Olivine Dolerite

Olivine dolerite, a variant of dolerite, also known as diabase, is an intrusive fine grained igneous mafic rock with a crystalline texture. It is sub-volcanic in origin, forming at medium to shallow depths within the earths crust, usually less than 2 Km deep and similar in composition to gabbros and basalts, sometimes being confused as such upon first inspection. The rock is black in colour and is composed of expected mafic minerals such as augite and magnetite as well as plagioclase feldspar laths. As expected from the name, olivine is also present in quantity in these samples which can be viewed under hand magnification as dark green crystals within the rock but can be visible with the naked eye as some are up to 3 mm in size. These samples come from Clee Hill, Shropshire, UK in an area rich in carboniferous coal seams which have been intruded by numerous olivine dolerite sills which are suspected to have formed during the Westphalian stage of the Permian period, 295 million years ago. Due to the hardness of the dolerite, the surrounding rocks were more susceptible to erosion, resulting in the igneous rock being visible the most, with its black colouration, the stone is known locally as ‘Dhustone’, derived from the Welsh word for ‘Black stone’. Samples are fairly dense due to the abundance of iron bearing pyroxene minerals, overall, they are suitable for collectors, particularly those interested in igneous activity as well as higher level education to indicate various igneous rocks and chemistries one may come across in the field, especially among rocks which may look like another upon first glance. Specimens are available in various sizes and come supplied in a card tray with an information label.