- Aplite
Aplite (pronEng|ˈæplaɪt) in
petrology , the name given to intrusive rock in whichquartz andfeldspar are the dominantmineral s. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens. Dykes and threads of aplite are commonly observed traversinggranitic bosses; they occur also, though less frequently, insyenite s,diorite s,quartz -diabase s andgabbro s.Aplites usually have a genetic affinity to the rocks they intersect. The aplites of
granite areas, for example, are the last part of themagma to crystallize, and correspond in composition to the quartzo-feldspathic aggregates that fill up the interspaces between the early minerals in the main body of the rock. They bear a considerable resemblance to theeutectic mixtures which are formed on the cooling of solutions of mineral salts, and remain liquid till the excess of either of the components has separated out, finally solidifying "en masse" when the proper proportions of the constituents and a suitable temperature are reached.The essential components of aplites are quartz and alkali feldspar (the latter usually
orthoclase or microperthite ),microcline andalbite .Crystallization has been apparently rapid (as the rocks are so fine-grained), and the ingredients have solidified almost at the same time. Hence theircrystal s are rather imperfect and fit closely to one another in a sort of fine mosaic of nearly equi-dimensional grains.Porphyritic feldspar s occur occasionally and quartz more seldom; but the relation of the aplites to quartz-porphyries,granophyre s andfelsite s is very close, as all these rocks have nearly the same chemical composition.The aplites associated with diorites and quartz-diabases differ in minor respects from the common aplites which accompany granites. The accessory minerals of these rocks are principally
oligoclase ,muscovite ,apatite andzircon .Biotite and all ferromagnesian minerals rarely appear in them, and never in considerable amounts.Riebeckite -granites have close affinities to aplites, shown especially in the prevalence of alkali feldspars.Tourmaline also occurs in some aplites.The rocks of this group are very frequent in all areas where masses of granite are known. They form
dykes and irregular veins which may be only a few inches or many feet indiameter . Less frequently aplite forms stocks or bosses, or occupies the edges or irregular portions of the interior ofoutcrop s of granite. Thesyenite -aplites consist mainly of alkali feldspar; the diorite-aplites ofplagioclase ; there arenepheline -bearing aplites which intersect some elaeolite-syenites. In all cases they bear the same relation to the parent masses. By increase of quartz, aplites pass gradually, in a few localities, through highly quartzose modifications into quartz veins.References
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