Picrite basalt or picrobasalt is a variety of high-magnesium olivine

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

that is very rich in the mineral

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

. It is dark with yellow-green olivine

phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...

s (20-50%) and black to dark brown

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

, mostly augite. The olivine-rich picrite basalts that occur with the more common tholeiitic basalts of Kīlauea and other volcanoes of the

Hawaiian Islands The Hawaiian Islands () are an archipelago of eight major volcanic islands, several atolls, and numerous smaller islets in the Pacific Ocean, North Pacific Ocean, extending some from the Hawaii (island), island of Hawaii in the south to nort ...

are the result of accumulation of olivine crystals either in a portion of the

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

or in a caldera lava lake. The compositions of these rocks are well represented by mixes of olivine and more typical tholeiitic basalt. The name "picrite" can also be applied to an olivine-rich alkali basalt: such picrite consists largely of

s of olivine and titanium-rich augite pyroxene with minor

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

set in a groundmass of augite and more sodic plagioclase and perhaps analcite and

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

. More generally the classification of fine grained rocks recognises a group known as 'picritic rocks' that are characterised by high magnesium content and low SiO2 content. They fit in the

TAS classification TAS stands for Total Alkali Silica. The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali and silica contents. These chemical parameters are useful because ...

system only at the lowest level of Si02 (41 to 43% by weight) and Na2O + K2O (up to 3% by weight). They include picrite, komatiite and meimechite. Picrites and komatiites are somewhat similar chemically (defined as >18% MgO), but differ in having 1 to 3% total alkalis and less than 1% total alkalis respectively. Komatiite lavas are products of more magnesium-rich melts, and good examples exhibit the spinifex texture. They are largely restricted to the

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

. In contrast, picrites are magnesium-rich because crystals of olivine have accumulated in more normal melts by magmatic processes. Picrite basalt is found in the

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s of

Mauna Kea Mauna Kea (, ; abbreviation for ''Mauna a Wākea''); is a dormant Shield volcano, shield volcano on the Hawaii (island), island of Hawaii. Its peak is above sea level, making it the List of U.S. states by elevation, highest point in Hawaii a ...

and Mauna Loa in

Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...

Curaçao Curaçao, officially the Country of Curaçao, is a constituent island country within the Kingdom of the Netherlands, located in the southern Caribbean Sea (specifically the Dutch Caribbean region), about north of Venezuela. Curaçao includ ...

, in the Piton de la Fournaise volcano on Réunion Island and various other oceanic island volcanoes. *Picrite basalt has erupted in historical times from Mauna Loa during the eruptions of 1852 and 1868 (from different flanks of Mauna Loa). *Picrite basalt with 30% olivine commonly erupts from the Piton de la Fournaise. In addition to extrusive occurrences, it also occurs in minor intrusions.

Oceanite

Oceanite is a variety of picritic basalt characterized by its large amounts of olivine phenocrysts and lesser amounts of augite and by having a groundmass of olivine, plagioclase and augite. The term was coined by Antoine Lacroix in 1923 for rare basalts with more than 50% olivine.

Common uses

Olivine basalt is commonly used by foundries, boilermakers and boiler users to protect the area around a burner tip or to protect a floor from molten metal and other slag. Its use in this fashion is appropriate since olivine is a highly refractory, high-melting-temperature mineral.

References

* * * . * * * * {{DEFAULTSORT:Picrite Basalt Aphanitic rocks Mafic rocks Ultramafic rocks Volcanology Basalt