Dolerite

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

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. They contain a small amount of quartz, and often micropegmatite, as the last element to consolidate, filling up little angular interspaces between the feldspars and pyroxenes, which had previously crystallized.

Allied to these are olivine-free dolerites with more or less of interstitial glassy base. 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. Transitions to the porphyritic dolerites and basalts arise by increase in the proportion of this ground-mass. 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. They may be ophitic, though often they are not, and they usually contain olivine, while their augite has distinctly purple shades, and a feeble dichroism. 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. Usually it can be clearly shown to be secondary to some extent, but there is reason to suppose that it is really a pneumatolytic deposit. These rocks are known as teschenites, and have a wide distribution in Europe and in The Americas. Often they are comparatively rich in brown hornblende. 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. Typical nepheline-dolerites are scarce, and consist of idiomorphic augite, surrounded by nepheline. Examples are known front the Tertiary volcanic districts of the Rhine.

Dolerites have a very wide distribution, as they are found wherever basalts occur in any number. It is superfluous to cite localities for them as they are among the commonest of igneous rocks. 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.

[edit] References

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.