms of mass: 1.9 parts per million (ppm) for beryllium versus 1.8 ppm for uranium. In terms of molar abundance, they aren't so similar. In this sense, beryllium's abundance is 4.3 ppm; for uranium, it's only 0.15 ppm.