Use of Oxide Data

Mineralogists frequently report percent composition data for minerals that contain oxygen in terms of the oxides of the metals (and sometimes some of the non-metals, see below). For example, the data for spinel would be reported as MgO 28.2%, AI2O3 71.8%. This does not mean that spinel is a mixture of these oxides. 

In order to calculate the empirical formula from oxide data, we need to know what anion is present and we must convert the oxide percentages to moles of each cation. Spinel is an oxide, which means that we can assume that the anion is the ion. 

convert the percentages of MgO and AI2O3 to relative numbers of moles of Mg and Al. 

we assume our usual lOOg sample, which would contain 28.2g of MgO and 71.8g AI2O3. Next, we determine the number of moles of each of these oxides by dividing each mass by the molar mass of the oxide  

Finally, we convert these mole amounts to the number of moles of Mg and Al. Because one mole of MgO contains one mole of Mg, 0.700 mole of MgO must contain 0.700 mole of Mg atoms. One mole of AI2O3 contains two moles of aluminum per mole of AI2O3, and therefore 0.704 mole of AI2O3 is equivalent to 2 x 0.704 = 1.408 mole Al. 

---