Picrite basalt orpicrobasalt is a variety of high-magnesium olivinebasalt that is very rich in the mineralolivine. It is dark with yellow-green olivinephenocrysts (20-50%) and black to dark brownpyroxene, mostlyaugite.
The olivine-rich picrite basalts that occur with the more commontholeiitic basalts ofKīlauea and other volcanoes of theHawaiian Islands are the result of accumulation of olivine crystals either in a portion of themagma chamber or in acaldera lava lake.[1] 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-richalkali basalt: such picrite consists largely ofphenocrysts of olivine andtitanium-richaugite pyroxene with minorplagioclase set in a groundmass of augite and more sodic plagioclase and perhapsanalcite andbiotite.
More generally the classification of fine grained rocks recognizes a group known as 'picritic rocks' that are characterised by high magnesium content and low SiO2 content. They fit in theTAS classification system only at the lowest level of SiO2 (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.[2] They are largely restricted to theArchean. In contrast, picrites are magnesium-rich because crystals of olivine have accumulated in more normal melts by magmatic processes.[1]
Picrite basalt is found in thelavas ofMauna Kea andMauna Loa inHawaiʻi[3],Curaçao, in thePiton de la Fournaise[4] volcano onRéunion Island and various other oceanic island volcanoes.
In addition to extrusive occurrences, it also occurs in minor intrusions.
Oceanite is a variety of picritic basalt characterized by its large amounts of olivine phenocrysts and lesser amounts ofaugite and by having a groundmass of olivine, plagioclase and augite. The term was coined byAntoine Lacroix in 1923 for rare basalts with more than 50% olivine.[7]
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.[2][3]
{{cite journal}}:Cite journal requires|journal= (help)Types ofbasalts | |
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| Basalts by tectonic setting | |
| Basalts by form and flow | |
| Basalts by chemistry | |
| Important minerals | |
