Normative mineralogy is an estimate of the mineralogy of the rock. It usually differs from the visually observable mineralogy, at least as much as the types of mineral species, especially amongst the ferromagnesian minerals and feldspars, where it is possible to have many solid solution series of minerals, or minerals with similar Fe and Mg ratios substituting, especially with water (e.g.; amphibole and biotite replacing pyroxene).
However, in aphanites, or rocks with phenocrysts clearly out of equilibrium with the groundmass, a normative mineral calculation is often the best to understand the evolution of the rock and its relationship to other igneous rocks in the region.