10.5 A Composition Axis View

The bulk chemical composition of the two-component MgO-SiO2 system can be shown on a composition axis in mass units or mole units. Both axes are shown here with sliders that allow you to change the bulk composition and see the results of the norm calculation (the software does the algebra, still using the approximate gfw values for MgO (40g/mol) and SiO2 (60g/mol)). Move the slider.



Mineral Formula Mole % Wt. %
forsterite Mg2SiO4   0.00 0.00
enstatite MgSiO3   0.00 0.00
quartz SiO2   0.00 0.00
    Total 0.00

The composition axis is a graph of the CIPW norm algebra. If the bulk composition (the slider position) is between enstatite (en) and quartz (Q), the CIPW norm will have (en) and (Q) as the normative mineralogy. If the bulk composition is between forsterite (fo) and enstatite (en), the CIPW norm will have (fo) and (en) as the normative mineralogy. Notice that if the bulk compositon is between MgO and (fo), the result is a negative value for (en).

Adding more oxides to the chemical analysis add more steps to the norm calculation, but the principles are the same. An example using using three oxides and a ternary diagram is shown on the   .