Braid perthite in nepheline syenite pegmatite, Langesundsfjorden, Oslo Region (Norway)

Three occurrences of braid perthite (lamellae parallel to (110) and ( )) and micro braid perthite (lamellae parallel to ( ) and ( )) from Permian nepheline syenite pegmatites in monzonite (larvikite) of the Oslo Graben are described. All the crystals studied show a zoned arrangement with either alternating mmacro- and microperthitic bands parallel to (010) or with microperthitic cores surrounded by macroperthite and a marginal zone of plagioclase. Both types of perthite are mesoperthitic and composed of maximum microcline and low albite. Chemical analyses suggest that the alkali feldspar is stoichiometric, while precession exposures imply unusual cell parameters in the host K-feldspar phase. The macroperthite seems to have crystallized at the expense of the microperthite. The braided texture of exsolved albite lamellae may be due to some kind of strain causing cracking along the prism and pyramidal planes.