Larvikite is a variety of monzonite

Monzonite

Monzonite is an intermediate igneous intrusive rock composed of approximately equal amounts of sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende, biotite and other minerals...

, 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

Perthite

Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar . Typically the host grain is orthoclase or microcline, and the lamellae are albite...

, and the presence of the alternating alkali feldspar

Alkali feldspar

The alkali feldspar group are those feldspar minerals rich in the alkali elements like potassium. The alkali feldspars include: anorthoclase, microcline, orthoclase and sanidine....

 and plagioclase layers give its characteristic silver blue sheen (Schiller effect) on polished surfaces. Olivine

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

 can be present along with apatite

Apatite

Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...

, and locally quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

. Usually titanium rich, with titanaugite

Augite

Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...

 and/or titanomagnetite

Magnetite

Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

 present.

The name originates from the town of Larvik

Larvik

is a city and municipality in Vestfold county, Norway. The administrative centre of the municipality is the city of Larvik. Larvik kommune - has about 41 364 inhabitants and covers 530 km2....

 in Norway

Norway

Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

, where this type of igneous rock is found.

Formation

Intrusions of Larvikite form part of the suite of igneous rock

Igneous rock

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

s that were emplaced during the Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "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.

Uses

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

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

, although this is not an accurate description.

External links

• Larvikitt.no
• The Larvik Plutonic Complex
• R. V. Dietrich, 2006, Gemrocks
• 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