# What are the formula weight and gram-formula unit (gfu)?

I'm reading Shaw's "Trace Elements in Magmas" and in his page 16 he writes the following:

...It is necessary to look at the relationship between $c_i$ and $x_i$, i.e. the weight and molar concentrations. In any phase of weight $W$ the weight of component $i$ is $w_i$ and if the formula weight of $i$ is $M_i$, then the number of gfu (gram-formula unit concentration) of $i$ is $n_i$, where $n_i = \frac{w_i}{M_i}$...

What does he mean when he mentions the "formula weight" ($M_i$) and the gfu (gram formula unit)?

$M_i$ 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.