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Olivine tholeiite 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.
Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine.
☞ It is usually of a greenish black color, or of some dull brown shade, or black.
It constitutes immense beds in some regions, and also occurs in veins or dikes cutting through other rocks.
Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform.
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.The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.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 (coarse) groundmass are generally referred to as diabase (also called dolerite) or gabbro.Note white plagioclase 'microlites' in cross-polarized light picture, surrounded by very fine grained volcanic glass.The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvospinel, and ilmenite.Olivine may also be a phenocryst, and when present, may have rims of pigeonite.The groundmass contains interstitial quartz or tridymite or cristobalite.Feldspar phenocrysts typically are labradorite to andesine in composition.Augite is rich in titanium compared to augite in tholeiitic basalt.The majority of tholeiites are formed at approximately 50–100 km depth within the mantle.Many alkali basalts may be formed at greater depths, perhaps as deep as 150–200 km.