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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)?

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$M_i$ 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.

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