Petrology of the soda-minette dikes from Håøya, Langesundsfjord, S. Norway

The soda-minette dikes from Håöya are dark porphyritic rocks essentially composed of plagioclase (An23 to An30), orthoclase-antiperthite, aegirinediopside, biotite, sphene, apatite, rutile and ore. Chemically, the soda-minettes contain a high percentage of soda in comparison with the minettcs described elsewhere. The sequence of formation of feldspars in the phenocryst and groundmass is traced in the light of the optical and X-ray properties of the mineral and the chemistry of the environment. Emphasis is particularly put on the conditions necessary for the formation of ocellar structure in the rock. These dikes were emplaced mainly in the nepheline syenite pegmatites and were locally deformed while still semiconsolidated.