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.