Microgabbro is a medium-grained intrusive igneous rock. It contains crystals, smaller than rice grains, which are interlocking and randomly oriented. It is dark green to dark grey, with occasional rare paler crystals.
How is it formed?
Dolerite is the medium-grained equivalent of gabbro. The crystals are slightly smaller than gabbro, indicating that the magma cooled more quickly. It usually occurs as small intrusions called ‘dykes’ or ‘sills’ which are sheet-like and cut through the surrounding rocks.
Like gabbro, dolerite forms from magma that is rich in iron and magnesium, and poor in silica (quartz).