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A dark-coloured, medium-grained igneous rock which contains plagioclase feldspar of labradorite composition and pyroxene of augite or titanaugite composition as essential minerals, and magnetite, titanomagnetite, or ilmenite as accessory minerals. Where olivine also occurs as an additional mineral, the rock is termed an ‘olivine dolerite’. Where quartz occurs as an additional mineral in the groundmass, the rock is termed a ‘quartz dolerite’. Dolerites are the medium-grained equivalents of basalts and, like the basalts, can be divided into alkali and tholeiitic types. Diabase (dolerite) are commonly found in shallow level intrusions such as dykes, sills, or plugs. — A Dictionary of Geology and Earth Sciences (4 ed.), Oxford University Press