The groundmass contains interstitial quartz or tridymite or cristobalite.
Olivine tholeiitic basalt has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass.
The modern petrological term basalt describing a particular composition of lava-derived rock originates from its use by Georgius Agricola in 1556 in his famous work of mining and mineralogy De re metallica, libri XII.
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.
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.
Gabbro is often marketed commercially as "black granite." In the Hadean, Archean, and early Proterozoic eras of Earth's history, the chemistry of erupted magmas was significantly different from today's, due to immature crustal and asthenosphere differentiation.
In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly.
They are tholeiitic basalts particularly low in total alkalis and in incompatible trace elements, and they have relatively flat rare earth element (REE) patterns normalized to mantle or chondrite values.In tholeiitic basalt, pyroxene (augite and orthopyroxene or pigeonite) and calcium-rich plagioclase are common phenocryst minerals.Olivine may also be a phenocryst, and when present, may have rims of pigeonite.Photomicrograph of a volcanic (basaltic) sand grain; upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.Note white plagioclase "microlites" in cross-polarized light picture, surrounded by very fine grained volcanic glass.These ultramafic volcanic rocks, with silica (Si O) contents below 45% are usually classified as komatiites.