Larvikite

[Monzonite]

Photographed by Michael P. Klimetz

Permian

Oslo Graben

Larvik

NORWAY

Larvikite is a variety of monzonite, notable for the presence of handsome, thumbnail-sized crystals of feldspar. These feldspars are known as ternary because they contain significant components of all three end-member feldspars. The feldspar has partly unmixed on the micro-scale to form a perthite, and the presence of the alternating alkali feldspar and plagioclase layers give its characteristic silver blue sheen (Schiller effect) on polished surfaces. Olivine can be present along with apatite, and locally quartz. Usually titanium rich, with titanaugite and/or titanomagnetite present. The name originates from the town of Larvik in Norway, where this type of igneous rock is found. Intrusions of Larvikite form part of the suite of igneous rocks that were emplaced during the Permian, associated with the formation of the Oslo Rift. The crystallization of a ternary feldspar indicates that this rock began to crystallize under lower crustal conditions.