Larvikite is an igneous rock, specifically 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, 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 (a.k.a. Larvik Plutonic Complex), a suite of 10 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. Larvikite is also found in the Killala Lake Alkalic Rock Complex near Thunder Bay in Ontario, Canada.
Intrusions of Larvikite form part of the suite of igneous rocks that were emplaced during the Permian period, associated with the formation of the Oslo rift. The crystallisation of a ternary feldspar indicates that this rock began to crystallise under lower crustal conditions.
This rock is very popular in the High Street in the UK, for its high polish and the spectacular reflections from its feldspars and is frequently found fronting the facades of banks. It is sometimes known informally as Blue Granite, although this is not an accurate description. It is sometimes used as a gemstone.
- The Making of a Land: Geology of Norway. 2008. Ramberg, I.B., Bryhni, I., Nottvedt, A. & Rangnes, K. (eds). Geological Society of Norway, 640pp.
- Larvikite geology
- Sage, R.P. (1988). Geology of Carbonatite - Alkalic Rock Complexes in Ontario: Killala Lake Alkalic Rock Complex, District of Thunder Bay, Ontario Geological Survey Study 45 (PDF). Toronto: Ontario Geological Survey and the Ministry of Northern Development and Mines. pp. 9–18. ISBN 0-7729-0580-0.
- Rock Roles: Facts, Properties, and Lore of Gemstones By Suzanne Bettonville, p. 57
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- Petrogenesis of the Oslo Region Larvikites and Associated Rocks (abstract) Journal of Petrology, 1980, volume 21, Number 3, pages 499-531
- Structure of the larvikite-lardalite complex, Oslo-region, Norway, and its evolution (abstract) International Journal of Earth Sciences, 1978, volume 67, number 1, pages 330-342
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