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Encyclopedia Britannica - Main :: DIO-DRO |
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DOLERITE (from Gr. SoXepos, deceptive) , in petrology., the name given by Hauy to those basaltic rocks which are comparatively coarse grained and nearly, if not. quite, holocrystalline. 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, viz. olivine, augite and plagioclase felspar, while hornblende, ilmenite, apatite and hiotite are their commonest accessory ingredients. The chemical and microscopic features of these minerals agree generally with those presented in the basalts, and only their exceptional peculiarities need be mentioned here. Many dolerites are porphyritic and carry phenocrysts of olivine, augite and plagioclase felspar (or of one or more of these). Others, probably the majority , are non-porphyritic,and these are generally coarser grained than the ground-mass of the former group; thoughDOLET 3$7 Iacking their large conspicuous phenocrysts. The commonest type of structure in dolerite is the ophitic, which results from the felspar of the rock
mineral
mineral
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
pale
Allied to these are olivine-free dolerites with more or less of interstitial glassy base (tholeites,&c.). 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 interspaces 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 England, Scotland , on the continent and in America. Often they are comparatively rich in brown hornblende. This last-named mineral is not usually abundant in "dolerites, but in a special
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 kerb-stones, 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
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