The relation between haematite and potash feldspar is discussed. These parageneses are often encountered in Norway. The solubility of Fe2O3 in microcline in solid state is determined by means of radioactive iron (fig. 5.) The solubility is normally low. It is conduded that e. g. the aventurine feldspar originally crystallized from a magma, poor in aluminum or rich in alcali. Thus the feldspars became rich in iron. A subsequent aluminum metasomatism replaced the iron atoms in the feldspar lattices. The iron thus expelled, formed the iron oxide lamellae in and between the feldspar crystals.