dolerite

A medium-grained basic igneous rock, dark-green in colour and composed mainly of plagioclase feldspar and pyroxene crystals. It is the common rock of dykes and sills throughout the world.

Dolerite (IPA: /ˈdɒlərʌɪt/, Greek: doleros, meaning "deceptive"), in petrology is the name given by Hauy to those basaltic rocks which are comparatively coarse grained.

As may be inferred from their highly crystalline state they are very often intrusive, and occur as dikes and sills, but many of them form lava flows. Their essential minerals are those of basalt, namely olivine, augite and plagioclase feldspar, while hornblende, ilmenite, apatite and biotite are their most common accessory ingredients. The chemical and microscopic features of these minerals agree generally with those presented in the basalts.

The quartz-dolerites are an important group, hardly less common than the olivine-dolerites. In the rocks of this group ophitic structure is typically absent, and the presence of an interstitial finely crystalline or amorphous material gives rise to the structure which is known as intersertal. The edges of dolerite sills and dikes often contain, much dark brown glass, and pass into tachylytes, in which this material preponderates.

Another interesting group of doleritic rocks contains analcite. Their characteristic feature is the presence of a small amount of analcite, which never shows crystalline outlines but fills up the intersp.aces between the other minerals. Some writers held that this mineral has resulted from the decomposition of nepheline; others regard it as a primary mineral. These rocks are known as teschenites, and have a wide distribution in Europe and in The Americas. This last-named mineral is not usually abundant in dolerites, but in a special group, the proterobases, it to a large extent replaces the customary augite. A few dolerites contain much brown mica (mica-dolerites). Nepheline may appear in these rocks, as in the basalts.

Dolerites have a very wide distribution, as they are found wherever basalts occur in any number. They are much employed for road-mending and for kerbstones, though their dark colour and the tendency they have to weather with a dingy brown crust make them unsuitable for the better classes of architectural work.