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DIABASE

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Originally appearing in Volume V08, Page 145 of the 1911 Encyclopedia Britannica.
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DIABASE , in See also:

petrology, a See also:rock which is a weathered See also:form of See also:dolerite. It was See also:long widely accepted that the pre-See also:Tertiary rocks of this See also:group differed from their Tertiary and See also:Recent representatives in certain essential respects, but this is now admitted to be untenable, and the See also:differences are known to be merely the result of the longer exposure to decomposition, pressure and shearing, which the older rocks have experienced. Their See also:olivine tends to become serpentinized; their See also:augite changes to See also:chlorite and uralite; their felspars are clouded by formation of See also:zeolites, See also:calcite, sericite and See also:epidote. The rocks acquire a See also:green See also:colour (from the development of chlorite, uralite and epidote); hence the older name of " greenstones," which is now little used. Many of them become somewhat schistose from pressure (" greenstone-See also:schists," See also:meta-diabase, &c.). Although the See also:original See also:definition of the group can no longer be justified, the name is so well established in current usage that it can hardly be discarded. The terms diabase and dolerite are employed really to designate distinct facies of the same set of rocks. The minerals of diabase are the same as those of dolerite, viz. olivine, augite, and See also:plagioclase See also:felspar, with subordinate quantities of See also:hornblende, See also:biotite, See also:iron oxides and See also:apatite. There are olivine-diabases and diabases without olivine; See also:quartz diabases, See also:analcite-diabases (or teschenites) and hornblende diabases (or proterobases). See also:Hypersthene (or See also:bronzite) is characteristic of another group. Many of them are ophitic, especially those which contain olivine, but others are intersertal, like the intersertal dolerites. The last include most quartz-diabases, hypersthenediabases and the rocks which have been described as tholeites.

Porphyritic structure appears in the diabase-porphyrites, some of which are highly vesicular and contain remains of an abundant See also:

fine-grained or partly glassy ground-See also:mass (diabas-mandelstein, amygdaloidal diabase). The somewhat See also:ill-defined spilites are regarded by many as modifications of diabase-porphyrite. In the intersertal and porphyrite diabases, fresh or devitrified glassy See also:base is not infrequent. It is especially conspicuous in some tholeites (hyalo-tholeites) and in weisselbergites. These rocks consist of augite and plagioclase, with little or no olivine, on a See also:brown, vitreous, interstitial See also:matrix. Devitrified forms of tachylyte (sordawilite, &c.) occur at the rapidly chilled margins of dolerite sills and dikes, and fine-grained spotted rocks with large See also:spherulites of See also:grey or greenish felspar, and branching growths of brownish-green augite (See also:variolites). To nearly every variety in See also:composition and structure presented by the diabases, a counterpart can be found among the Tertiary dolerites. In the older rocks, however, certain minerals are more See also:common than in the newer. Hornblende, mostly of See also:pale green See also:colours and somewhat fibrous See also:habit, is very frequent in diabase; it is in most cases secondary after See also:pyroxene, and is then known as uralite; often it forms pseudomorphs which retain the shape of the original augite. Where diabases have been crushed or sheared, hornblende readily develops at the expense of pyroxene, sometimes replacing it completely. In the later stages of alteration the See also:amphibole becomes compact and well crystallized; the rocks consist of green hornblende and plagioclase felspar, and are then generally known as epidiorites or amphibolites. At the same See also:time a schistose structure is produced.

But transition forms are very common, having more or less of the augite remaining, surrounded by newly formed hornblende which at first is rather fibrous and tends to spread outwards through the surrounding felspar. Chlorite also is abundant both in sheared and unsheared diabases, and with it calcite may make its See also:

appearance, or the See also:lime set See also:free from the augite may combine with the See also:titanium of the iron See also:oxide and with See also:silica to form incrustations or See also:borders of See also:sphene around the original crystals of See also:ilmenite. Epidote is another secondary lime-bearing See also:mineral which results from the decomposition of the soda lime felspars and the pyroxenes. Many diabases, especially those of the teschenite sub-group, are filled with zeolites. Diabases are exceedingly abundant among the older rocks of all parts of the globe. Popular names for them are " whinstone," " greenstone," " toadstone " and " See also:trap." They form excellent road-mending stones and are much quarried for this purpose, being tough, durable and resistant to See also:wear, so long as they are not extremely decomposed. Many of them are to be preferred to the fresher dolerites as being less brittle. The'quality of the Cornish greenstones appears to have been distinctly improved by a smaller amount of recrystallization where they have been heated by contact with intrusive masses of See also:granite. (J. S.

End of Article: DIABASE

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