Colour When fresh it is black, dark grey or green; may be mottled black and white. Grain size Medium. Texture Occasionally ophitic texture (see gabbro) can be distinguished in hand specimen. May be porphyritic. Structure Vesicles and amygdales occur. Sometimes has segregations of coarser rock enriched in feldspar. Mineralogy Phenocrysts comprise olivine (olivine diabase) and/or pyroxene or plagioclase. The groundmass comprises the same minerals with iron oxide, and sometimes with some quartz, hornblende or biotite. Field relations Dykes and sills. These may form swarms of hundreds or perhaps thousands of individual dykes or sills which often radiate from a single volcanic centre.
Igneous rock formed below the Earth's surface, a form of basalt, containing relatively little silica (mafic in composition). Dolerite is a medium-gra
(Dolerite) Colour When fresh it is black, dark grey or green; may be mottled black and white. Grain size Medium. Texture Occasionally...
n 1 a type of rock made up of large crystals of, among other constituents, olivine and amphibole and which is found in an analcite groundmass