Larvikite

Larvikite

Igneous rock
Larvikite from Larvik, Norway
Composition
Plagioclase, alkali feldspar, amphibole

Larvikite is an igneous rock, specifically a variety of monzonite,^[1] notable for the presence of thumbnail-sized crystals of feldspar. These feldspars are known as ternary because they contain significant components of all three endmember feldspars.^[1] 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 schiller effect (called labradorescence) on polished surfaces. Olivine can be present along with apatite, and locally quartz. Larvikite is usually rich in titanium, with titanaugite and/or titanomagnetite present.

Larvikite occurs in the Larvik Batholith (also called the Larvik Plutonic Complex), a suite of ten igneous plutons emplaced in the Oslo Rift (Oslo Graben) surrounded by ~1.1 billion year old Sveconorwegian gneisses. The Larvik Batholith is of Permian age, about 292–298 million years old.^[2] Larvikite is also found in the Killala Lake Alkalic Rock Complex near Thunder Bay in Ontario, Canada.^[3]

The name originates from the town of Larvik in Norway, where this type of igneous rock is found. Many quarries exploit larvikite in the vicinity of Larvik.