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Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into hematite and other iron oxides and hydroxides.
Although usually characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes.
Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt.
In tholeiitic basalt, pyroxene (augite and orthopyroxene or pigeonite) and calcium-rich plagioclase are common phenocryst minerals.
and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase.
This is as per definition of the International Union of Geological Sciences (IUGS) classification scheme.
Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are generally referred to as diabase (also called dolerite) or, when more coarse-grained (crystals over 2 mm across), as gabbro.
Agricola applied "basalt" to the volcanic black rock of the Schloßberg (local castle hill) at Stolpen, believing it to be the same as the "very hard stone" described by Pliny the Elder in Naturalis Historiae.
and has also formed on the Moon, Mars, Venus, and the asteroid Vesta.
Many alkali basalts may be formed at greater depths, perhaps as deep as 150–200 km. The abundances of the lanthanide or rare-earth elements (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled.
In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly.