A chemical formula that represents the ideal mineral composition of a rock, with each constituent expressed as the percentage by weight (wt. %). This is used in the normative classification of rocks (see cipw norm calculation). The minerals most commonly used in normative classification are: quartz (Q); corundum (C); orthoclase (Or, see alkali feldspar); albite (Ab, see alkali feldspar; plagioclase feldspar); anorthite (An, see alkali feldspar); nepheline (Ne); wollastonite (Wo); enstatite (En); ferrosilite (Fs, see orthopyroxene); forsterite (Fo, see olivine); fayalite (Fa, see olivine); ilmenite (Il); magnetite (Mt); and apatite (Ap).