OXIDE
Geologists and geochemists typically report their data in wt.% oxides, as oxygen is the dominant element in earth materials and many elements in nature are bonded with oxygen in crystal lattices or glass. Thus, silicon (Si) is reported as silicon dioxide or silica (SiO2), Ca as CaO, Na as Na2O, phosphorus as P2O5. These are discrete, stoichiometric ratios with charge balance.
Some elements can easily exist in different valence states (e.g. Fe, Cr) but for convenience sake are nominally assumed to exist in one form (e.g., FeO, Cr2O3). They can be easily "translated" to elemental weight values; simply create a fraction using atomic weights from the periodic table, with numerator the element wt. % and denominator the oxide wt.% summed up. Thus to determine Si from SiO2, one would divide 28.09 by (28.09 + [2 x 16.00]), yielding the fraction 0.4675. Thus, something at is 100 wt.% SiO2 is actually 46.75 wt.% Si and the balance oxygen by weight. And elemental wt.% values can be converted to oxide wt.% by the inverse. In a Probe for EPMA run, the sample compositions are reported as oxides if the Display As Oxides option is selected for that sample.